Auto merge of #141860 - RalfJung:miri-sync, r=RalfJung

Miri subtree update

r? `@ghost`

Includes https://github.com/rust-lang/miri/pull/4370 to unbreak PR CI. (So we're committing to having bda28aa38 in the Miri history by landing this, whether or not that Miri PR lands.)

Cc `@Noratrieb` `@tiif`

41 files changed, 1300 insertions, 246 deletions

diff --git a/src/tools/miri/cargo-miri/src/phases.rs b/src/tools/miri/cargo-miri/src/phases.rs
index 4857f62cd3a..a5e019a8ea9 100644
--- a/src/tools/miri/cargo-miri/src/phases.rs
+++ b/src/tools/miri/cargo-miri/src/phases.rs
@@ -90,7 +90,7 @@ pub fn phase_cargo_miri(mut args: impl Iterator<Item = String>) {
                 "`cargo miri` supports the following subcommands: `run`, `test`, `nextest`, `clean`, and `setup`."
             ),
     };
-    let verbose = num_arg_flag("-v");
+    let verbose = num_arg_flag("-v") + num_arg_flag("--verbose");
     let quiet = has_arg_flag("-q") || has_arg_flag("--quiet");
 
     // Determine the involved architectures.
diff --git a/src/tools/miri/ci/ci.sh b/src/tools/miri/ci/ci.sh
index 9ae15739dcb..8941af681a4 100755
--- a/src/tools/miri/ci/ci.sh
+++ b/src/tools/miri/ci/ci.sh
@@ -142,12 +142,12 @@ case $HOST_TARGET in
     # Host
     GC_STRESS=1 MIR_OPT=1 MANY_SEEDS=64 TEST_BENCH=1 CARGO_MIRI_ENV=1 run_tests
     # Extra tier 1
-    # With reduced many-seed count to avoid spending too much time on that.
-    # (All OSes and ABIs are run with 64 seeds at least once though via the macOS runner.)
-    MANY_SEEDS=16 TEST_TARGET=i686-unknown-linux-gnu run_tests
-    MANY_SEEDS=16 TEST_TARGET=aarch64-unknown-linux-gnu run_tests
-    MANY_SEEDS=16 TEST_TARGET=x86_64-apple-darwin run_tests
-    MANY_SEEDS=16 TEST_TARGET=x86_64-pc-windows-gnu run_tests
+    MANY_SEEDS=64 TEST_TARGET=i686-unknown-linux-gnu run_tests
+    MANY_SEEDS=64 TEST_TARGET=aarch64-unknown-linux-gnu run_tests
+    MANY_SEEDS=64 TEST_TARGET=x86_64-apple-darwin run_tests
+    MANY_SEEDS=64 TEST_TARGET=x86_64-pc-windows-gnu run_tests
+    # Extra tier 1 candidate
+    MANY_SEEDS=64 TEST_TARGET=aarch64-pc-windows-msvc run_tests
     ;;
   aarch64-apple-darwin)
     # Host
@@ -156,7 +156,8 @@ case $HOST_TARGET in
     MANY_SEEDS=64 TEST_TARGET=i686-pc-windows-gnu run_tests
     MANY_SEEDS=64 TEST_TARGET=x86_64-pc-windows-msvc CARGO_MIRI_ENV=1 run_tests
     # Extra tier 2
-    MANY_SEEDS=16 TEST_TARGET=arm-unknown-linux-gnueabi run_tests
+    MANY_SEEDS=16 TEST_TARGET=arm-unknown-linux-gnueabi run_tests # 32bit ARM
+    MANY_SEEDS=16 TEST_TARGET=aarch64-pc-windows-gnullvm run_tests # gnullvm ABI
     MANY_SEEDS=16 TEST_TARGET=s390x-unknown-linux-gnu run_tests # big-endian architecture of choice
     # Not officially supported tier 2
     MANY_SEEDS=16 TEST_TARGET=mips-unknown-linux-gnu run_tests # a 32bit big-endian target, and also a target without 64bit atomics
@@ -178,7 +179,7 @@ case $HOST_TARGET in
     # Host
     # Without GC_STRESS and with reduced many-seeds count as this is the slowest runner.
     # (The macOS runner checks windows-msvc with full many-seeds count.)
-    MIR_OPT=1 MANY_SEEDS=16 TEST_BENCH=1 run_tests
+    MIR_OPT=1 MANY_SEEDS=64 TEST_BENCH=1 run_tests
     # Extra tier 1
     # We really want to ensure a Linux target works on a Windows host,
     # and a 64bit target works on a 32bit host.
diff --git a/src/tools/miri/miri-script/src/commands.rs b/src/tools/miri/miri-script/src/commands.rs
index 3b7b159aeab..86362145d47 100644
--- a/src/tools/miri/miri-script/src/commands.rs
+++ b/src/tools/miri/miri-script/src/commands.rs
@@ -1,4 +1,4 @@
-use std::collections::HashMap;
+use std::collections::BTreeMap;
 use std::ffi::{OsStr, OsString};
 use std::fmt::Write as _;
 use std::fs::{self, File};
@@ -404,7 +404,28 @@ impl Command {
         // We want to forward the host stdin so apparently we cannot use `cmd!`.
         let mut cmd = process::Command::new("git");
         cmd.arg("rebase").arg(&base).arg("--interactive");
-        cmd.env("GIT_SEQUENCE_EDITOR", env::current_exe()?);
+        let current_exe = {
+            if cfg!(windows) {
+                // Apparently git-for-Windows gets confused by backslashes if we just use
+                // `current_exe()` here. So replace them by forward slashes if this is not a "magic"
+                // path starting with "\\". This is clearly a git bug but we work around it here.
+                // Also see <https://github.com/rust-lang/miri/issues/4340>.
+                let bin = env::current_exe()?;
+                match bin.into_os_string().into_string() {
+                    Err(not_utf8) => not_utf8.into(), // :shrug:
+                    Ok(str) => {
+                        if str.starts_with(r"\\") {
+                            str.into() // don't touch these magic paths, they must use backslashes
+                        } else {
+                            str.replace('\\', "/").into()
+                        }
+                    }
+                }
+            } else {
+                env::current_exe()?
+            }
+        };
+        cmd.env("GIT_SEQUENCE_EDITOR", current_exe);
         cmd.env("MIRI_SCRIPT_IS_GIT_SEQUENCE_EDITOR", "1");
         cmd.current_dir(sh.current_dir());
         let result = cmd.status()?;
@@ -489,7 +510,9 @@ impl Command {
             sh.read_dir(benches_dir)?
                 .into_iter()
                 .filter(|path| path.is_dir())
-                .map(|path| path.into_os_string().into_string().unwrap())
+                // Only keep the basename: that matches the usage with a manual bench list,
+                // and it ensure the path concatenations below work as intended.
+                .map(|path| path.file_name().unwrap().to_owned().into_string().unwrap())
                 .collect()
         } else {
             benches.into_iter().collect()
@@ -530,14 +553,16 @@ impl Command {
             stddev: f64,
         }
 
-        let gather_results = || -> Result<HashMap<&str, BenchResult>> {
+        let gather_results = || -> Result<BTreeMap<&str, BenchResult>> {
             let baseline_temp_dir = results_json_dir.unwrap();
-            let mut results = HashMap::new();
+            let mut results = BTreeMap::new();
             for bench in &benches {
-                let result = File::open(path!(baseline_temp_dir / format!("{bench}.bench.json")))?;
-                let mut result: serde_json::Value =
-                    serde_json::from_reader(BufReader::new(result))?;
-                let result: BenchResult = serde_json::from_value(result["results"][0].take())?;
+                let result = File::open(path!(baseline_temp_dir / format!("{bench}.bench.json")))
+                    .context("failed to read hyperfine JSON")?;
+                let mut result: serde_json::Value = serde_json::from_reader(BufReader::new(result))
+                    .context("failed to parse hyperfine JSON")?;
+                let result: BenchResult = serde_json::from_value(result["results"][0].take())
+                    .context("failed to interpret hyperfine JSON")?;
                 results.insert(bench as &str, result);
             }
             Ok(results)
@@ -549,15 +574,15 @@ impl Command {
             serde_json::to_writer_pretty(BufWriter::new(baseline), &results)?;
         } else if let Some(baseline_file) = load_baseline {
             let new_results = gather_results()?;
-            let baseline_results: HashMap<String, BenchResult> = {
+            let baseline_results: BTreeMap<String, BenchResult> = {
                 let f = File::open(baseline_file)?;
                 serde_json::from_reader(BufReader::new(f))?
             };
             println!(
                 "Comparison with baseline (relative speed, lower is better for the new results):"
             );
-            for (bench, new_result) in new_results.iter() {
-                let Some(baseline_result) = baseline_results.get(*bench) else { continue };
+            for (bench, new_result) in new_results {
+                let Some(baseline_result) = baseline_results.get(bench) else { continue };
 
                 // Compare results (inspired by hyperfine)
                 let ratio = new_result.mean / baseline_result.mean;
diff --git a/src/tools/miri/rust-version b/src/tools/miri/rust-version
index 46989695302..553d410b2bc 100644
--- a/src/tools/miri/rust-version
+++ b/src/tools/miri/rust-version
@@ -1 +1 @@
-2b96ddca1272960623e41829439df8dae82d20af
+337c11e5932275e7d450c1f2e26f289f0ddfa717
diff --git a/src/tools/miri/src/alloc_bytes.rs b/src/tools/miri/src/alloc/alloc_bytes.rs
index 2bac2659ec0..2a253952b27 100644
--- a/src/tools/miri/src/alloc_bytes.rs
+++ b/src/tools/miri/src/alloc/alloc_bytes.rs
@@ -1,12 +1,23 @@
 use std::alloc::Layout;
 use std::borrow::Cow;
 use std::{alloc, slice};
+#[cfg(target_os = "linux")]
+use std::{cell::RefCell, rc::Rc};
 
 use rustc_abi::{Align, Size};
 use rustc_middle::mir::interpret::AllocBytes;
 
+#[cfg(target_os = "linux")]
+use crate::alloc::isolated_alloc::IsolatedAlloc;
 use crate::helpers::ToU64 as _;
 
+#[derive(Clone, Debug)]
+pub enum MiriAllocParams {
+    Global,
+    #[cfg(target_os = "linux")]
+    Isolated(Rc<RefCell<IsolatedAlloc>>),
+}
+
 /// Allocation bytes that explicitly handle the layout of the data they're storing.
 /// This is necessary to interface with native code that accesses the program store in Miri.
 #[derive(Debug)]
@@ -18,13 +29,16 @@ pub struct MiriAllocBytes {
     /// * If `self.layout.size() == 0`, then `self.ptr` was allocated with the equivalent layout with size 1.
     /// * Otherwise, `self.ptr` points to memory allocated with `self.layout`.
     ptr: *mut u8,
+    /// Whether this instance of `MiriAllocBytes` had its allocation created by calling `alloc::alloc()`
+    /// (`Global`) or the discrete allocator (`Isolated`)
+    params: MiriAllocParams,
 }
 
 impl Clone for MiriAllocBytes {
     fn clone(&self) -> Self {
         let bytes: Cow<'_, [u8]> = Cow::Borrowed(self);
         let align = Align::from_bytes(self.layout.align().to_u64()).unwrap();
-        MiriAllocBytes::from_bytes(bytes, align, ())
+        MiriAllocBytes::from_bytes(bytes, align, self.params.clone())
     }
 }
 
@@ -37,8 +51,16 @@ impl Drop for MiriAllocBytes {
         } else {
             self.layout
         };
+
         // SAFETY: Invariant, `self.ptr` points to memory allocated with `self.layout`.
-        unsafe { alloc::dealloc(self.ptr, alloc_layout) }
+        unsafe {
+            match self.params.clone() {
+                MiriAllocParams::Global => alloc::dealloc(self.ptr, alloc_layout),
+                #[cfg(target_os = "linux")]
+                MiriAllocParams::Isolated(alloc) =>
+                    alloc.borrow_mut().dealloc(self.ptr, alloc_layout),
+            }
+        }
     }
 }
 
@@ -67,7 +89,8 @@ impl MiriAllocBytes {
     fn alloc_with(
         size: u64,
         align: u64,
-        alloc_fn: impl FnOnce(Layout) -> *mut u8,
+        params: MiriAllocParams,
+        alloc_fn: impl FnOnce(Layout, &MiriAllocParams) -> *mut u8,
     ) -> Result<MiriAllocBytes, ()> {
         let size = usize::try_from(size).map_err(|_| ())?;
         let align = usize::try_from(align).map_err(|_| ())?;
@@ -75,27 +98,36 @@ impl MiriAllocBytes {
         // When size is 0 we allocate 1 byte anyway, to ensure each allocation has a unique address.
         let alloc_layout =
             if size == 0 { Layout::from_size_align(1, align).unwrap() } else { layout };
-        let ptr = alloc_fn(alloc_layout);
+        let ptr = alloc_fn(alloc_layout, &params);
         if ptr.is_null() {
             Err(())
         } else {
             // SAFETY: All `MiriAllocBytes` invariants are fulfilled.
-            Ok(Self { ptr, layout })
+            Ok(Self { ptr, layout, params })
         }
     }
 }
 
 impl AllocBytes for MiriAllocBytes {
-    /// Placeholder!
-    type AllocParams = ();
+    type AllocParams = MiriAllocParams;
 
-    fn from_bytes<'a>(slice: impl Into<Cow<'a, [u8]>>, align: Align, _params: ()) -> Self {
+    fn from_bytes<'a>(
+        slice: impl Into<Cow<'a, [u8]>>,
+        align: Align,
+        params: MiriAllocParams,
+    ) -> Self {
         let slice = slice.into();
         let size = slice.len();
         let align = align.bytes();
         // SAFETY: `alloc_fn` will only be used with `size != 0`.
-        let alloc_fn = |layout| unsafe { alloc::alloc(layout) };
-        let alloc_bytes = MiriAllocBytes::alloc_with(size.to_u64(), align, alloc_fn)
+        let alloc_fn = |layout, params: &MiriAllocParams| unsafe {
+            match params {
+                MiriAllocParams::Global => alloc::alloc(layout),
+                #[cfg(target_os = "linux")]
+                MiriAllocParams::Isolated(alloc) => alloc.borrow_mut().alloc(layout),
+            }
+        };
+        let alloc_bytes = MiriAllocBytes::alloc_with(size.to_u64(), align, params, alloc_fn)
             .unwrap_or_else(|()| {
                 panic!("Miri ran out of memory: cannot create allocation of {size} bytes")
             });
@@ -105,12 +137,18 @@ impl AllocBytes for MiriAllocBytes {
         alloc_bytes
     }
 
-    fn zeroed(size: Size, align: Align, _params: ()) -> Option<Self> {
+    fn zeroed(size: Size, align: Align, params: MiriAllocParams) -> Option<Self> {
         let size = size.bytes();
         let align = align.bytes();
         // SAFETY: `alloc_fn` will only be used with `size != 0`.
-        let alloc_fn = |layout| unsafe { alloc::alloc_zeroed(layout) };
-        MiriAllocBytes::alloc_with(size, align, alloc_fn).ok()
+        let alloc_fn = |layout, params: &MiriAllocParams| unsafe {
+            match params {
+                MiriAllocParams::Global => alloc::alloc_zeroed(layout),
+                #[cfg(target_os = "linux")]
+                MiriAllocParams::Isolated(alloc) => alloc.borrow_mut().alloc_zeroed(layout),
+            }
+        };
+        MiriAllocBytes::alloc_with(size, align, params, alloc_fn).ok()
     }
 
     fn as_mut_ptr(&mut self) -> *mut u8 {
diff --git a/src/tools/miri/src/alloc/isolated_alloc.rs b/src/tools/miri/src/alloc/isolated_alloc.rs
new file mode 100644
index 00000000000..7b74d171373
--- /dev/null
+++ b/src/tools/miri/src/alloc/isolated_alloc.rs
@@ -0,0 +1,389 @@
+use std::alloc::{self, Layout};
+
+use rustc_index::bit_set::DenseBitSet;
+
+/// How many bytes of memory each bit in the bitset represents.
+const COMPRESSION_FACTOR: usize = 4;
+
+/// A dedicated allocator for interpreter memory contents, ensuring they are stored on dedicated
+/// pages (not mixed with Miri's own memory). This is used in native-lib mode.
+#[derive(Debug)]
+pub struct IsolatedAlloc {
+    /// Pointers to page-aligned memory that has been claimed by the allocator.
+    /// Every pointer here must point to a page-sized allocation claimed via
+    /// the global allocator. These pointers are used for "small" allocations.
+    page_ptrs: Vec<*mut u8>,
+    /// Metadata about which bytes have been allocated on each page. The length
+    /// of this vector must be the same as that of `page_ptrs`, and the domain
+    /// size of the bitset must be exactly `page_size / COMPRESSION_FACTOR`.
+    ///
+    /// Conceptually, each bit of the bitset represents the allocation status of
+    /// one n-byte chunk on the corresponding element of `page_ptrs`. Thus,
+    /// indexing into it should be done with a value one-nth of the corresponding
+    /// offset on the matching `page_ptrs` element (n = `COMPRESSION_FACTOR`).
+    page_infos: Vec<DenseBitSet<usize>>,
+    /// Pointers to multiple-page-sized allocations. These must also be page-aligned,
+    /// with their size stored as the second element of the vector.
+    huge_ptrs: Vec<(*mut u8, usize)>,
+    /// The host (not emulated) page size.
+    page_size: usize,
+}
+
+impl IsolatedAlloc {
+    /// Creates an empty allocator.
+    pub fn new() -> Self {
+        Self {
+            page_ptrs: Vec::new(),
+            huge_ptrs: Vec::new(),
+            page_infos: Vec::new(),
+            // SAFETY: `sysconf(_SC_PAGESIZE)` is always safe to call at runtime
+            // See https://www.man7.org/linux/man-pages/man3/sysconf.3.html
+            page_size: unsafe { libc::sysconf(libc::_SC_PAGESIZE).try_into().unwrap() },
+        }
+    }
+
+    /// For simplicity, we serve small allocations in multiples of COMPRESSION_FACTOR
+    /// bytes with at least that alignment.
+    #[inline]
+    fn normalized_layout(layout: Layout) -> Layout {
+        let align =
+            if layout.align() < COMPRESSION_FACTOR { COMPRESSION_FACTOR } else { layout.align() };
+        let size = layout.size().next_multiple_of(COMPRESSION_FACTOR);
+        Layout::from_size_align(size, align).unwrap()
+    }
+
+    /// Returns the layout used to allocate the pages that hold small allocations.
+    #[inline]
+    fn page_layout(&self) -> Layout {
+        Layout::from_size_align(self.page_size, self.page_size).unwrap()
+    }
+
+    /// If the allocation is greater than a page, then round to the nearest page #.
+    #[inline]
+    fn huge_normalized_layout(layout: Layout, page_size: usize) -> Layout {
+        // Allocate in page-sized chunks
+        let size = layout.size().next_multiple_of(page_size);
+        // And make sure the align is at least one page
+        let align = std::cmp::max(layout.align(), page_size);
+        Layout::from_size_align(size, align).unwrap()
+    }
+
+    /// Determined whether a given normalized (size, align) should be sent to
+    /// `alloc_huge` / `dealloc_huge`.
+    #[inline]
+    fn is_huge_alloc(&self, layout: &Layout) -> bool {
+        layout.align() > self.page_size / 2 || layout.size() >= self.page_size / 2
+    }
+
+    /// Allocates memory as described in `Layout`. This memory should be deallocated
+    /// by calling `dealloc` on this same allocator.
+    ///
+    /// SAFETY: See `alloc::alloc()`
+    pub unsafe fn alloc(&mut self, layout: Layout) -> *mut u8 {
+        // SAFETY: Upheld by caller
+        unsafe { self.allocate(layout, false) }
+    }
+
+    /// Same as `alloc`, but zeroes out the memory.
+    ///
+    /// SAFETY: See `alloc::alloc_zeroed()`
+    pub unsafe fn alloc_zeroed(&mut self, layout: Layout) -> *mut u8 {
+        // SAFETY: Upheld by caller
+        unsafe { self.allocate(layout, true) }
+    }
+
+    /// Abstracts over the logic of `alloc_zeroed` vs `alloc`, as determined by
+    /// the `zeroed` argument.
+    ///
+    /// SAFETY: See `alloc::alloc()`, with the added restriction that `page_size`
+    /// corresponds to the host pagesize.
+    unsafe fn allocate(&mut self, layout: Layout, zeroed: bool) -> *mut u8 {
+        let layout = IsolatedAlloc::normalized_layout(layout);
+        if self.is_huge_alloc(&layout) {
+            // SAFETY: Validity of `layout` upheld by caller; we checked that
+            // the size and alignment are appropriate for being a huge alloc
+            unsafe { self.alloc_huge(layout, zeroed) }
+        } else {
+            for (&mut page, pinfo) in std::iter::zip(&mut self.page_ptrs, &mut self.page_infos) {
+                // SAFETY: The value in `self.page_size` is used to allocate
+                // `page`, with page alignment
+                if let Some(ptr) =
+                    unsafe { Self::alloc_small(self.page_size, layout, page, pinfo, zeroed) }
+                {
+                    return ptr;
+                }
+            }
+
+            // We get here only if there's no space in our existing pages
+            let page_size = self.page_size;
+            // Add another page and allocate from it; this cannot fail since the
+            // new page is empty and we already asserted it fits into a page
+            let (page, pinfo) = self.add_page();
+
+            // SAFETY: See comment on `alloc_from_page` above
+            unsafe { Self::alloc_small(page_size, layout, page, pinfo, zeroed).unwrap() }
+        }
+    }
+
+    /// Used internally by `allocate` to abstract over some logic.
+    ///
+    /// SAFETY: `page` must be a page-aligned pointer to an allocated page,
+    /// where the allocation is (at least) `page_size` bytes.
+    unsafe fn alloc_small(
+        page_size: usize,
+        layout: Layout,
+        page: *mut u8,
+        pinfo: &mut DenseBitSet<usize>,
+        zeroed: bool,
+    ) -> Option<*mut u8> {
+        // Check every alignment-sized block and see if there exists a `size`
+        // chunk of empty space i.e. forall idx . !pinfo.contains(idx / n)
+        for offset in (0..page_size).step_by(layout.align()) {
+            let offset_pinfo = offset / COMPRESSION_FACTOR;
+            let size_pinfo = layout.size() / COMPRESSION_FACTOR;
+            // DenseBitSet::contains() panics if the index is out of bounds
+            if pinfo.domain_size() < offset_pinfo + size_pinfo {
+                break;
+            }
+            // FIXME: is there a more efficient way to check whether the entire range is unset
+            // in the bitset?
+            let range_avail = !(offset_pinfo..offset_pinfo + size_pinfo).any(|i| pinfo.contains(i));
+            if range_avail {
+                pinfo.insert_range(offset_pinfo..offset_pinfo + size_pinfo);
+                // SAFETY: We checked the available bytes after `idx` in the call
+                // to `domain_size` above and asserted there are at least `idx +
+                // layout.size()` bytes available and unallocated after it.
+                // `page` must point to the start of the page, so adding `idx`
+                // is safe per the above.
+                unsafe {
+                    let ptr = page.add(offset);
+                    if zeroed {
+                        // Only write the bytes we were specifically asked to
+                        // zero out, even if we allocated more
+                        ptr.write_bytes(0, layout.size());
+                    }
+                    return Some(ptr);
+                }
+            }
+        }
+        None
+    }
+
+    /// Expands the available memory pool by adding one page.
+    fn add_page(&mut self) -> (*mut u8, &mut DenseBitSet<usize>) {
+        // SAFETY: The system page size, which is the layout size, cannot be 0
+        let page_ptr = unsafe { alloc::alloc(self.page_layout()) };
+        // `page_infos` has to have one bit for each `COMPRESSION_FACTOR`-sized chunk of bytes in the page.
+        assert!(self.page_size % COMPRESSION_FACTOR == 0);
+        self.page_infos.push(DenseBitSet::new_empty(self.page_size / COMPRESSION_FACTOR));
+        self.page_ptrs.push(page_ptr);
+        (page_ptr, self.page_infos.last_mut().unwrap())
+    }
+
+    /// Allocates in multiples of one page on the host system.
+    ///
+    /// SAFETY: Same as `alloc()`.
+    unsafe fn alloc_huge(&mut self, layout: Layout, zeroed: bool) -> *mut u8 {
+        let layout = IsolatedAlloc::huge_normalized_layout(layout, self.page_size);
+        // SAFETY: Upheld by caller
+        let ret =
+            unsafe { if zeroed { alloc::alloc_zeroed(layout) } else { alloc::alloc(layout) } };
+        self.huge_ptrs.push((ret, layout.size()));
+        ret
+    }
+
+    /// Deallocates a pointer from this allocator.
+    ///
+    /// SAFETY: This pointer must have been allocated by calling `alloc()` (or
+    /// `alloc_zeroed()`) with the same layout as the one passed on this same
+    /// `IsolatedAlloc`.
+    pub unsafe fn dealloc(&mut self, ptr: *mut u8, layout: Layout) {
+        let layout = IsolatedAlloc::normalized_layout(layout);
+
+        if self.is_huge_alloc(&layout) {
+            // SAFETY: Partly upheld by caller, and we checked that the size
+            // and align, meaning this must have been allocated via `alloc_huge`
+            unsafe {
+                self.dealloc_huge(ptr, layout);
+            }
+        } else {
+            // SAFETY: It's not a huge allocation, therefore it is a small one.
+            let idx = unsafe { self.dealloc_small(ptr, layout) };
+
+            // This may have been the last allocation on this page. If so, free the entire page.
+            // FIXME: this can lead to threshold effects, we should probably add some form
+            // of hysteresis.
+            if self.page_infos[idx].is_empty() {
+                self.page_infos.remove(idx);
+                let page_ptr = self.page_ptrs.remove(idx);
+                // SAFETY: We checked that there are no outstanding allocations
+                // from us pointing to this page, and we know it was allocated
+                // with this layout
+                unsafe {
+                    alloc::dealloc(page_ptr, self.page_layout());
+                }
+            }
+        }
+    }
+
+    /// Returns the index of the page that this was deallocated from
+    ///
+    /// SAFETY: the pointer must have been allocated with `alloc_small`.
+    unsafe fn dealloc_small(&mut self, ptr: *mut u8, layout: Layout) -> usize {
+        // Offset of the pointer in the current page
+        let offset = ptr.addr() % self.page_size;
+        // And then the page's base address
+        let page_addr = ptr.addr() - offset;
+
+        // Find the page this allocation belongs to.
+        // This could be made faster if the list was sorted -- the allocator isn't fully optimized at the moment.
+        let pinfo = std::iter::zip(&mut self.page_ptrs, &mut self.page_infos)
+            .enumerate()
+            .find(|(_, (page, _))| page.addr() == page_addr);
+        let Some((idx_of_pinfo, (_, pinfo))) = pinfo else {
+            panic!("Freeing in an unallocated page: {ptr:?}\nHolding pages {:?}", self.page_ptrs)
+        };
+        // Mark this range as available in the page.
+        let ptr_idx_pinfo = offset / COMPRESSION_FACTOR;
+        let size_pinfo = layout.size() / COMPRESSION_FACTOR;
+        for idx in ptr_idx_pinfo..ptr_idx_pinfo + size_pinfo {
+            pinfo.remove(idx);
+        }
+        idx_of_pinfo
+    }
+
+    /// SAFETY: Same as `dealloc()` with the added requirement that `layout`
+    /// must ask for a size larger than the host pagesize.
+    unsafe fn dealloc_huge(&mut self, ptr: *mut u8, layout: Layout) {
+        let layout = IsolatedAlloc::huge_normalized_layout(layout, self.page_size);
+        // Find the pointer matching in address with the one we got
+        let idx = self
+            .huge_ptrs
+            .iter()
+            .position(|pg| ptr.addr() == pg.0.addr())
+            .expect("Freeing unallocated pages");
+        // And kick it from the list
+        self.huge_ptrs.remove(idx);
+        // SAFETY: Caller ensures validity of the layout
+        unsafe {
+            alloc::dealloc(ptr, layout);
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    /// Helper function to assert that all bytes from `ptr` to `ptr.add(layout.size())`
+    /// are zeroes.
+    ///
+    /// SAFETY: `ptr` must have been allocated with `layout`.
+    unsafe fn assert_zeroes(ptr: *mut u8, layout: Layout) {
+        // SAFETY: Caller ensures this is valid
+        unsafe {
+            for ofs in 0..layout.size() {
+                assert_eq!(0, ptr.add(ofs).read());
+            }
+        }
+    }
+
+    /// Check that small (sub-pagesize) allocations are properly zeroed out.
+    #[test]
+    fn small_zeroes() {
+        let mut alloc = IsolatedAlloc::new();
+        // 256 should be less than the pagesize on *any* system
+        let layout = Layout::from_size_align(256, 32).unwrap();
+        // SAFETY: layout size is the constant above, not 0
+        let ptr = unsafe { alloc.alloc_zeroed(layout) };
+        // SAFETY: `ptr` was just allocated with `layout`
+        unsafe {
+            assert_zeroes(ptr, layout);
+            alloc.dealloc(ptr, layout);
+        }
+    }
+
+    /// Check that huge (> 1 page) allocations are properly zeroed out also.
+    #[test]
+    fn huge_zeroes() {
+        let mut alloc = IsolatedAlloc::new();
+        // 16k is about as big as pages get e.g. on macos aarch64
+        let layout = Layout::from_size_align(16 * 1024, 128).unwrap();
+        // SAFETY: layout size is the constant above, not 0
+        let ptr = unsafe { alloc.alloc_zeroed(layout) };
+        // SAFETY: `ptr` was just allocated with `layout`
+        unsafe {
+            assert_zeroes(ptr, layout);
+            alloc.dealloc(ptr, layout);
+        }
+    }
+
+    /// Check that repeatedly reallocating the same memory will still zero out
+    /// everything properly
+    #[test]
+    fn repeated_allocs() {
+        let mut alloc = IsolatedAlloc::new();
+        // Try both sub-pagesize allocs and those larger than / equal to a page
+        for sz in (1..=(16 * 1024)).step_by(128) {
+            let layout = Layout::from_size_align(sz, 1).unwrap();
+            // SAFETY: all sizes in the range above are nonzero as we start from 1
+            let ptr = unsafe { alloc.alloc_zeroed(layout) };
+            // SAFETY: `ptr` was just allocated with `layout`, which was used
+            // to bound the access size
+            unsafe {
+                assert_zeroes(ptr, layout);
+                ptr.write_bytes(255, sz);
+                alloc.dealloc(ptr, layout);
+            }
+        }
+    }
+
+    /// Checks that allocations of different sizes do not overlap, then for memory
+    /// leaks that might have occurred.
+    #[test]
+    fn check_leaks_and_overlaps() {
+        let mut alloc = IsolatedAlloc::new();
+
+        // Some random sizes and aligns
+        let mut sizes = vec![32; 10];
+        sizes.append(&mut vec![15; 4]);
+        sizes.append(&mut vec![256; 12]);
+        // Give it some multi-page ones too
+        sizes.append(&mut vec![32 * 1024; 4]);
+
+        // Matching aligns for the sizes
+        let mut aligns = vec![16; 12];
+        aligns.append(&mut vec![256; 2]);
+        aligns.append(&mut vec![64; 12]);
+        aligns.append(&mut vec![4096; 4]);
+
+        // Make sure we didn't mess up in the test itself!
+        assert_eq!(sizes.len(), aligns.len());
+
+        // Aggregate the sizes and aligns into a vec of layouts, then allocate them
+        let layouts: Vec<_> = std::iter::zip(sizes, aligns)
+            .map(|(sz, al)| Layout::from_size_align(sz, al).unwrap())
+            .collect();
+        // SAFETY: all sizes specified in `sizes` are nonzero
+        let ptrs: Vec<_> =
+            layouts.iter().map(|layout| unsafe { alloc.alloc_zeroed(*layout) }).collect();
+
+        for (&ptr, &layout) in std::iter::zip(&ptrs, &layouts) {
+            // We requested zeroed allocations, so check that that's true
+            // Then write to the end of the current size, so if the allocs
+            // overlap (or the zeroing is wrong) then `assert_zeroes` will panic.
+            // Also check that the alignment we asked for was respected
+            assert_eq!(ptr.addr().strict_rem(layout.align()), 0);
+            // SAFETY: each `ptr` was allocated with its corresponding `layout`,
+            // which is used to bound the access size
+            unsafe {
+                assert_zeroes(ptr, layout);
+                ptr.write_bytes(255, layout.size());
+                alloc.dealloc(ptr, layout);
+            }
+        }
+
+        // And then verify that no memory was leaked after all that
+        assert!(alloc.page_ptrs.is_empty() && alloc.huge_ptrs.is_empty());
+    }
+}
diff --git a/src/tools/miri/src/alloc/mod.rs b/src/tools/miri/src/alloc/mod.rs
new file mode 100644
index 00000000000..3be885920d2
--- /dev/null
+++ b/src/tools/miri/src/alloc/mod.rs
@@ -0,0 +1,5 @@
+mod alloc_bytes;
+#[cfg(target_os = "linux")]
+pub mod isolated_alloc;
+
+pub use self::alloc_bytes::{MiriAllocBytes, MiriAllocParams};
diff --git a/src/tools/miri/src/alloc_addresses/mod.rs b/src/tools/miri/src/alloc_addresses/mod.rs
index d2977a55e46..12a320b9676 100644
--- a/src/tools/miri/src/alloc_addresses/mod.rs
+++ b/src/tools/miri/src/alloc_addresses/mod.rs
@@ -135,11 +135,12 @@ trait EvalContextExtPriv<'tcx>: crate::MiriInterpCxExt<'tcx> {
         if this.machine.native_lib.is_some() {
             // In native lib mode, we use the "real" address of the bytes for this allocation.
             // This ensures the interpreted program and native code have the same view of memory.
+            let params = this.machine.get_default_alloc_params();
             let base_ptr = match info.kind {
                 AllocKind::LiveData => {
                     if memory_kind == MiriMemoryKind::Global.into() {
                         // For new global allocations, we always pre-allocate the memory to be able use the machine address directly.
-                        let prepared_bytes = MiriAllocBytes::zeroed(info.size, info.align, ())
+                        let prepared_bytes = MiriAllocBytes::zeroed(info.size, info.align, params)
                             .unwrap_or_else(|| {
                                 panic!("Miri ran out of memory: cannot create allocation of {size:?} bytes", size = info.size)
                             });
@@ -158,8 +159,11 @@ trait EvalContextExtPriv<'tcx>: crate::MiriInterpCxExt<'tcx> {
                 }
                 AllocKind::Function | AllocKind::VTable => {
                     // Allocate some dummy memory to get a unique address for this function/vtable.
-                    let alloc_bytes =
-                        MiriAllocBytes::from_bytes(&[0u8; 1], Align::from_bytes(1).unwrap(), ());
+                    let alloc_bytes = MiriAllocBytes::from_bytes(
+                        &[0u8; 1],
+                        Align::from_bytes(1).unwrap(),
+                        params,
+                    );
                     let ptr = alloc_bytes.as_ptr();
                     // Leak the underlying memory to ensure it remains unique.
                     std::mem::forget(alloc_bytes);
@@ -429,7 +433,8 @@ pub trait EvalContextExt<'tcx>: crate::MiriInterpCxExt<'tcx> {
             prepared_alloc_bytes.copy_from_slice(bytes);
             interp_ok(prepared_alloc_bytes)
         } else {
-            interp_ok(MiriAllocBytes::from_bytes(std::borrow::Cow::Borrowed(bytes), align, ()))
+            let params = this.machine.get_default_alloc_params();
+            interp_ok(MiriAllocBytes::from_bytes(std::borrow::Cow::Borrowed(bytes), align, params))
         }
     }
 
diff --git a/src/tools/miri/src/bin/miri.rs b/src/tools/miri/src/bin/miri.rs
index 7098ef5130d..0121472d330 100644
--- a/src/tools/miri/src/bin/miri.rs
+++ b/src/tools/miri/src/bin/miri.rs
@@ -281,7 +281,9 @@ impl rustc_driver::Callbacks for MiriBeRustCompilerCalls {
                             }
                             let codegen_fn_attrs = tcx.codegen_fn_attrs(local_def_id);
                             if codegen_fn_attrs.contains_extern_indicator()
-                                || codegen_fn_attrs.flags.contains(CodegenFnAttrFlags::USED_COMPILER)
+                                || codegen_fn_attrs
+                                    .flags
+                                    .contains(CodegenFnAttrFlags::USED_COMPILER)
                                 || codegen_fn_attrs.flags.contains(CodegenFnAttrFlags::USED_LINKER)
                             {
                                 Some((
diff --git a/src/tools/miri/src/borrow_tracker/tree_borrows/diagnostics.rs b/src/tools/miri/src/borrow_tracker/tree_borrows/diagnostics.rs
index f5a0013047a..7b4c533cfae 100644
--- a/src/tools/miri/src/borrow_tracker/tree_borrows/diagnostics.rs
+++ b/src/tools/miri/src/borrow_tracker/tree_borrows/diagnostics.rs
@@ -504,7 +504,7 @@ impl DisplayFmt {
         if let Some(perm) = perm {
             format!(
                 "{ac}{st}",
-                ac = if perm.is_initialized() { self.accessed.yes } else { self.accessed.no },
+                ac = if perm.is_accessed() { self.accessed.yes } else { self.accessed.no },
                 st = perm.permission().short_name(),
             )
         } else {
diff --git a/src/tools/miri/src/borrow_tracker/tree_borrows/mod.rs b/src/tools/miri/src/borrow_tracker/tree_borrows/mod.rs
index f3e32e75f2f..411ae89da90 100644
--- a/src/tools/miri/src/borrow_tracker/tree_borrows/mod.rs
+++ b/src/tools/miri/src/borrow_tracker/tree_borrows/mod.rs
@@ -3,6 +3,8 @@ use rustc_middle::mir::{Mutability, RetagKind};
 use rustc_middle::ty::layout::HasTypingEnv;
 use rustc_middle::ty::{self, Ty};
 
+use self::foreign_access_skipping::IdempotentForeignAccess;
+use self::tree::LocationState;
 use crate::borrow_tracker::{GlobalState, GlobalStateInner, ProtectorKind};
 use crate::concurrency::data_race::NaReadType;
 use crate::*;
@@ -95,7 +97,7 @@ impl<'tcx> Tree {
     /// A tag just lost its protector.
     ///
     /// This emits a special kind of access that is only applied
-    /// to initialized locations, as a protection against other
+    /// to accessed locations, as a protection against other
     /// tags not having been made aware of the existence of this
     /// protector.
     pub fn release_protector(
@@ -113,16 +115,19 @@ impl<'tcx> Tree {
 
 /// Policy for a new borrow.
 #[derive(Debug, Clone, Copy)]
-struct NewPermission {
-    /// Which permission should the pointer start with.
-    initial_state: Permission,
+pub struct NewPermission {
+    /// Permission for the frozen part of the range.
+    freeze_perm: Permission,
+    /// Whether a read access should be performed on the frozen part on a retag.
+    freeze_access: bool,
+    /// Permission for the non-frozen part of the range.
+    nonfreeze_perm: Permission,
+    /// Whether a read access should be performed on the non-frozen
+    /// part on a retag.
+    nonfreeze_access: bool,
     /// Whether this pointer is part of the arguments of a function call.
     /// `protector` is `Some(_)` for all pointers marked `noalias`.
     protector: Option<ProtectorKind>,
-    /// Whether a read should be performed on a retag.  This should be `false`
-    /// for `Cell` because this could cause data races when using thread-safe
-    /// data types like `Mutex<T>`.
-    initial_read: bool,
 }
 
 impl<'tcx> NewPermission {
@@ -133,27 +138,42 @@ impl<'tcx> NewPermission {
         kind: RetagKind,
         cx: &crate::MiriInterpCx<'tcx>,
     ) -> Option<Self> {
-        let ty_is_freeze = pointee.is_freeze(*cx.tcx, cx.typing_env());
         let ty_is_unpin = pointee.is_unpin(*cx.tcx, cx.typing_env());
         let is_protected = kind == RetagKind::FnEntry;
-        // As demonstrated by `tests/fail/tree_borrows/reservedim_spurious_write.rs`,
-        // interior mutability and protectors interact poorly.
-        // To eliminate the case of Protected Reserved IM we override interior mutability
-        // in the case of a protected reference: protected references are always considered
-        // "freeze" in their reservation phase.
-        let (initial_state, initial_read) = match mutability {
+        let protector = is_protected.then_some(ProtectorKind::StrongProtector);
+
+        Some(match mutability {
             Mutability::Mut if ty_is_unpin =>
-                (Permission::new_reserved(ty_is_freeze, is_protected), true),
-            Mutability::Not if ty_is_freeze => (Permission::new_frozen(), true),
-            Mutability::Not if !ty_is_freeze => (Permission::new_cell(), false),
-            // Raw pointers never enter this function so they are not handled.
-            // However raw pointers are not the only pointers that take the parent
-            // tag, this also happens for `!Unpin` `&mut`s, which are excluded above.
+                NewPermission {
+                    freeze_perm: Permission::new_reserved(
+                        /* ty_is_freeze */ true,
+                        is_protected,
+                    ),
+                    freeze_access: true,
+                    nonfreeze_perm: Permission::new_reserved(
+                        /* ty_is_freeze */ false,
+                        is_protected,
+                    ),
+                    // If we have a mutable reference, then the non-frozen part will
+                    // have state `ReservedIM` or `Reserved`, which can have an initial read access
+                    // performed on it because you cannot have multiple mutable borrows.
+                    nonfreeze_access: true,
+                    protector,
+                },
+            Mutability::Not =>
+                NewPermission {
+                    freeze_perm: Permission::new_frozen(),
+                    freeze_access: true,
+                    nonfreeze_perm: Permission::new_cell(),
+                    // If it is a shared reference, then the non-frozen
+                    // part will have state `Cell`, which should not have an initial access,
+                    // as this can cause data races when using thread-safe data types like
+                    // `Mutex<T>`.
+                    nonfreeze_access: false,
+                    protector,
+                },
             _ => return None,
-        };
-
-        let protector = is_protected.then_some(ProtectorKind::StrongProtector);
-        Some(Self { initial_state, protector, initial_read })
+        })
     }
 
     /// Compute permission for `Box`-like type (`Box` always, and also `Unique` if enabled).
@@ -168,13 +188,17 @@ impl<'tcx> NewPermission {
         pointee.is_unpin(*cx.tcx, cx.typing_env()).then_some(()).map(|()| {
             // Regular `Unpin` box, give it `noalias` but only a weak protector
             // because it is valid to deallocate it within the function.
-            let ty_is_freeze = pointee.is_freeze(*cx.tcx, cx.typing_env());
-            let protected = kind == RetagKind::FnEntry;
-            let initial_state = Permission::new_reserved(ty_is_freeze, protected);
-            Self {
-                initial_state,
-                protector: protected.then_some(ProtectorKind::WeakProtector),
-                initial_read: true,
+            let is_protected = kind == RetagKind::FnEntry;
+            let protector = is_protected.then_some(ProtectorKind::WeakProtector);
+            NewPermission {
+                freeze_perm: Permission::new_reserved(/* ty_is_freeze */ true, is_protected),
+                freeze_access: true,
+                nonfreeze_perm: Permission::new_reserved(
+                    /* ty_is_freeze */ false,
+                    is_protected,
+                ),
+                nonfreeze_access: true,
+                protector,
             }
         })
     }
@@ -194,8 +218,6 @@ trait EvalContextPrivExt<'tcx>: crate::MiriInterpCxExt<'tcx> {
         new_tag: BorTag,
     ) -> InterpResult<'tcx, Option<Provenance>> {
         let this = self.eval_context_mut();
-        // Make sure the new permission makes sense as the initial permission of a fresh tag.
-        assert!(new_perm.initial_state.is_initial());
         // Ensure we bail out if the pointer goes out-of-bounds (see miri#1050).
         this.check_ptr_access(place.ptr(), ptr_size, CheckInAllocMsg::Dereferenceable)?;
 
@@ -206,7 +228,13 @@ trait EvalContextPrivExt<'tcx>: crate::MiriInterpCxExt<'tcx> {
             let global = this.machine.borrow_tracker.as_ref().unwrap().borrow();
             let ty = place.layout.ty;
             if global.tracked_pointer_tags.contains(&new_tag) {
-                let kind_str = format!("initial state {} (pointee type {ty})", new_perm.initial_state);
+                 let ty_is_freeze = ty.is_freeze(*this.tcx, this.typing_env());
+                 let kind_str =
+                     if ty_is_freeze {
+                         format!("initial state {} (pointee type {ty})", new_perm.freeze_perm)
+                     } else {
+                         format!("initial state {}/{} outside/inside UnsafeCell (pointee type {ty})", new_perm.freeze_perm, new_perm.nonfreeze_perm)
+                     };
                 this.emit_diagnostic(NonHaltingDiagnostic::CreatedPointerTag(
                     new_tag.inner(),
                     Some(kind_str),
@@ -285,43 +313,103 @@ trait EvalContextPrivExt<'tcx>: crate::MiriInterpCxExt<'tcx> {
 
         let span = this.machine.current_span();
         let alloc_extra = this.get_alloc_extra(alloc_id)?;
-        let range = alloc_range(base_offset, ptr_size);
         let mut tree_borrows = alloc_extra.borrow_tracker_tb().borrow_mut();
 
-        // All reborrows incur a (possibly zero-sized) read access to the parent
-        if new_perm.initial_read {
-            tree_borrows.perform_access(
-                orig_tag,
-                Some((range, AccessKind::Read, diagnostics::AccessCause::Reborrow)),
-                this.machine.borrow_tracker.as_ref().unwrap(),
-                alloc_id,
-                this.machine.current_span(),
-            )?;
-        }
+        // Store initial permissions and their corresponding range.
+        let mut perms_map: RangeMap<LocationState> = RangeMap::new(
+            ptr_size,
+            LocationState::new_accessed(Permission::new_disabled(), IdempotentForeignAccess::None), // this will be overwritten
+        );
+        // Keep track of whether the node has any part that allows for interior mutability.
+        // FIXME: This misses `PhantomData<UnsafeCell<T>>` which could be considered a marker
+        // for requesting interior mutability.
+        let mut has_unsafe_cell = false;
+
+        // When adding a new node, the SIFA of its parents needs to be updated, potentially across
+        // the entire memory range. For the parts that are being accessed below, the access itself
+        // trivially takes care of that. However, we have to do some more work to also deal with
+        // the parts that are not being accessed. Specifically what we do is that we
+        // call `update_last_accessed_after_retag` on the SIFA of the permission set for the part of
+        // memory outside `perm_map` -- so that part is definitely taken care of. The remaining concern
+        // is the part of memory that is in the range of `perms_map`, but not accessed below.
+        // There we have two cases:
+        // * If we do have an `UnsafeCell` (`has_unsafe_cell` becomes true), then the non-accessed part
+        //   uses `nonfreeze_perm`, so the `nonfreeze_perm` initialized parts are also fine. We enforce
+        //   the `freeze_perm` parts to be accessed, and thus everything is taken care of.
+        // * If there is no `UnsafeCell`, then `freeze_perm` is used everywhere (both inside and outside the initial range),
+        //   and we update everything to have the `freeze_perm`'s SIFA, so there are no issues. (And this assert below is not
+        //   actually needed in this case).
+        assert!(new_perm.freeze_access);
+
+        let protected = new_perm.protector.is_some();
+        this.visit_freeze_sensitive(place, ptr_size, |range, frozen| {
+            has_unsafe_cell = has_unsafe_cell || !frozen;
+
+            // We are only ever `Frozen` inside the frozen bits.
+            let (perm, access) = if frozen {
+                (new_perm.freeze_perm, new_perm.freeze_access)
+            } else {
+                (new_perm.nonfreeze_perm, new_perm.nonfreeze_access)
+            };
+
+            // Store initial permissions.
+            for (_loc_range, loc) in perms_map.iter_mut(range.start, range.size) {
+                let sifa = perm.strongest_idempotent_foreign_access(protected);
+                // NOTE: Currently, `access` is false if and only if `perm` is Cell, so this `if`
+                // doesn't not change whether any code is UB or not. We could just always use
+                // `new_accessed` and everything would stay the same. But that seems conceptually
+                // odd, so we keep the initial "accessed" bit of the `LocationState` in sync with whether
+                // a read access is performed below.
+                if access {
+                    *loc = LocationState::new_accessed(perm, sifa);
+                } else {
+                    *loc = LocationState::new_non_accessed(perm, sifa);
+                }
+            }
+
+            // Some reborrows incur a read access to the parent.
+            if access {
+                // Adjust range to be relative to allocation start (rather than to `place`).
+                let mut range_in_alloc = range;
+                range_in_alloc.start += base_offset;
+
+                tree_borrows.perform_access(
+                    orig_tag,
+                    Some((range_in_alloc, AccessKind::Read, diagnostics::AccessCause::Reborrow)),
+                    this.machine.borrow_tracker.as_ref().unwrap(),
+                    alloc_id,
+                    this.machine.current_span(),
+                )?;
+
+                // Also inform the data race model (but only if any bytes are actually affected).
+                if range.size.bytes() > 0 {
+                    if let Some(data_race) = alloc_extra.data_race.as_vclocks_ref() {
+                        data_race.read(
+                            alloc_id,
+                            range_in_alloc,
+                            NaReadType::Retag,
+                            Some(place.layout.ty),
+                            &this.machine,
+                        )?
+                    }
+                }
+            }
+            interp_ok(())
+        })?;
+
         // Record the parent-child pair in the tree.
         tree_borrows.new_child(
+            base_offset,
             orig_tag,
             new_tag,
-            new_perm.initial_state,
-            range,
+            perms_map,
+            // Allow lazily writing to surrounding data if we found an `UnsafeCell`.
+            if has_unsafe_cell { new_perm.nonfreeze_perm } else { new_perm.freeze_perm },
+            protected,
             span,
-            new_perm.protector.is_some(),
         )?;
         drop(tree_borrows);
 
-        // Also inform the data race model (but only if any bytes are actually affected).
-        if range.size.bytes() > 0 && new_perm.initial_read {
-            if let Some(data_race) = alloc_extra.data_race.as_vclocks_ref() {
-                data_race.read(
-                    alloc_id,
-                    range,
-                    NaReadType::Retag,
-                    Some(place.layout.ty),
-                    &this.machine,
-                )?;
-            }
-        }
-
         interp_ok(Some(Provenance::Concrete { alloc_id, tag: new_tag }))
     }
 
@@ -508,15 +596,21 @@ pub trait EvalContextExt<'tcx>: crate::MiriInterpCxExt<'tcx> {
     fn tb_protect_place(&mut self, place: &MPlaceTy<'tcx>) -> InterpResult<'tcx, MPlaceTy<'tcx>> {
         let this = self.eval_context_mut();
 
-        // Note: if we were to inline `new_reserved` below we would find out that
-        // `ty_is_freeze` is eventually unused because it appears in a `ty_is_freeze || true`.
-        // We are nevertheless including it here for clarity.
-        let ty_is_freeze = place.layout.ty.is_freeze(*this.tcx, this.typing_env());
         // Retag it. With protection! That is the entire point.
         let new_perm = NewPermission {
-            initial_state: Permission::new_reserved(ty_is_freeze, /* protected */ true),
+            // Note: If we are creating a protected Reserved, which can
+            // never be ReservedIM, the value of the `ty_is_freeze`
+            // argument doesn't matter
+            // (`ty_is_freeze || true` in `new_reserved` will always be `true`).
+            freeze_perm: Permission::new_reserved(
+                /* ty_is_freeze */ true, /* protected */ true,
+            ),
+            freeze_access: true,
+            nonfreeze_perm: Permission::new_reserved(
+                /* ty_is_freeze */ false, /* protected */ true,
+            ),
+            nonfreeze_access: true,
             protector: Some(ProtectorKind::StrongProtector),
-            initial_read: true,
         };
         this.tb_retag_place(place, new_perm)
     }
diff --git a/src/tools/miri/src/borrow_tracker/tree_borrows/perms.rs b/src/tools/miri/src/borrow_tracker/tree_borrows/perms.rs
index 087f6fc3f24..38863ca0734 100644
--- a/src/tools/miri/src/borrow_tracker/tree_borrows/perms.rs
+++ b/src/tools/miri/src/borrow_tracker/tree_borrows/perms.rs
@@ -94,6 +94,7 @@ impl PermissionPriv {
     }
 
     /// Reject `ReservedIM` that cannot exist in the presence of a protector.
+    #[cfg(test)]
     fn compatible_with_protector(&self) -> bool {
         // FIXME(TB-Cell): It is unclear what to do here.
         // `Cell` will occur with a protector but won't provide the guarantees
@@ -253,10 +254,6 @@ impl Permission {
     pub fn is_disabled(&self) -> bool {
         self.inner == Disabled
     }
-    /// Check if `self` is the post-child-write state of a pointer (is `Active`).
-    pub fn is_active(&self) -> bool {
-        self.inner == Active
-    }
     /// Check if `self` is the never-allow-writes-again state of a pointer (is `Frozen`).
     pub fn is_frozen(&self) -> bool {
         self.inner == Frozen
@@ -289,6 +286,11 @@ impl Permission {
     /// is a protector is relevant because being protected takes priority over being
     /// interior mutable)
     pub fn new_reserved(ty_is_freeze: bool, protected: bool) -> Self {
+        // As demonstrated by `tests/fail/tree_borrows/reservedim_spurious_write.rs`,
+        // interior mutability and protectors interact poorly.
+        // To eliminate the case of Protected Reserved IM we override interior mutability
+        // in the case of a protected reference: protected references are always considered
+        // "freeze" in their reservation phase.
         if ty_is_freeze || protected { Self::new_reserved_frz() } else { Self::new_reserved_im() }
     }
 
@@ -309,6 +311,7 @@ impl Permission {
     }
 
     /// Reject `ReservedIM` that cannot exist in the presence of a protector.
+    #[cfg(test)]
     pub fn compatible_with_protector(&self) -> bool {
         self.inner.compatible_with_protector()
     }
@@ -393,11 +396,6 @@ impl PermTransition {
         self.from <= self.to
     }
 
-    pub fn from(from: Permission, to: Permission) -> Option<Self> {
-        let t = Self { from: from.inner, to: to.inner };
-        t.is_possible().then_some(t)
-    }
-
     pub fn is_noop(self) -> bool {
         self.from == self.to
     }
@@ -407,11 +405,6 @@ impl PermTransition {
         (starting_point.inner == self.from).then_some(Permission { inner: self.to })
     }
 
-    /// Extract starting point of a transition
-    pub fn started(self) -> Permission {
-        Permission { inner: self.from }
-    }
-
     /// Determines if this transition would disable the permission.
     pub fn produces_disabled(self) -> bool {
         self.to == Disabled
diff --git a/src/tools/miri/src/borrow_tracker/tree_borrows/tree.rs b/src/tools/miri/src/borrow_tracker/tree_borrows/tree.rs
index 47ccaadbb9e..48e4a19e263 100644
--- a/src/tools/miri/src/borrow_tracker/tree_borrows/tree.rs
+++ b/src/tools/miri/src/borrow_tracker/tree_borrows/tree.rs
@@ -10,6 +10,7 @@
 //!   and the relative position of the access;
 //! - idempotency properties asserted in `perms.rs` (for optimizations)
 
+use std::ops::Range;
 use std::{fmt, mem};
 
 use rustc_abi::Size;
@@ -32,18 +33,18 @@ mod tests;
 /// Data for a single *location*.
 #[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
 pub(super) struct LocationState {
-    /// A location is initialized when it is child-accessed for the first time (and the initial
+    /// A location is "accessed" when it is child-accessed for the first time (and the initial
     /// retag initializes the location for the range covered by the type), and it then stays
-    /// initialized forever.
-    /// For initialized locations, "permission" is the current permission. However, for
-    /// uninitialized locations, we still need to track the "future initial permission": this will
+    /// accessed forever.
+    /// For accessed locations, "permission" is the current permission. However, for
+    /// non-accessed locations, we still need to track the "future initial permission": this will
     /// start out to be `default_initial_perm`, but foreign accesses need to be taken into account.
     /// Crucially however, while transitions to `Disabled` would usually be UB if this location is
-    /// protected, that is *not* the case for uninitialized locations. Instead we just have a latent
+    /// protected, that is *not* the case for non-accessed locations. Instead we just have a latent
     /// "future initial permission" of `Disabled`, causing UB only if an access is ever actually
     /// performed.
-    /// Note that the tree root is also always initialized, as if the allocation was a write access.
-    initialized: bool,
+    /// Note that the tree root is also always accessed, as if the allocation was a write access.
+    accessed: bool,
     /// This pointer's current permission / future initial permission.
     permission: Permission,
     /// See `foreign_access_skipping.rs`.
@@ -58,30 +59,30 @@ impl LocationState {
     /// to any foreign access yet.
     /// The permission is not allowed to be `Active`.
     /// `sifa` is the (strongest) idempotent foreign access, see `foreign_access_skipping.rs`
-    fn new_uninit(permission: Permission, sifa: IdempotentForeignAccess) -> Self {
+    pub fn new_non_accessed(permission: Permission, sifa: IdempotentForeignAccess) -> Self {
         assert!(permission.is_initial() || permission.is_disabled());
-        Self { permission, initialized: false, idempotent_foreign_access: sifa }
+        Self { permission, accessed: false, idempotent_foreign_access: sifa }
     }
 
     /// Constructs a new initial state. It has not yet been subjected
     /// to any foreign access. However, it is already marked as having been accessed.
     /// `sifa` is the (strongest) idempotent foreign access, see `foreign_access_skipping.rs`
-    fn new_init(permission: Permission, sifa: IdempotentForeignAccess) -> Self {
-        Self { permission, initialized: true, idempotent_foreign_access: sifa }
+    pub fn new_accessed(permission: Permission, sifa: IdempotentForeignAccess) -> Self {
+        Self { permission, accessed: true, idempotent_foreign_access: sifa }
     }
 
-    /// Check if the location has been initialized, i.e. if it has
+    /// Check if the location has been accessed, i.e. if it has
     /// ever been accessed through a child pointer.
-    pub fn is_initialized(&self) -> bool {
-        self.initialized
+    pub fn is_accessed(&self) -> bool {
+        self.accessed
     }
 
     /// Check if the state can exist as the initial permission of a pointer.
     ///
-    /// Do not confuse with `is_initialized`, the two are almost orthogonal
-    /// as apart from `Active` which is not initial and must be initialized,
+    /// Do not confuse with `is_accessed`, the two are almost orthogonal
+    /// as apart from `Active` which is not initial and must be accessed,
     /// any other permission can have an arbitrary combination of being
-    /// initial/initialized.
+    /// initial/accessed.
     /// FIXME: when the corresponding `assert` in `tree_borrows/mod.rs` finally
     /// passes and can be uncommented, remove this `#[allow(dead_code)]`.
     #[cfg_attr(not(test), allow(dead_code))]
@@ -95,8 +96,8 @@ impl LocationState {
 
     /// Apply the effect of an access to one location, including
     /// - applying `Permission::perform_access` to the inner `Permission`,
-    /// - emitting protector UB if the location is initialized,
-    /// - updating the initialized status (child accesses produce initialized locations).
+    /// - emitting protector UB if the location is accessed,
+    /// - updating the accessed status (child accesses produce accessed locations).
     fn perform_access(
         &mut self,
         access_kind: AccessKind,
@@ -106,14 +107,14 @@ impl LocationState {
         let old_perm = self.permission;
         let transition = Permission::perform_access(access_kind, rel_pos, old_perm, protected)
             .ok_or(TransitionError::ChildAccessForbidden(old_perm))?;
-        self.initialized |= !rel_pos.is_foreign();
+        self.accessed |= !rel_pos.is_foreign();
         self.permission = transition.applied(old_perm).unwrap();
-        // Why do only initialized locations cause protector errors?
+        // Why do only accessed locations cause protector errors?
         // Consider two mutable references `x`, `y` into disjoint parts of
         // the same allocation. A priori, these may actually both be used to
         // access the entire allocation, as long as only reads occur. However,
         // a write to `y` needs to somehow record that `x` can no longer be used
-        // on that location at all. For these uninitialized locations (i.e., locations
+        // on that location at all. For these non-accessed locations (i.e., locations
         // that haven't been accessed with `x` yet), we track the "future initial state":
         // it defaults to whatever the initial state of the tag is,
         // but the access to `y` moves that "future initial state" of `x` to `Disabled`.
@@ -121,8 +122,8 @@ impl LocationState {
         // So clearly protectors shouldn't fire for such "future initial state" transitions.
         //
         // See the test `two_mut_protected_same_alloc` in `tests/pass/tree_borrows/tree-borrows.rs`
-        // for an example of safe code that would be UB if we forgot to check `self.initialized`.
-        if protected && self.initialized && transition.produces_disabled() {
+        // for an example of safe code that would be UB if we forgot to check `self.accessed`.
+        if protected && self.accessed && transition.produces_disabled() {
             return Err(TransitionError::ProtectedDisabled(old_perm));
         }
         Ok(transition)
@@ -157,11 +158,11 @@ impl LocationState {
                 self.idempotent_foreign_access.can_skip_foreign_access(happening_now);
             if self.permission.is_disabled() {
                 // A foreign access to a `Disabled` tag will have almost no observable effect.
-                // It's a theorem that `Disabled` node have no protected initialized children,
+                // It's a theorem that `Disabled` node have no protected accessed children,
                 // and so this foreign access will never trigger any protector.
-                // (Intuition: You're either protected initialized, and thus can't become Disabled
-                // or you're already Disabled protected, but not initialized, and then can't
-                // become initialized since that requires a child access, which Disabled blocks.)
+                // (Intuition: You're either protected accessed, and thus can't become Disabled
+                // or you're already Disabled protected, but not accessed, and then can't
+                // become accessed since that requires a child access, which Disabled blocks.)
                 // Further, the children will never be able to read or write again, since they
                 // have a `Disabled` parent. So this only affects diagnostics, such that the
                 // blocking write will still be identified directly, just at a different tag.
@@ -217,7 +218,7 @@ impl LocationState {
 impl fmt::Display for LocationState {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         write!(f, "{}", self.permission)?;
-        if !self.initialized {
+        if !self.accessed {
             write!(f, "?")?;
         }
         Ok(())
@@ -598,12 +599,15 @@ impl Tree {
         let rperms = {
             let mut perms = UniValMap::default();
             // We manually set it to `Active` on all in-bounds positions.
-            // We also ensure that it is initialized, so that no `Active` but
-            // not yet initialized nodes exist. Essentially, we pretend there
+            // We also ensure that it is accessed, so that no `Active` but
+            // not yet accessed nodes exist. Essentially, we pretend there
             // was a write that initialized these to `Active`.
             perms.insert(
                 root_idx,
-                LocationState::new_init(Permission::new_active(), IdempotentForeignAccess::None),
+                LocationState::new_accessed(
+                    Permission::new_active(),
+                    IdempotentForeignAccess::None,
+                ),
             );
             RangeMap::new(size, perms)
         };
@@ -612,20 +616,32 @@ impl Tree {
 }
 
 impl<'tcx> Tree {
-    /// Insert a new tag in the tree
-    pub fn new_child(
+    /// Insert a new tag in the tree.
+    ///
+    /// `initial_perms` defines the initial permissions for the part of memory
+    /// that is already considered "initialized" immediately. The ranges in this
+    /// map are relative to `base_offset`.
+    /// `default_perm` defines the initial permission for the rest of the allocation.
+    ///
+    /// For all non-accessed locations in the RangeMap (those that haven't had an
+    /// implicit read), their SIFA must be weaker than or as weak as the SIFA of
+    /// `default_perm`.
+    pub(super) fn new_child(
         &mut self,
+        base_offset: Size,
         parent_tag: BorTag,
         new_tag: BorTag,
-        default_initial_perm: Permission,
-        reborrow_range: AllocRange,
+        initial_perms: RangeMap<LocationState>,
+        default_perm: Permission,
+        protected: bool,
         span: Span,
-        prot: bool,
     ) -> InterpResult<'tcx> {
-        assert!(!self.tag_mapping.contains_key(&new_tag));
         let idx = self.tag_mapping.insert(new_tag);
         let parent_idx = self.tag_mapping.get(&parent_tag).unwrap();
-        let strongest_idempotent = default_initial_perm.strongest_idempotent_foreign_access(prot);
+        assert!(default_perm.is_initial());
+
+        let default_strongest_idempotent =
+            default_perm.strongest_idempotent_foreign_access(protected);
         // Create the node
         self.nodes.insert(
             idx,
@@ -633,25 +649,36 @@ impl<'tcx> Tree {
                 tag: new_tag,
                 parent: Some(parent_idx),
                 children: SmallVec::default(),
-                default_initial_perm,
-                default_initial_idempotent_foreign_access: strongest_idempotent,
-                debug_info: NodeDebugInfo::new(new_tag, default_initial_perm, span),
+                default_initial_perm: default_perm,
+                default_initial_idempotent_foreign_access: default_strongest_idempotent,
+                debug_info: NodeDebugInfo::new(new_tag, default_perm, span),
             },
         );
         // Register new_tag as a child of parent_tag
         self.nodes.get_mut(parent_idx).unwrap().children.push(idx);
-        // Initialize perms
-        let perm = LocationState::new_init(default_initial_perm, strongest_idempotent);
-        for (_perms_range, perms) in self.rperms.iter_mut(reborrow_range.start, reborrow_range.size)
+
+        for (Range { start, end }, &perm) in
+            initial_perms.iter(Size::from_bytes(0), initial_perms.size())
         {
-            perms.insert(idx, perm);
+            assert!(perm.is_initial());
+            for (_perms_range, perms) in self
+                .rperms
+                .iter_mut(Size::from_bytes(start) + base_offset, Size::from_bytes(end - start))
+            {
+                assert!(
+                    default_strongest_idempotent
+                        >= perm.permission.strongest_idempotent_foreign_access(protected)
+                );
+                perms.insert(idx, perm);
+            }
         }
 
         // Inserting the new perms might have broken the SIFA invariant (see `foreign_access_skipping.rs`).
         // We now weaken the recorded SIFA for our parents, until the invariant is restored.
         // We could weaken them all to `LocalAccess`, but it is more efficient to compute the SIFA
         // for the new permission statically, and use that.
-        self.update_last_accessed_after_retag(parent_idx, strongest_idempotent);
+        // See the comment in `tb_reborrow` for why it is correct to use the SIFA of `default_uninit_perm`.
+        self.update_last_accessed_after_retag(parent_idx, default_strongest_idempotent);
 
         interp_ok(())
     }
@@ -758,14 +785,14 @@ impl<'tcx> Tree {
     ///
     /// If `access_range_and_kind` is `None`, this is interpreted as the special
     /// access that is applied on protector release:
-    /// - the access will be applied only to initialized locations of the allocation,
+    /// - the access will be applied only to accessed locations of the allocation,
     /// - it will not be visible to children,
     /// - it will be recorded as a `FnExit` diagnostic access
     /// - and it will be a read except if the location is `Active`, i.e. has been written to,
     ///   in which case it will be a write.
     ///
     /// `LocationState::perform_access` will take care of raising transition
-    /// errors and updating the `initialized` status of each location,
+    /// errors and updating the `accessed` status of each location,
     /// this traversal adds to that:
     /// - inserting into the map locations that do not exist yet,
     /// - trimming the traversal,
@@ -858,7 +885,7 @@ impl<'tcx> Tree {
             }
         } else {
             // This is a special access through the entire allocation.
-            // It actually only affects `initialized` locations, so we need
+            // It actually only affects `accessed` locations, so we need
             // to filter on those before initiating the traversal.
             //
             // In addition this implicit access should not be visible to children,
@@ -868,10 +895,10 @@ impl<'tcx> Tree {
             // why this is important.
             for (perms_range, perms) in self.rperms.iter_mut_all() {
                 let idx = self.tag_mapping.get(&tag).unwrap();
-                // Only visit initialized permissions
+                // Only visit accessed permissions
                 if let Some(p) = perms.get(idx)
                     && let Some(access_kind) = p.permission.protector_end_access()
-                    && p.initialized
+                    && p.accessed
                 {
                     let access_cause = diagnostics::AccessCause::FnExit(access_kind);
                     TreeVisitor { nodes: &mut self.nodes, tag_mapping: &self.tag_mapping, perms }
@@ -1035,7 +1062,7 @@ impl Tree {
 
 impl Node {
     pub fn default_location_state(&self) -> LocationState {
-        LocationState::new_uninit(
+        LocationState::new_non_accessed(
             self.default_initial_perm,
             self.default_initial_idempotent_foreign_access,
         )
@@ -1073,15 +1100,4 @@ impl AccessRelatedness {
     pub fn is_foreign(self) -> bool {
         matches!(self, AccessRelatedness::AncestorAccess | AccessRelatedness::CousinAccess)
     }
-
-    /// Given the AccessRelatedness for the parent node, compute the AccessRelatedness
-    /// for the child node. This function assumes that we propagate away from the initial
-    /// access.
-    pub fn for_child(self) -> Self {
-        use AccessRelatedness::*;
-        match self {
-            AncestorAccess | This => AncestorAccess,
-            StrictChildAccess | CousinAccess => CousinAccess,
-        }
-    }
 }
diff --git a/src/tools/miri/src/borrow_tracker/tree_borrows/tree/tests.rs b/src/tools/miri/src/borrow_tracker/tree_borrows/tree/tests.rs
index dbfa9807e3b..bb3fc2d80b3 100644
--- a/src/tools/miri/src/borrow_tracker/tree_borrows/tree/tests.rs
+++ b/src/tools/miri/src/borrow_tracker/tree_borrows/tree/tests.rs
@@ -9,10 +9,10 @@ use crate::borrow_tracker::tree_borrows::exhaustive::{Exhaustive, precondition};
 impl Exhaustive for LocationState {
     fn exhaustive() -> Box<dyn Iterator<Item = Self>> {
         // We keep `latest_foreign_access` at `None` as that's just a cache.
-        Box::new(<(Permission, bool)>::exhaustive().map(|(permission, initialized)| {
+        Box::new(<(Permission, bool)>::exhaustive().map(|(permission, accessed)| {
             Self {
                 permission,
-                initialized,
+                accessed,
                 idempotent_foreign_access: IdempotentForeignAccess::default(),
             }
         }))
@@ -76,8 +76,8 @@ fn as_protected(b: bool) -> &'static str {
     if b { " (protected)" } else { "" }
 }
 
-fn as_lazy_or_init(b: bool) -> &'static str {
-    if b { "initialized" } else { "lazy" }
+fn as_lazy_or_accessed(b: bool) -> &'static str {
+    if b { "accessed" } else { "lazy" }
 }
 
 /// Test that tree compacting (as performed by the GC) is sound.
@@ -106,7 +106,7 @@ fn tree_compacting_is_sound() {
                         as_foreign_or_child(rel),
                         kind,
                         parent.permission(),
-                        as_lazy_or_init(child.is_initialized()),
+                        as_lazy_or_accessed(child.is_accessed()),
                         child.permission(),
                         as_protected(child_protected),
                         np.permission(),
@@ -122,7 +122,7 @@ fn tree_compacting_is_sound() {
                         as_foreign_or_child(rel),
                         kind,
                         parent.permission(),
-                        as_lazy_or_init(child.is_initialized()),
+                        as_lazy_or_accessed(child.is_accessed()),
                         child.permission(),
                         as_protected(child_protected),
                         nc.permission()
@@ -435,19 +435,19 @@ mod spurious_read {
             Ok(Self { x, y, ..self })
         }
 
-        /// Perform a read on the given pointer if its state is `initialized`.
+        /// Perform a read on the given pointer if its state is `accessed`.
         /// Must be called just after reborrowing a pointer, and just after
         /// removing a protector.
-        fn read_if_initialized(self, ptr: PtrSelector) -> Result<Self, ()> {
-            let initialized = match ptr {
-                PtrSelector::X => self.x.state.initialized,
-                PtrSelector::Y => self.y.state.initialized,
+        fn read_if_accessed(self, ptr: PtrSelector) -> Result<Self, ()> {
+            let accessed = match ptr {
+                PtrSelector::X => self.x.state.accessed,
+                PtrSelector::Y => self.y.state.accessed,
                 PtrSelector::Other =>
                     panic!(
-                        "the `initialized` status of `PtrSelector::Other` is unknown, do not pass it to `read_if_initialized`"
+                        "the `accessed` status of `PtrSelector::Other` is unknown, do not pass it to `read_if_accessed`"
                     ),
             };
-            if initialized {
+            if accessed {
                 self.perform_test_access(&TestAccess { ptr, kind: AccessKind::Read })
             } else {
                 Ok(self)
@@ -457,13 +457,13 @@ mod spurious_read {
         /// Remove the protector of `x`, including the implicit read on function exit.
         fn end_protector_x(self) -> Result<Self, ()> {
             let x = self.x.end_protector();
-            Self { x, ..self }.read_if_initialized(PtrSelector::X)
+            Self { x, ..self }.read_if_accessed(PtrSelector::X)
         }
 
         /// Remove the protector of `y`, including the implicit read on function exit.
         fn end_protector_y(self) -> Result<Self, ()> {
             let y = self.y.end_protector();
-            Self { y, ..self }.read_if_initialized(PtrSelector::Y)
+            Self { y, ..self }.read_if_accessed(PtrSelector::Y)
         }
 
         fn retag_y(self, new_y: LocStateProt) -> Result<Self, ()> {
@@ -473,7 +473,7 @@ mod spurious_read {
             }
             // `xy_rel` changes to "mutually foreign" now: `y` can no longer be a parent of `x`.
             Self { y: new_y, xy_rel: RelPosXY::MutuallyForeign, ..self }
-                .read_if_initialized(PtrSelector::Y)
+                .read_if_accessed(PtrSelector::Y)
         }
 
         fn perform_test_event<RetX, RetY>(self, evt: &TestEvent<RetX, RetY>) -> Result<Self, ()> {
@@ -602,14 +602,14 @@ mod spurious_read {
             xy_rel: RelPosXY::MutuallyForeign,
             x: LocStateProt {
                 // For the tests, the strongest idempotent foreign access does not matter, so we use `Default::default`
-                state: LocationState::new_init(
+                state: LocationState::new_accessed(
                     Permission::new_frozen(),
                     IdempotentForeignAccess::default(),
                 ),
                 prot: true,
             },
             y: LocStateProt {
-                state: LocationState::new_uninit(
+                state: LocationState::new_non_accessed(
                     Permission::new_reserved(/* freeze */ true, /* protected */ true),
                     IdempotentForeignAccess::default(),
                 ),
@@ -650,8 +650,8 @@ mod spurious_read {
             for xy_rel in RelPosXY::exhaustive() {
                 for (x_retag_perm, y_current_perm) in <(LocationState, LocationState)>::exhaustive()
                 {
-                    // We can only do spurious reads for initialized locations anyway.
-                    precondition!(x_retag_perm.initialized);
+                    // We can only do spurious reads for accessed locations anyway.
+                    precondition!(x_retag_perm.accessed);
                     // And `x` just got retagged, so it must be initial.
                     precondition!(x_retag_perm.permission.is_initial());
                     // As stated earlier, `x` is always protected in the patterns we consider here.
@@ -696,7 +696,7 @@ mod spurious_read {
         fn initial_state(&self) -> Result<LocStateProtPair, ()> {
             let (x, y) = self.retag_permissions();
             let state = LocStateProtPair { xy_rel: self.xy_rel, x, y };
-            state.read_if_initialized(PtrSelector::X)
+            state.read_if_accessed(PtrSelector::X)
         }
     }
 
diff --git a/src/tools/miri/src/concurrency/thread.rs b/src/tools/miri/src/concurrency/thread.rs
index 15f15572c93..ba1436b77b8 100644
--- a/src/tools/miri/src/concurrency/thread.rs
+++ b/src/tools/miri/src/concurrency/thread.rs
@@ -897,12 +897,17 @@ pub trait EvalContextExt<'tcx>: crate::MiriInterpCxExt<'tcx> {
             if tcx.is_foreign_item(def_id) {
                 throw_unsup_format!("foreign thread-local statics are not supported");
             }
+            let params = this.machine.get_default_alloc_params();
             let alloc = this.ctfe_query(|tcx| tcx.eval_static_initializer(def_id))?;
             // We make a full copy of this allocation.
             let mut alloc = alloc.inner().adjust_from_tcx(
                 &this.tcx,
                 |bytes, align| {
-                    interp_ok(MiriAllocBytes::from_bytes(std::borrow::Cow::Borrowed(bytes), align, ()))
+                    interp_ok(MiriAllocBytes::from_bytes(
+                        std::borrow::Cow::Borrowed(bytes),
+                        align,
+                        params,
+                    ))
                 },
                 |ptr| this.global_root_pointer(ptr),
             )?;
diff --git a/src/tools/miri/src/diagnostics.rs b/src/tools/miri/src/diagnostics.rs
index 10570a37e5d..1728a9cfd6d 100644
--- a/src/tools/miri/src/diagnostics.rs
+++ b/src/tools/miri/src/diagnostics.rs
@@ -255,8 +255,7 @@ pub fn report_error<'tcx>(
                 ],
             UnsupportedForeignItem(_) => {
                 vec![
-                    note!("if this is a basic API commonly used on this target, please report an issue with Miri"),
-                    note!("however, note that Miri does not aim to support every FFI function out there; for instance, we will not support APIs for things such as GUIs, scripting languages, or databases"),
+                    note!("this means the program tried to do something Miri does not support; it does not indicate a bug in the program"),
                 ]
             }
             StackedBorrowsUb { help, history, .. } => {
diff --git a/src/tools/miri/src/lib.rs b/src/tools/miri/src/lib.rs
index 9d663ca9edf..8802216448f 100644
--- a/src/tools/miri/src/lib.rs
+++ b/src/tools/miri/src/lib.rs
@@ -11,6 +11,7 @@
 #![feature(nonzero_ops)]
 #![feature(strict_overflow_ops)]
 #![feature(pointer_is_aligned_to)]
+#![feature(ptr_metadata)]
 #![feature(unqualified_local_imports)]
 #![feature(derive_coerce_pointee)]
 #![feature(arbitrary_self_types)]
@@ -69,8 +70,8 @@ extern crate rustc_target;
 #[allow(unused_extern_crates)]
 extern crate rustc_driver;
 
+mod alloc;
 mod alloc_addresses;
-mod alloc_bytes;
 mod borrow_tracker;
 mod clock;
 mod concurrency;
@@ -105,8 +106,8 @@ pub type OpTy<'tcx> = interpret::OpTy<'tcx, machine::Provenance>;
 pub type PlaceTy<'tcx> = interpret::PlaceTy<'tcx, machine::Provenance>;
 pub type MPlaceTy<'tcx> = interpret::MPlaceTy<'tcx, machine::Provenance>;
 
+pub use crate::alloc::MiriAllocBytes;
 pub use crate::alloc_addresses::{EvalContextExt as _, ProvenanceMode};
-pub use crate::alloc_bytes::MiriAllocBytes;
 pub use crate::borrow_tracker::stacked_borrows::{
     EvalContextExt as _, Item, Permission, Stack, Stacks,
 };
diff --git a/src/tools/miri/src/machine.rs b/src/tools/miri/src/machine.rs
index e7a2cb25159..15b3653d7ae 100644
--- a/src/tools/miri/src/machine.rs
+++ b/src/tools/miri/src/machine.rs
@@ -532,6 +532,10 @@ pub struct MiriMachine<'tcx> {
     /// Needs to be queried by ptr_to_int, hence needs interior mutability.
     pub(crate) rng: RefCell<StdRng>,
 
+    /// The allocator used for the machine's `AllocBytes` in native-libs mode.
+    #[cfg(target_os = "linux")]
+    pub(crate) allocator: Option<Rc<RefCell<crate::alloc::isolated_alloc::IsolatedAlloc>>>,
+
     /// The allocation IDs to report when they are being allocated
     /// (helps for debugging memory leaks and use after free bugs).
     tracked_alloc_ids: FxHashSet<AllocId>,
@@ -715,6 +719,10 @@ impl<'tcx> MiriMachine<'tcx> {
             local_crates,
             extern_statics: FxHashMap::default(),
             rng: RefCell::new(rng),
+            #[cfg(target_os = "linux")]
+            allocator: if config.native_lib.is_some() {
+                Some(Rc::new(RefCell::new(crate::alloc::isolated_alloc::IsolatedAlloc::new())))
+            } else { None },
             tracked_alloc_ids: config.tracked_alloc_ids.clone(),
             track_alloc_accesses: config.track_alloc_accesses,
             check_alignment: config.check_alignment,
@@ -917,6 +925,8 @@ impl VisitProvenance for MiriMachine<'_> {
             backtrace_style: _,
             local_crates: _,
             rng: _,
+            #[cfg(target_os = "linux")]
+            allocator: _,
             tracked_alloc_ids: _,
             track_alloc_accesses: _,
             check_alignment: _,
@@ -1637,7 +1647,7 @@ impl<'tcx> Machine<'tcx> for MiriMachine<'tcx> {
     fn before_stack_pop(ecx: &mut InterpCx<'tcx, Self>) -> InterpResult<'tcx> {
         let frame = ecx.frame();
         // We want this *before* the return value copy, because the return place itself is protected
-        // until we do `end_call` here.
+        // until we do `on_stack_pop` here, and we need to un-protect it to copy the return value.
         if ecx.machine.borrow_tracker.is_some() {
             ecx.on_stack_pop(frame)?;
         }
@@ -1804,8 +1814,17 @@ impl<'tcx> Machine<'tcx> for MiriMachine<'tcx> {
         Cow::Borrowed(ecx.machine.union_data_ranges.entry(ty).or_insert_with(compute_range))
     }
 
-    /// Placeholder!
-    fn get_default_alloc_params(&self) -> <Self::Bytes as AllocBytes>::AllocParams { () }
+    fn get_default_alloc_params(&self) -> <Self::Bytes as AllocBytes>::AllocParams {
+        use crate::alloc::MiriAllocParams;
+
+        #[cfg(target_os = "linux")]
+        match &self.allocator {
+            Some(alloc) => MiriAllocParams::Isolated(alloc.clone()),
+            None => MiriAllocParams::Global,
+        }
+        #[cfg(not(target_os = "linux"))]
+        MiriAllocParams::Global
+    }
 }
 
 /// Trait for callbacks handling asynchronous machine operations.
diff --git a/src/tools/miri/src/range_map.rs b/src/tools/miri/src/range_map.rs
index 2c2484cd0bc..29a5a8537a4 100644
--- a/src/tools/miri/src/range_map.rs
+++ b/src/tools/miri/src/range_map.rs
@@ -31,6 +31,11 @@ impl<T> RangeMap<T> {
         RangeMap { v }
     }
 
+    pub fn size(&self) -> Size {
+        let size = self.v.last().map(|x| x.range.end).unwrap_or(0);
+        Size::from_bytes(size)
+    }
+
     /// Finds the index containing the given offset.
     fn find_offset(&self, offset: u64) -> usize {
         self.v
@@ -71,10 +76,7 @@ impl<T> RangeMap<T> {
         };
         // The first offset that is not included any more.
         let end = offset + len;
-        assert!(
-            end <= self.v.last().unwrap().range.end,
-            "iterating beyond the bounds of this RangeMap"
-        );
+        assert!(end <= self.size().bytes(), "iterating beyond the bounds of this RangeMap");
         slice
             .iter()
             .take_while(move |elem| elem.range.start < end)
@@ -327,4 +329,16 @@ mod tests {
         let map = RangeMap::<i32>::new(Size::from_bytes(20), -1);
         let _ = map.iter(Size::from_bytes(11), Size::from_bytes(11));
     }
+
+    #[test]
+    fn empty_map_iter() {
+        let map = RangeMap::<i32>::new(Size::from_bytes(0), -1);
+        let _ = map.iter(Size::from_bytes(0), Size::from_bytes(0));
+    }
+
+    #[test]
+    fn empty_map_iter_mut() {
+        let mut map = RangeMap::<i32>::new(Size::from_bytes(0), -1);
+        let _ = map.iter_mut(Size::from_bytes(0), Size::from_bytes(0));
+    }
 }
diff --git a/src/tools/miri/src/shims/files.rs b/src/tools/miri/src/shims/files.rs
index 31142431247..606d1ffbea6 100644
--- a/src/tools/miri/src/shims/files.rs
+++ b/src/tools/miri/src/shims/files.rs
@@ -202,6 +202,20 @@ pub trait FileDescription: std::fmt::Debug + FileDescriptionExt {
     fn as_unix<'tcx>(&self, _ecx: &MiriInterpCx<'tcx>) -> &dyn UnixFileDescription {
         panic!("Not a unix file descriptor: {}", self.name());
     }
+
+    /// Implementation of fcntl(F_GETFL) for this FD.
+    fn get_flags<'tcx>(&self, _ecx: &mut MiriInterpCx<'tcx>) -> InterpResult<'tcx, Scalar> {
+        throw_unsup_format!("fcntl: {} is not supported for F_GETFL", self.name());
+    }
+
+    /// Implementation of fcntl(F_SETFL) for this FD.
+    fn set_flags<'tcx>(
+        &self,
+        _flag: i32,
+        _ecx: &mut MiriInterpCx<'tcx>,
+    ) -> InterpResult<'tcx, Scalar> {
+        throw_unsup_format!("fcntl: {} is not supported for F_SETFL", self.name());
+    }
 }
 
 impl FileDescription for io::Stdin {
diff --git a/src/tools/miri/src/shims/unix/fd.rs b/src/tools/miri/src/shims/unix/fd.rs
index 156814a26fa..71102d9f2f3 100644
--- a/src/tools/miri/src/shims/unix/fd.rs
+++ b/src/tools/miri/src/shims/unix/fd.rs
@@ -141,6 +141,8 @@ pub trait EvalContextExt<'tcx>: crate::MiriInterpCxExt<'tcx> {
         let f_getfd = this.eval_libc_i32("F_GETFD");
         let f_dupfd = this.eval_libc_i32("F_DUPFD");
         let f_dupfd_cloexec = this.eval_libc_i32("F_DUPFD_CLOEXEC");
+        let f_getfl = this.eval_libc_i32("F_GETFL");
+        let f_setfl = this.eval_libc_i32("F_SETFL");
 
         // We only support getting the flags for a descriptor.
         match cmd {
@@ -175,6 +177,25 @@ pub trait EvalContextExt<'tcx>: crate::MiriInterpCxExt<'tcx> {
                     this.set_last_error_and_return_i32(LibcError("EBADF"))
                 }
             }
+            cmd if cmd == f_getfl => {
+                // Check if this is a valid open file descriptor.
+                let Some(fd) = this.machine.fds.get(fd_num) else {
+                    return this.set_last_error_and_return_i32(LibcError("EBADF"));
+                };
+
+                fd.get_flags(this)
+            }
+            cmd if cmd == f_setfl => {
+                // Check if this is a valid open file descriptor.
+                let Some(fd) = this.machine.fds.get(fd_num) else {
+                    return this.set_last_error_and_return_i32(LibcError("EBADF"));
+                };
+
+                let [flag] = check_min_vararg_count("fcntl(fd, F_SETFL, ...)", varargs)?;
+                let flag = this.read_scalar(flag)?.to_i32()?;
+
+                fd.set_flags(flag, this)
+            }
             cmd if this.tcx.sess.target.os == "macos"
                 && cmd == this.eval_libc_i32("F_FULLFSYNC") =>
             {
diff --git a/src/tools/miri/src/shims/unix/unnamed_socket.rs b/src/tools/miri/src/shims/unix/unnamed_socket.rs
index 135d8f6bee7..817ddd7954d 100644
--- a/src/tools/miri/src/shims/unix/unnamed_socket.rs
+++ b/src/tools/miri/src/shims/unix/unnamed_socket.rs
@@ -20,6 +20,16 @@ use crate::*;
 /// be configured in the real system.
 const MAX_SOCKETPAIR_BUFFER_CAPACITY: usize = 212992;
 
+#[derive(Debug, PartialEq)]
+enum AnonSocketType {
+    // Either end of the socketpair fd.
+    Socketpair,
+    // Read end of the pipe.
+    PipeRead,
+    // Write end of the pipe.
+    PipeWrite,
+}
+
 /// One end of a pair of connected unnamed sockets.
 #[derive(Debug)]
 struct AnonSocket {
@@ -40,7 +50,10 @@ struct AnonSocket {
     /// A list of thread ids blocked because the buffer was full.
     /// Once another thread reads some bytes, these threads will be unblocked.
     blocked_write_tid: RefCell<Vec<ThreadId>>,
-    is_nonblock: bool,
+    /// Whether this fd is non-blocking or not.
+    is_nonblock: Cell<bool>,
+    // Differentiate between different AnonSocket fd types.
+    fd_type: AnonSocketType,
 }
 
 #[derive(Debug)]
@@ -63,7 +76,10 @@ impl AnonSocket {
 
 impl FileDescription for AnonSocket {
     fn name(&self) -> &'static str {
-        "socketpair"
+        match self.fd_type {
+            AnonSocketType::Socketpair => "socketpair",
+            AnonSocketType::PipeRead | AnonSocketType::PipeWrite => "pipe",
+        }
     }
 
     fn close<'tcx>(
@@ -110,6 +126,66 @@ impl FileDescription for AnonSocket {
     fn as_unix<'tcx>(&self, _ecx: &MiriInterpCx<'tcx>) -> &dyn UnixFileDescription {
         self
     }
+
+    fn get_flags<'tcx>(&self, ecx: &mut MiriInterpCx<'tcx>) -> InterpResult<'tcx, Scalar> {
+        let mut flags = 0;
+
+        // Get flag for file access mode.
+        // The flag for both socketpair and pipe will remain the same even when the peer
+        // fd is closed, so we need to look at the original type of this socket, not at whether
+        // the peer socket still exists.
+        match self.fd_type {
+            AnonSocketType::Socketpair => {
+                flags |= ecx.eval_libc_i32("O_RDWR");
+            }
+            AnonSocketType::PipeRead => {
+                flags |= ecx.eval_libc_i32("O_RDONLY");
+            }
+            AnonSocketType::PipeWrite => {
+                flags |= ecx.eval_libc_i32("O_WRONLY");
+            }
+        }
+
+        // Get flag for blocking status.
+        if self.is_nonblock.get() {
+            flags |= ecx.eval_libc_i32("O_NONBLOCK");
+        }
+
+        interp_ok(Scalar::from_i32(flags))
+    }
+
+    fn set_flags<'tcx>(
+        &self,
+        mut flag: i32,
+        ecx: &mut MiriInterpCx<'tcx>,
+    ) -> InterpResult<'tcx, Scalar> {
+        // FIXME: File creation flags should be ignored.
+
+        let o_nonblock = ecx.eval_libc_i32("O_NONBLOCK");
+        let o_rdonly = ecx.eval_libc_i32("O_RDONLY");
+        let o_wronly = ecx.eval_libc_i32("O_WRONLY");
+        let o_rdwr = ecx.eval_libc_i32("O_RDWR");
+
+        // O_NONBLOCK flag can be set / unset by user.
+        if flag & o_nonblock == o_nonblock {
+            self.is_nonblock.set(true);
+            flag &= !o_nonblock;
+        } else {
+            self.is_nonblock.set(false);
+        }
+
+        // Ignore all file access mode flags.
+        flag &= !(o_rdonly | o_wronly | o_rdwr);
+
+        // Throw error if there is any unsupported flag.
+        if flag != 0 {
+            throw_unsup_format!(
+                "fcntl: only O_NONBLOCK is supported for F_SETFL on socketpairs and pipes"
+            )
+        }
+
+        interp_ok(Scalar::from_i32(0))
+    }
 }
 
 /// Write to AnonSocket based on the space available and return the written byte size.
@@ -141,7 +217,7 @@ fn anonsocket_write<'tcx>(
     // Let's see if we can write.
     let available_space = MAX_SOCKETPAIR_BUFFER_CAPACITY.strict_sub(writebuf.borrow().buf.len());
     if available_space == 0 {
-        if self_ref.is_nonblock {
+        if self_ref.is_nonblock.get() {
             // Non-blocking socketpair with a full buffer.
             return finish.call(ecx, Err(ErrorKind::WouldBlock.into()));
         } else {
@@ -223,7 +299,7 @@ fn anonsocket_read<'tcx>(
             // Socketpair with no peer and empty buffer.
             // 0 bytes successfully read indicates end-of-file.
             return finish.call(ecx, Ok(0));
-        } else if self_ref.is_nonblock {
+        } else if self_ref.is_nonblock.get() {
             // Non-blocking socketpair with writer and empty buffer.
             // https://linux.die.net/man/2/read
             // EAGAIN or EWOULDBLOCK can be returned for socket,
@@ -407,7 +483,8 @@ pub trait EvalContextExt<'tcx>: crate::MiriInterpCxExt<'tcx> {
             peer_lost_data: Cell::new(false),
             blocked_read_tid: RefCell::new(Vec::new()),
             blocked_write_tid: RefCell::new(Vec::new()),
-            is_nonblock: is_sock_nonblock,
+            is_nonblock: Cell::new(is_sock_nonblock),
+            fd_type: AnonSocketType::Socketpair,
         });
         let fd1 = fds.new_ref(AnonSocket {
             readbuf: Some(RefCell::new(Buffer::new())),
@@ -415,7 +492,8 @@ pub trait EvalContextExt<'tcx>: crate::MiriInterpCxExt<'tcx> {
             peer_lost_data: Cell::new(false),
             blocked_read_tid: RefCell::new(Vec::new()),
             blocked_write_tid: RefCell::new(Vec::new()),
-            is_nonblock: is_sock_nonblock,
+            is_nonblock: Cell::new(is_sock_nonblock),
+            fd_type: AnonSocketType::Socketpair,
         });
 
         // Make the file descriptions point to each other.
@@ -475,7 +553,8 @@ pub trait EvalContextExt<'tcx>: crate::MiriInterpCxExt<'tcx> {
             peer_lost_data: Cell::new(false),
             blocked_read_tid: RefCell::new(Vec::new()),
             blocked_write_tid: RefCell::new(Vec::new()),
-            is_nonblock,
+            is_nonblock: Cell::new(is_nonblock),
+            fd_type: AnonSocketType::PipeRead,
         });
         let fd1 = fds.new_ref(AnonSocket {
             readbuf: None,
@@ -483,7 +562,8 @@ pub trait EvalContextExt<'tcx>: crate::MiriInterpCxExt<'tcx> {
             peer_lost_data: Cell::new(false),
             blocked_read_tid: RefCell::new(Vec::new()),
             blocked_write_tid: RefCell::new(Vec::new()),
-            is_nonblock,
+            is_nonblock: Cell::new(is_nonblock),
+            fd_type: AnonSocketType::PipeWrite,
         });
 
         // Make the file descriptions point to each other.
diff --git a/src/tools/miri/src/shims/windows/foreign_items.rs b/src/tools/miri/src/shims/windows/foreign_items.rs
index d822dd07fcd..98099e07b2e 100644
--- a/src/tools/miri/src/shims/windows/foreign_items.rs
+++ b/src/tools/miri/src/shims/windows/foreign_items.rs
@@ -572,6 +572,14 @@ pub trait EvalContextExt<'tcx>: crate::MiriInterpCxExt<'tcx> {
                 let ret = this.WaitForSingleObject(handle, timeout)?;
                 this.write_scalar(ret, dest)?;
             }
+            "GetCurrentProcess" => {
+                let [] = this.check_shim(abi, sys_conv, link_name, args)?;
+
+                this.write_scalar(
+                    Handle::Pseudo(PseudoHandle::CurrentProcess).to_scalar(this),
+                    dest,
+                )?;
+            }
             "GetCurrentThread" => {
                 let [] = this.check_shim(abi, sys_conv, link_name, args)?;
 
@@ -693,6 +701,20 @@ pub trait EvalContextExt<'tcx>: crate::MiriInterpCxExt<'tcx> {
                 let res = this.GetStdHandle(which)?;
                 this.write_scalar(res, dest)?;
             }
+            "DuplicateHandle" => {
+                let [src_proc, src_handle, target_proc, target_handle, access, inherit, options] =
+                    this.check_shim(abi, sys_conv, link_name, args)?;
+                let res = this.DuplicateHandle(
+                    src_proc,
+                    src_handle,
+                    target_proc,
+                    target_handle,
+                    access,
+                    inherit,
+                    options,
+                )?;
+                this.write_scalar(res, dest)?;
+            }
             "CloseHandle" => {
                 let [handle] = this.check_shim(abi, sys_conv, link_name, args)?;
 
diff --git a/src/tools/miri/src/shims/windows/handle.rs b/src/tools/miri/src/shims/windows/handle.rs
index 5c04271fac5..1e30bf25ed9 100644
--- a/src/tools/miri/src/shims/windows/handle.rs
+++ b/src/tools/miri/src/shims/windows/handle.rs
@@ -9,6 +9,7 @@ use crate::*;
 #[derive(Clone, Copy, Debug, PartialEq, Eq, Hash)]
 pub enum PseudoHandle {
     CurrentThread,
+    CurrentProcess,
 }
 
 /// Miri representation of a Windows `HANDLE`
@@ -23,16 +24,19 @@ pub enum Handle {
 
 impl PseudoHandle {
     const CURRENT_THREAD_VALUE: u32 = 0;
+    const CURRENT_PROCESS_VALUE: u32 = 1;
 
     fn value(self) -> u32 {
         match self {
             Self::CurrentThread => Self::CURRENT_THREAD_VALUE,
+            Self::CurrentProcess => Self::CURRENT_PROCESS_VALUE,
         }
     }
 
     fn from_value(value: u32) -> Option<Self> {
         match value {
             Self::CURRENT_THREAD_VALUE => Some(Self::CurrentThread),
+            Self::CURRENT_PROCESS_VALUE => Some(Self::CurrentProcess),
             _ => None,
         }
     }
@@ -244,6 +248,76 @@ pub trait EvalContextExt<'tcx>: crate::MiriInterpCxExt<'tcx> {
         interp_ok(handle.to_scalar(this))
     }
 
+    fn DuplicateHandle(
+        &mut self,
+        src_proc: &OpTy<'tcx>,       // HANDLE
+        src_handle: &OpTy<'tcx>,     // HANDLE
+        target_proc: &OpTy<'tcx>,    // HANDLE
+        target_handle: &OpTy<'tcx>,  // LPHANDLE
+        desired_access: &OpTy<'tcx>, // DWORD
+        inherit: &OpTy<'tcx>,        // BOOL
+        options: &OpTy<'tcx>,        // DWORD
+    ) -> InterpResult<'tcx, Scalar> {
+        // ^ Returns BOOL (i32 on Windows)
+        let this = self.eval_context_mut();
+
+        let src_proc = this.read_handle(src_proc, "DuplicateHandle")?;
+        let src_handle = this.read_handle(src_handle, "DuplicateHandle")?;
+        let target_proc = this.read_handle(target_proc, "DuplicateHandle")?;
+        let target_handle_ptr = this.read_pointer(target_handle)?;
+        // Since we only support DUPLICATE_SAME_ACCESS, this value is ignored, but should be valid
+        let _ = this.read_scalar(desired_access)?.to_u32()?;
+        // We don't support the CreateProcess API, so inheritable or not means nothing.
+        // If we ever add CreateProcess support, this will need to be implemented.
+        let _ = this.read_scalar(inherit)?;
+        let options = this.read_scalar(options)?;
+
+        if src_proc != Handle::Pseudo(PseudoHandle::CurrentProcess) {
+            throw_unsup_format!(
+                "`DuplicateHandle` `hSourceProcessHandle` parameter is not the current process, which is unsupported"
+            );
+        }
+
+        if target_proc != Handle::Pseudo(PseudoHandle::CurrentProcess) {
+            throw_unsup_format!(
+                "`DuplicateHandle` `hSourceProcessHandle` parameter is not the current process, which is unsupported"
+            );
+        }
+
+        if this.ptr_is_null(target_handle_ptr)? {
+            throw_unsup_format!(
+                "`DuplicateHandle` `lpTargetHandle` parameter is null, which is unsupported"
+            );
+        }
+
+        if options != this.eval_windows("c", "DUPLICATE_SAME_ACCESS") {
+            throw_unsup_format!(
+                "`DuplicateHandle` `dwOptions` parameter is not `DUPLICATE_SAME_ACCESS`, which is unsupported"
+            );
+        }
+
+        let new_handle = match src_handle {
+            Handle::File(old_fd_num) => {
+                let Some(fd) = this.machine.fds.get(old_fd_num) else {
+                    this.invalid_handle("DuplicateHandle")?
+                };
+                Handle::File(this.machine.fds.insert(fd))
+            }
+            Handle::Thread(_) => {
+                throw_unsup_format!(
+                    "`DuplicateHandle` called on a thread handle, which is unsupported"
+                );
+            }
+            Handle::Pseudo(pseudo) => Handle::Pseudo(pseudo),
+            Handle::Null | Handle::Invalid => this.invalid_handle("DuplicateHandle")?,
+        };
+
+        let target_place = this.deref_pointer_as(target_handle, this.machine.layouts.usize)?;
+        this.write_scalar(new_handle.to_scalar(this), &target_place)?;
+
+        interp_ok(this.eval_windows("c", "TRUE"))
+    }
+
     fn CloseHandle(&mut self, handle_op: &OpTy<'tcx>) -> InterpResult<'tcx, Scalar> {
         let this = self.eval_context_mut();
 
diff --git a/src/tools/miri/tests/fail-dep/libc/fcntl_fsetfl_while_blocking.rs b/src/tools/miri/tests/fail-dep/libc/fcntl_fsetfl_while_blocking.rs
new file mode 100644
index 00000000000..eef32136a0a
--- /dev/null
+++ b/src/tools/miri/tests/fail-dep/libc/fcntl_fsetfl_while_blocking.rs
@@ -0,0 +1,20 @@
+//@ignore-target: windows # Sockets/pipes are not implemented yet
+//~^ ERROR: deadlock: the evaluated program deadlocked
+//@compile-flags: -Zmiri-deterministic-concurrency
+use std::thread;
+
+/// If an O_NONBLOCK flag is set while the fd is blocking, that fd will not be woken up.
+fn main() {
+    let mut fds = [-1, -1];
+    let res = unsafe { libc::pipe(fds.as_mut_ptr()) };
+    assert_eq!(res, 0);
+    let mut buf: [u8; 5] = [0; 5];
+    let _thread1 = thread::spawn(move || {
+        // Add O_NONBLOCK flag while pipe is still block on read.
+        let res = unsafe { libc::fcntl(fds[0], libc::F_SETFL, libc::O_NONBLOCK) };
+        assert_eq!(res, 0);
+    });
+    // Main thread will block on read.
+    let _res = unsafe { libc::read(fds[0], buf.as_mut_ptr().cast(), buf.len() as libc::size_t) };
+    //~^ ERROR: deadlock: the evaluated program deadlocked
+}
diff --git a/src/tools/miri/tests/fail-dep/libc/fcntl_fsetfl_while_blocking.stderr b/src/tools/miri/tests/fail-dep/libc/fcntl_fsetfl_while_blocking.stderr
new file mode 100644
index 00000000000..9ca5598abae
--- /dev/null
+++ b/src/tools/miri/tests/fail-dep/libc/fcntl_fsetfl_while_blocking.stderr
@@ -0,0 +1,19 @@
+error: deadlock: the evaluated program deadlocked
+   |
+   = note: the evaluated program deadlocked
+   = note: (no span available)
+   = note: BACKTRACE on thread `unnamed-ID`:
+
+error: deadlock: the evaluated program deadlocked
+  --> tests/fail-dep/libc/fcntl_fsetfl_while_blocking.rs:LL:CC
+   |
+LL |     let _res = unsafe { libc::read(fds[0], buf.as_mut_ptr().cast(), buf.len() as libc::size_t) };
+   |                                                                                              ^ the evaluated program deadlocked
+   |
+   = note: BACKTRACE:
+   = note: inside `main` at tests/fail-dep/libc/fcntl_fsetfl_while_blocking.rs:LL:CC
+
+note: some details are omitted, run with `MIRIFLAGS=-Zmiri-backtrace=full` for a verbose backtrace
+
+error: aborting due to 2 previous errors
+
diff --git a/src/tools/miri/tests/fail-dep/libc/unsupported_incomplete_function.stderr b/src/tools/miri/tests/fail-dep/libc/unsupported_incomplete_function.stderr
index a92a97cef3b..52a93ab263d 100644
--- a/src/tools/miri/tests/fail-dep/libc/unsupported_incomplete_function.stderr
+++ b/src/tools/miri/tests/fail-dep/libc/unsupported_incomplete_function.stderr
@@ -4,8 +4,7 @@ error: unsupported operation: can't call foreign function `signal` on $OS
 LL |         libc::signal(libc::SIGPIPE, libc::SIG_IGN);
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ can't call foreign function `signal` on $OS
    |
-   = help: if this is a basic API commonly used on this target, please report an issue with Miri
-   = help: however, note that Miri does not aim to support every FFI function out there; for instance, we will not support APIs for things such as GUIs, scripting languages, or databases
+   = help: this means the program tried to do something Miri does not support; it does not indicate a bug in the program
    = note: BACKTRACE:
    = note: inside `main` at tests/fail-dep/libc/unsupported_incomplete_function.rs:LL:CC
 
diff --git a/src/tools/miri/tests/fail/alloc/no_global_allocator.stderr b/src/tools/miri/tests/fail/alloc/no_global_allocator.stderr
index 541af64b894..e80a3646714 100644
--- a/src/tools/miri/tests/fail/alloc/no_global_allocator.stderr
+++ b/src/tools/miri/tests/fail/alloc/no_global_allocator.stderr
@@ -4,8 +4,7 @@ error: unsupported operation: can't call foreign function `__rust_alloc` on $OS
 LL |         __rust_alloc(1, 1);
    |         ^^^^^^^^^^^^^^^^^^ can't call foreign function `__rust_alloc` on $OS
    |
-   = help: if this is a basic API commonly used on this target, please report an issue with Miri
-   = help: however, note that Miri does not aim to support every FFI function out there; for instance, we will not support APIs for things such as GUIs, scripting languages, or databases
+   = help: this means the program tried to do something Miri does not support; it does not indicate a bug in the program
    = note: BACKTRACE:
    = note: inside `miri_start` at tests/fail/alloc/no_global_allocator.rs:LL:CC
 
diff --git a/src/tools/miri/tests/fail/tree_borrows/cell-inside-struct.rs b/src/tools/miri/tests/fail/tree_borrows/cell-inside-struct.rs
new file mode 100644
index 00000000000..ff797877682
--- /dev/null
+++ b/src/tools/miri/tests/fail/tree_borrows/cell-inside-struct.rs
@@ -0,0 +1,33 @@
+//! A version of `cell_inside_struct` that dumps the tree so that we can see what is happening.
+//@compile-flags: -Zmiri-tree-borrows
+#[path = "../../utils/mod.rs"]
+#[macro_use]
+mod utils;
+
+use std::cell::Cell;
+
+struct Foo {
+    field1: u32,
+    field2: Cell<u32>,
+}
+
+pub fn main() {
+    let root = Foo { field1: 42, field2: Cell::new(88) };
+    unsafe {
+        let a = &root;
+
+        name!(a as *const Foo, "a");
+
+        let a: *const Foo = a as *const Foo;
+        let a: *mut Foo = a as *mut Foo;
+
+        let alloc_id = alloc_id!(a);
+        print_state!(alloc_id);
+
+        // Writing to `field2`, which is interior mutable, should be allowed.
+        (*a).field2.set(10);
+
+        // Writing to `field1`, which is frozen, should not be allowed.
+        (*a).field1 = 88; //~ ERROR: /write access through .* is forbidden/
+    }
+}
diff --git a/src/tools/miri/tests/fail/tree_borrows/cell-inside-struct.stderr b/src/tools/miri/tests/fail/tree_borrows/cell-inside-struct.stderr
new file mode 100644
index 00000000000..717f1419452
--- /dev/null
+++ b/src/tools/miri/tests/fail/tree_borrows/cell-inside-struct.stderr
@@ -0,0 +1,26 @@
+──────────────────────────────────────────────────
+Warning: this tree is indicative only. Some tags may have been hidden.
+0..   4..   8
+| Act | Act |    └─┬──<TAG=root of the allocation>
+| Frz |?Cel |      └────<TAG=a>
+──────────────────────────────────────────────────
+error: Undefined Behavior: write access through <TAG> (a) at ALLOC[0x0] is forbidden
+  --> tests/fail/tree_borrows/cell-inside-struct.rs:LL:CC
+   |
+LL |         (*a).field1 = 88;
+   |         ^^^^^^^^^^^^^^^^ write access through <TAG> (a) at ALLOC[0x0] is forbidden
+   |
+   = help: this indicates a potential bug in the program: it performed an invalid operation, but the Tree Borrows rules it violated are still experimental
+   = help: the accessed tag <TAG> (a) has state Frozen which forbids this child write access
+help: the accessed tag <TAG> was created here, in the initial state Cell
+  --> tests/fail/tree_borrows/cell-inside-struct.rs:LL:CC
+   |
+LL |         let a = &root;
+   |                 ^^^^^
+   = note: BACKTRACE (of the first span):
+   = note: inside `main` at tests/fail/tree_borrows/cell-inside-struct.rs:LL:CC
+
+note: some details are omitted, run with `MIRIFLAGS=-Zmiri-backtrace=full` for a verbose backtrace
+
+error: aborting due to 1 previous error
+
diff --git a/src/tools/miri/tests/fail/unsupported_foreign_function.stderr b/src/tools/miri/tests/fail/unsupported_foreign_function.stderr
index 4fe45b0868a..bbfc5c31256 100644
--- a/src/tools/miri/tests/fail/unsupported_foreign_function.stderr
+++ b/src/tools/miri/tests/fail/unsupported_foreign_function.stderr
@@ -4,8 +4,7 @@ error: unsupported operation: can't call foreign function `foo` on $OS
 LL |         foo();
    |         ^^^^^ can't call foreign function `foo` on $OS
    |
-   = help: if this is a basic API commonly used on this target, please report an issue with Miri
-   = help: however, note that Miri does not aim to support every FFI function out there; for instance, we will not support APIs for things such as GUIs, scripting languages, or databases
+   = help: this means the program tried to do something Miri does not support; it does not indicate a bug in the program
    = note: BACKTRACE:
    = note: inside `main` at tests/fail/unsupported_foreign_function.rs:LL:CC
 
diff --git a/src/tools/miri/tests/many-seeds/reentrant-lock.rs b/src/tools/miri/tests/many-seeds/reentrant-lock.rs
index 8a363179a9c..4c2dc463f48 100644
--- a/src/tools/miri/tests/many-seeds/reentrant-lock.rs
+++ b/src/tools/miri/tests/many-seeds/reentrant-lock.rs
@@ -1,6 +1,6 @@
 #![feature(reentrant_lock)]
 //! This is a regression test for
-//! <https://rust-lang.zulipchat.com/#narrow/channel/269128-miri/topic/reentrant.20lock.20failure.20on.20musl>.
+//! <https://rust-lang.zulipchat.com/#narrow/channel/269128-miri/topic/reentrant.20lock.20failure.20on.20mips>.
 
 use std::cell::Cell;
 use std::sync::ReentrantLock;
diff --git a/src/tools/miri/tests/native-lib/fail/function_not_in_so.stderr b/src/tools/miri/tests/native-lib/fail/function_not_in_so.stderr
index bf1cfd573b8..b663fd41457 100644
--- a/src/tools/miri/tests/native-lib/fail/function_not_in_so.stderr
+++ b/src/tools/miri/tests/native-lib/fail/function_not_in_so.stderr
@@ -4,8 +4,7 @@ error: unsupported operation: can't call foreign function `foo` on $OS
 LL |         foo();
    |         ^^^^^ can't call foreign function `foo` on $OS
    |
-   = help: if this is a basic API commonly used on this target, please report an issue with Miri
-   = help: however, note that Miri does not aim to support every FFI function out there; for instance, we will not support APIs for things such as GUIs, scripting languages, or databases
+   = help: this means the program tried to do something Miri does not support; it does not indicate a bug in the program
    = note: BACKTRACE:
    = note: inside `main` at tests/native-lib/fail/function_not_in_so.rs:LL:CC
 
diff --git a/src/tools/miri/tests/native-lib/fail/private_function.stderr b/src/tools/miri/tests/native-lib/fail/private_function.stderr
index 2cfc062212b..03681240015 100644
--- a/src/tools/miri/tests/native-lib/fail/private_function.stderr
+++ b/src/tools/miri/tests/native-lib/fail/private_function.stderr
@@ -4,8 +4,7 @@ error: unsupported operation: can't call foreign function `not_exported` on $OS
 LL |         not_exported();
    |         ^^^^^^^^^^^^^^ can't call foreign function `not_exported` on $OS
    |
-   = help: if this is a basic API commonly used on this target, please report an issue with Miri
-   = help: however, note that Miri does not aim to support every FFI function out there; for instance, we will not support APIs for things such as GUIs, scripting languages, or databases
+   = help: this means the program tried to do something Miri does not support; it does not indicate a bug in the program
    = note: BACKTRACE:
    = note: inside `main` at tests/native-lib/fail/private_function.rs:LL:CC
 
diff --git a/src/tools/miri/tests/pass-dep/libc/libc-pipe.rs b/src/tools/miri/tests/pass-dep/libc/libc-pipe.rs
index 05f6c870c3d..bc755af864c 100644
--- a/src/tools/miri/tests/pass-dep/libc/libc-pipe.rs
+++ b/src/tools/miri/tests/pass-dep/libc/libc-pipe.rs
@@ -15,6 +15,8 @@ fn main() {
     ))]
     // `pipe2` only exists in some specific os.
     test_pipe2();
+    test_pipe_setfl_getfl();
+    test_pipe_fcntl_threaded();
 }
 
 fn test_pipe() {
@@ -127,3 +129,68 @@ fn test_pipe2() {
     let res = unsafe { libc::pipe2(fds.as_mut_ptr(), libc::O_NONBLOCK) };
     assert_eq!(res, 0);
 }
+
+/// Basic test for pipe fcntl's F_SETFL and F_GETFL flag.
+fn test_pipe_setfl_getfl() {
+    // Initialise pipe fds.
+    let mut fds = [-1, -1];
+    let res = unsafe { libc::pipe(fds.as_mut_ptr()) };
+    assert_eq!(res, 0);
+
+    // Both sides should either have O_RONLY or O_WRONLY.
+    let res = unsafe { libc::fcntl(fds[0], libc::F_GETFL) };
+    assert_eq!(res, libc::O_RDONLY);
+    let res = unsafe { libc::fcntl(fds[1], libc::F_GETFL) };
+    assert_eq!(res, libc::O_WRONLY);
+
+    // Add the O_NONBLOCK flag with F_SETFL.
+    let res = unsafe { libc::fcntl(fds[0], libc::F_SETFL, libc::O_NONBLOCK) };
+    assert_eq!(res, 0);
+
+    // Test if the O_NONBLOCK flag is successfully added.
+    let res = unsafe { libc::fcntl(fds[0], libc::F_GETFL) };
+    assert_eq!(res, libc::O_RDONLY | libc::O_NONBLOCK);
+
+    // The other side remains unchanged.
+    let res = unsafe { libc::fcntl(fds[1], libc::F_GETFL) };
+    assert_eq!(res, libc::O_WRONLY);
+
+    // Test if O_NONBLOCK flag can be unset.
+    let res = unsafe { libc::fcntl(fds[0], libc::F_SETFL, 0) };
+    assert_eq!(res, 0);
+    let res = unsafe { libc::fcntl(fds[0], libc::F_GETFL) };
+    assert_eq!(res, libc::O_RDONLY);
+}
+
+/// Test the behaviour of F_SETFL/F_GETFL when a fd is blocking.
+/// The expected execution is:
+/// 1. Main thread blocks on fds[0] `read`.
+/// 2. Thread 1 sets O_NONBLOCK flag on fds[0],
+///    checks the value of F_GETFL,
+///    then writes to fds[1] to unblock main thread's `read`.
+fn test_pipe_fcntl_threaded() {
+    let mut fds = [-1, -1];
+    let res = unsafe { libc::pipe(fds.as_mut_ptr()) };
+    assert_eq!(res, 0);
+    let mut buf: [u8; 5] = [0; 5];
+    let thread1 = thread::spawn(move || {
+        // Add O_NONBLOCK flag while pipe is still blocked on read.
+        let res = unsafe { libc::fcntl(fds[0], libc::F_SETFL, libc::O_NONBLOCK) };
+        assert_eq!(res, 0);
+
+        // Check the new flag value while the main thread is still blocked on fds[0].
+        let res = unsafe { libc::fcntl(fds[0], libc::F_GETFL) };
+        assert_eq!(res, libc::O_NONBLOCK);
+
+        // The write below will unblock the `read` in main thread: even though
+        // the socket is now "non-blocking", the shim needs to deal correctly
+        // with threads that were blocked before the socket was made non-blocking.
+        let data = "abcde".as_bytes().as_ptr();
+        let res = unsafe { libc::write(fds[1], data as *const libc::c_void, 5) };
+        assert_eq!(res, 5);
+    });
+    // The `read` below will block.
+    let res = unsafe { libc::read(fds[0], buf.as_mut_ptr().cast(), buf.len() as libc::size_t) };
+    thread1.join().unwrap();
+    assert_eq!(res, 5);
+}
diff --git a/src/tools/miri/tests/pass-dep/libc/libc-socketpair.rs b/src/tools/miri/tests/pass-dep/libc/libc-socketpair.rs
index 9e48410f704..c36f6b11224 100644
--- a/src/tools/miri/tests/pass-dep/libc/libc-socketpair.rs
+++ b/src/tools/miri/tests/pass-dep/libc/libc-socketpair.rs
@@ -12,6 +12,7 @@ fn main() {
     test_race();
     test_blocking_read();
     test_blocking_write();
+    test_socketpair_setfl_getfl();
 }
 
 fn test_socketpair() {
@@ -182,3 +183,35 @@ fn test_blocking_write() {
     thread1.join().unwrap();
     thread2.join().unwrap();
 }
+
+/// Basic test for socketpair fcntl's F_SETFL and F_GETFL flag.
+fn test_socketpair_setfl_getfl() {
+    // Initialise socketpair fds.
+    let mut fds = [-1, -1];
+    let res = unsafe { libc::socketpair(libc::AF_UNIX, libc::SOCK_STREAM, 0, fds.as_mut_ptr()) };
+    assert_eq!(res, 0);
+
+    // Test if both sides have O_RDWR.
+    let res = unsafe { libc::fcntl(fds[0], libc::F_GETFL) };
+    assert_eq!(res, libc::O_RDWR);
+    let res = unsafe { libc::fcntl(fds[1], libc::F_GETFL) };
+    assert_eq!(res, libc::O_RDWR);
+
+    // Add the O_NONBLOCK flag with F_SETFL.
+    let res = unsafe { libc::fcntl(fds[0], libc::F_SETFL, libc::O_NONBLOCK) };
+    assert_eq!(res, 0);
+
+    // Test if the O_NONBLOCK flag is successfully added.
+    let res = unsafe { libc::fcntl(fds[0], libc::F_GETFL) };
+    assert_eq!(res, libc::O_RDWR | libc::O_NONBLOCK);
+
+    // The other side remains unchanged.
+    let res = unsafe { libc::fcntl(fds[1], libc::F_GETFL) };
+    assert_eq!(res, libc::O_RDWR);
+
+    // Test if O_NONBLOCK flag can be unset.
+    let res = unsafe { libc::fcntl(fds[0], libc::F_SETFL, 0) };
+    assert_eq!(res, 0);
+    let res = unsafe { libc::fcntl(fds[0], libc::F_GETFL) };
+    assert_eq!(res, libc::O_RDWR);
+}
diff --git a/src/tools/miri/tests/pass-dep/tokio/file-io.rs b/src/tools/miri/tests/pass-dep/tokio/file-io.rs
index 6e88b907f5d..067753203bb 100644
--- a/src/tools/miri/tests/pass-dep/tokio/file-io.rs
+++ b/src/tools/miri/tests/pass-dep/tokio/file-io.rs
@@ -20,7 +20,11 @@ async fn test_create_and_write() -> io::Result<()> {
     let mut file = File::create(&path).await?;
 
     // Write 10 bytes to the file.
-    file.write(b"some bytes").await?;
+    file.write_all(b"some bytes").await?;
+    // For tokio's file I/O, `await` does not have its usual semantics of waiting until the
+    // operation is completed, so we have to wait some more to make sure the write is completed.
+    file.flush().await?;
+    // Check that 10 bytes have been written.
     assert_eq!(file.metadata().await.unwrap().len(), 10);
 
     remove_file(&path).unwrap();
@@ -31,10 +35,10 @@ async fn test_create_and_read() -> io::Result<()> {
     let bytes = b"more bytes";
     let path = utils::prepare_with_content("foo.txt", bytes);
     let mut file = OpenOptions::new().read(true).open(&path).await.unwrap();
-    let mut buffer = [0u8; 10];
+    let mut buffer = vec![];
 
     // Read the whole file.
-    file.read(&mut buffer[..]).await?;
+    file.read_to_end(&mut buffer).await?;
     assert_eq!(&buffer, b"more bytes");
 
     remove_file(&path).unwrap();
diff --git a/src/tools/miri/tests/pass/both_borrows/basic_aliasing_model.rs b/src/tools/miri/tests/pass/both_borrows/basic_aliasing_model.rs
index c76e7f2eebd..6a625e597df 100644
--- a/src/tools/miri/tests/pass/both_borrows/basic_aliasing_model.rs
+++ b/src/tools/miri/tests/pass/both_borrows/basic_aliasing_model.rs
@@ -1,6 +1,7 @@
 //@revisions: stack tree
 //@[tree]compile-flags: -Zmiri-tree-borrows
 #![feature(allocator_api)]
+use std::cell::Cell;
 use std::ptr;
 
 // Test various aliasing-model-related things.
@@ -22,6 +23,7 @@ fn main() {
     not_unpin_not_protected();
     write_does_not_invalidate_all_aliases();
     box_into_raw_allows_interior_mutable_alias();
+    cell_inside_struct()
 }
 
 // Make sure that reading from an `&mut` does, like reborrowing to `&`,
@@ -259,7 +261,7 @@ fn write_does_not_invalidate_all_aliases() {
 
 fn box_into_raw_allows_interior_mutable_alias() {
     unsafe {
-        let b = Box::new(std::cell::Cell::new(42));
+        let b = Box::new(Cell::new(42));
         let raw = Box::into_raw(b);
         let c = &*raw;
         let d = raw.cast::<i32>(); // bypassing `Cell` -- only okay in Miri tests
@@ -269,3 +271,19 @@ fn box_into_raw_allows_interior_mutable_alias() {
         drop(Box::from_raw(raw));
     }
 }
+
+fn cell_inside_struct() {
+    struct Foo {
+        field1: u32,
+        field2: Cell<u32>,
+    }
+
+    let mut root = Foo { field1: 42, field2: Cell::new(88) };
+    let a = &mut root;
+
+    // Writing to `field2`, which is interior mutable, should be allowed.
+    (*a).field2.set(10);
+
+    // Writing to `field1`, which is reserved, should also be allowed.
+    (*a).field1 = 88;
+}
diff --git a/src/tools/miri/tests/pass/shims/fs.rs b/src/tools/miri/tests/pass/shims/fs.rs
index 315637ff7ec..87df43ca7e5 100644
--- a/src/tools/miri/tests/pass/shims/fs.rs
+++ b/src/tools/miri/tests/pass/shims/fs.rs
@@ -23,9 +23,9 @@ fn main() {
     test_seek();
     test_errors();
     test_from_raw_os_error();
+    test_file_clone();
     // Windows file handling is very incomplete.
     if cfg!(not(windows)) {
-        test_file_clone();
         test_file_set_len();
         test_file_sync();
         test_rename();
diff --git a/src/tools/miri/tests/pass/tree_borrows/cell-alternate-writes.stderr b/src/tools/miri/tests/pass/tree_borrows/cell-alternate-writes.stderr
index 75a30c9a083..e09aed2cf5d 100644
--- a/src/tools/miri/tests/pass/tree_borrows/cell-alternate-writes.stderr
+++ b/src/tools/miri/tests/pass/tree_borrows/cell-alternate-writes.stderr
@@ -3,8 +3,8 @@ Warning: this tree is indicative only. Some tags may have been hidden.
 0..   1
 | Act |    └─┬──<TAG=root of the allocation>
 | ReIM|      └─┬──<TAG=data>
-| Cel |        ├────<TAG=x>
-| Cel |        └────<TAG=y>
+|?Cel |        ├────<TAG=x>
+|?Cel |        └────<TAG=y>
 ──────────────────────────────────────────────────
 ──────────────────────────────────────────────────
 Warning: this tree is indicative only. Some tags may have been hidden.
diff --git a/src/tools/miri/tests/pass/tree_borrows/cell-lazy-write-to-surrounding.rs b/src/tools/miri/tests/pass/tree_borrows/cell-lazy-write-to-surrounding.rs
new file mode 100644
index 00000000000..abe08f2cd22
--- /dev/null
+++ b/src/tools/miri/tests/pass/tree_borrows/cell-lazy-write-to-surrounding.rs
@@ -0,0 +1,22 @@
+//@compile-flags: -Zmiri-tree-borrows
+
+use std::cell::Cell;
+
+fn foo(x: &Cell<i32>) {
+    unsafe {
+        let ptr = x as *const Cell<i32> as *mut Cell<i32> as *mut i32;
+        ptr.offset(1).write(0);
+    }
+}
+
+fn main() {
+    let arr = [Cell::new(1), Cell::new(1)];
+    foo(&arr[0]);
+
+    let pair = (Cell::new(1), 1);
+    // TODO: Ideally, this would result in UB since the second element
+    // in `pair` is Frozen.  We would need some way to express a
+    // "shared reference with permission to access surrounding
+    // interior mutable data".
+    foo(&pair.0);
+}