FD: remove big surrounding RefCell, simplify socketpair

6 files changed, 169 insertions, 187 deletions

diff --git a/src/tools/miri/src/shims/unix/fd.rs b/src/tools/miri/src/shims/unix/fd.rs
index 98a124b9a56..1fd1cf4d99e 100644
--- a/src/tools/miri/src/shims/unix/fd.rs
+++ b/src/tools/miri/src/shims/unix/fd.rs
@@ -2,9 +2,9 @@
 //! standard file descriptors (stdin/stdout/stderr).
 
 use std::any::Any;
-use std::cell::{Ref, RefCell, RefMut};
 use std::collections::BTreeMap;
 use std::io::{self, ErrorKind, IsTerminal, Read, SeekFrom, Write};
+use std::ops::Deref;
 use std::rc::Rc;
 use std::rc::Weak;
 
@@ -27,7 +27,7 @@ pub trait FileDescription: std::fmt::Debug + Any {
 
     /// Reads as much as possible into the given buffer, and returns the number of bytes read.
     fn read<'tcx>(
-        &mut self,
+        &self,
         _communicate_allowed: bool,
         _fd_id: FdId,
         _bytes: &mut [u8],
@@ -38,7 +38,7 @@ pub trait FileDescription: std::fmt::Debug + Any {
 
     /// Writes as much as possible from the given buffer, and returns the number of bytes written.
     fn write<'tcx>(
-        &mut self,
+        &self,
         _communicate_allowed: bool,
         _fd_id: FdId,
         _bytes: &[u8],
@@ -50,7 +50,7 @@ pub trait FileDescription: std::fmt::Debug + Any {
     /// Reads as much as possible into the given buffer from a given offset,
     /// and returns the number of bytes read.
     fn pread<'tcx>(
-        &mut self,
+        &self,
         _communicate_allowed: bool,
         _bytes: &mut [u8],
         _offset: u64,
@@ -62,7 +62,7 @@ pub trait FileDescription: std::fmt::Debug + Any {
     /// Writes as much as possible from the given buffer starting at a given offset,
     /// and returns the number of bytes written.
     fn pwrite<'tcx>(
-        &mut self,
+        &self,
         _communicate_allowed: bool,
         _bytes: &[u8],
         _offset: u64,
@@ -74,7 +74,7 @@ pub trait FileDescription: std::fmt::Debug + Any {
     /// Seeks to the given offset (which can be relative to the beginning, end, or current position).
     /// Returns the new position from the start of the stream.
     fn seek<'tcx>(
-        &mut self,
+        &self,
         _communicate_allowed: bool,
         _offset: SeekFrom,
     ) -> InterpResult<'tcx, io::Result<u64>> {
@@ -111,14 +111,9 @@ pub trait FileDescription: std::fmt::Debug + Any {
 
 impl dyn FileDescription {
     #[inline(always)]
-    pub fn downcast_ref<T: Any>(&self) -> Option<&T> {
+    pub fn downcast<T: Any>(&self) -> Option<&T> {
         (self as &dyn Any).downcast_ref()
     }
-
-    #[inline(always)]
-    pub fn downcast_mut<T: Any>(&mut self) -> Option<&mut T> {
-        (self as &mut dyn Any).downcast_mut()
-    }
 }
 
 impl FileDescription for io::Stdin {
@@ -127,7 +122,7 @@ impl FileDescription for io::Stdin {
     }
 
     fn read<'tcx>(
-        &mut self,
+        &self,
         communicate_allowed: bool,
         _fd_id: FdId,
         bytes: &mut [u8],
@@ -137,7 +132,7 @@ impl FileDescription for io::Stdin {
             // We want isolation mode to be deterministic, so we have to disallow all reads, even stdin.
             helpers::isolation_abort_error("`read` from stdin")?;
         }
-        Ok(Read::read(self, bytes))
+        Ok(Read::read(&mut { self }, bytes))
     }
 
     fn is_tty(&self, communicate_allowed: bool) -> bool {
@@ -151,14 +146,14 @@ impl FileDescription for io::Stdout {
     }
 
     fn write<'tcx>(
-        &mut self,
+        &self,
         _communicate_allowed: bool,
         _fd_id: FdId,
         bytes: &[u8],
         _ecx: &mut MiriInterpCx<'tcx>,
     ) -> InterpResult<'tcx, io::Result<usize>> {
         // We allow writing to stderr even with isolation enabled.
-        let result = Write::write(self, bytes);
+        let result = Write::write(&mut { self }, bytes);
         // Stdout is buffered, flush to make sure it appears on the
         // screen.  This is the write() syscall of the interpreted
         // program, we want it to correspond to a write() syscall on
@@ -180,7 +175,7 @@ impl FileDescription for io::Stderr {
     }
 
     fn write<'tcx>(
-        &mut self,
+        &self,
         _communicate_allowed: bool,
         _fd_id: FdId,
         bytes: &[u8],
@@ -206,7 +201,7 @@ impl FileDescription for NullOutput {
     }
 
     fn write<'tcx>(
-        &mut self,
+        &self,
         _communicate_allowed: bool,
         _fd_id: FdId,
         bytes: &[u8],
@@ -221,26 +216,23 @@ impl FileDescription for NullOutput {
 #[derive(Clone, Debug)]
 pub struct FileDescWithId<T: FileDescription + ?Sized> {
     id: FdId,
-    file_description: RefCell<Box<T>>,
+    file_description: Box<T>,
 }
 
 #[derive(Clone, Debug)]
 pub struct FileDescriptionRef(Rc<FileDescWithId<dyn FileDescription>>);
 
-impl FileDescriptionRef {
-    fn new(fd: impl FileDescription, id: FdId) -> Self {
-        FileDescriptionRef(Rc::new(FileDescWithId {
-            id,
-            file_description: RefCell::new(Box::new(fd)),
-        }))
-    }
+impl Deref for FileDescriptionRef {
+    type Target = dyn FileDescription;
 
-    pub fn borrow(&self) -> Ref<'_, dyn FileDescription> {
-        Ref::map(self.0.file_description.borrow(), |fd| fd.as_ref())
+    fn deref(&self) -> &Self::Target {
+        &*self.0.file_description
     }
+}
 
-    pub fn borrow_mut(&self) -> RefMut<'_, dyn FileDescription> {
-        RefMut::map(self.0.file_description.borrow_mut(), |fd| fd.as_mut())
+impl FileDescriptionRef {
+    fn new(fd: impl FileDescription, id: FdId) -> Self {
+        FileDescriptionRef(Rc::new(FileDescWithId { id, file_description: Box::new(fd) }))
     }
 
     pub fn close<'tcx>(
@@ -256,7 +248,7 @@ impl FileDescriptionRef {
                 // Remove entry from the global epoll_event_interest table.
                 ecx.machine.epoll_interests.remove(id);
 
-                RefCell::into_inner(fd.file_description).close(communicate_allowed, ecx)
+                fd.file_description.close(communicate_allowed, ecx)
             }
             None => Ok(Ok(())),
         }
@@ -277,7 +269,7 @@ impl FileDescriptionRef {
         ecx: &mut InterpCx<'tcx, MiriMachine<'tcx>>,
     ) -> InterpResult<'tcx, ()> {
         use crate::shims::unix::linux::epoll::EvalContextExt;
-        ecx.check_and_update_readiness(self.get_id(), || self.borrow_mut().get_epoll_ready_events())
+        ecx.check_and_update_readiness(self.get_id(), || self.get_epoll_ready_events())
     }
 }
 
@@ -334,11 +326,20 @@ impl FdTable {
         fds
     }
 
-    /// Insert a new file description to the FdTable.
-    pub fn insert_new(&mut self, fd: impl FileDescription) -> i32 {
+    pub fn new_ref(&mut self, fd: impl FileDescription) -> FileDescriptionRef {
         let file_handle = FileDescriptionRef::new(fd, self.next_file_description_id);
         self.next_file_description_id = FdId(self.next_file_description_id.0.strict_add(1));
-        self.insert_ref_with_min_fd(file_handle, 0)
+        file_handle
+    }
+
+    /// Insert a new file description to the FdTable.
+    pub fn insert_new(&mut self, fd: impl FileDescription) -> i32 {
+        let fd_ref = self.new_ref(fd);
+        self.insert(fd_ref)
+    }
+
+    pub fn insert(&mut self, fd_ref: FileDescriptionRef) -> i32 {
+        self.insert_ref_with_min_fd(fd_ref, 0)
     }
 
     /// Insert a file description, giving it a file descriptor that is at least `min_fd`.
@@ -368,17 +369,7 @@ impl FdTable {
         new_fd
     }
 
-    pub fn get(&self, fd: i32) -> Option<Ref<'_, dyn FileDescription>> {
-        let fd = self.fds.get(&fd)?;
-        Some(fd.borrow())
-    }
-
-    pub fn get_mut(&self, fd: i32) -> Option<RefMut<'_, dyn FileDescription>> {
-        let fd = self.fds.get(&fd)?;
-        Some(fd.borrow_mut())
-    }
-
-    pub fn get_ref(&self, fd: i32) -> Option<FileDescriptionRef> {
+    pub fn get(&self, fd: i32) -> Option<FileDescriptionRef> {
         let fd = self.fds.get(&fd)?;
         Some(fd.clone())
     }
@@ -397,7 +388,7 @@ pub trait EvalContextExt<'tcx>: crate::MiriInterpCxExt<'tcx> {
     fn dup(&mut self, old_fd: i32) -> InterpResult<'tcx, Scalar> {
         let this = self.eval_context_mut();
 
-        let Some(dup_fd) = this.machine.fds.get_ref(old_fd) else {
+        let Some(dup_fd) = this.machine.fds.get(old_fd) else {
             return Ok(Scalar::from_i32(this.fd_not_found()?));
         };
         Ok(Scalar::from_i32(this.machine.fds.insert_ref_with_min_fd(dup_fd, 0)))
@@ -406,7 +397,7 @@ pub trait EvalContextExt<'tcx>: crate::MiriInterpCxExt<'tcx> {
     fn dup2(&mut self, old_fd: i32, new_fd: i32) -> InterpResult<'tcx, Scalar> {
         let this = self.eval_context_mut();
 
-        let Some(dup_fd) = this.machine.fds.get_ref(old_fd) else {
+        let Some(dup_fd) = this.machine.fds.get(old_fd) else {
             return Ok(Scalar::from_i32(this.fd_not_found()?));
         };
         if new_fd != old_fd {
@@ -492,7 +483,7 @@ pub trait EvalContextExt<'tcx>: crate::MiriInterpCxExt<'tcx> {
             }
             let start = this.read_scalar(&args[2])?.to_i32()?;
 
-            match this.machine.fds.get_ref(fd) {
+            match this.machine.fds.get(fd) {
                 Some(dup_fd) =>
                     Ok(Scalar::from_i32(this.machine.fds.insert_ref_with_min_fd(dup_fd, start))),
                 None => Ok(Scalar::from_i32(this.fd_not_found()?)),
@@ -565,7 +556,7 @@ pub trait EvalContextExt<'tcx>: crate::MiriInterpCxExt<'tcx> {
         let communicate = this.machine.communicate();
 
         // We temporarily dup the FD to be able to retain mutable access to `this`.
-        let Some(fd) = this.machine.fds.get_ref(fd) else {
+        let Some(fd) = this.machine.fds.get(fd) else {
             trace!("read: FD not found");
             return Ok(Scalar::from_target_isize(this.fd_not_found()?, this));
         };
@@ -576,14 +567,14 @@ pub trait EvalContextExt<'tcx>: crate::MiriInterpCxExt<'tcx> {
         // `usize::MAX` because it is bounded by the host's `isize`.
         let mut bytes = vec![0; usize::try_from(count).unwrap()];
         let result = match offset {
-            None => fd.borrow_mut().read(communicate, fd.get_id(), &mut bytes, this),
+            None => fd.read(communicate, fd.get_id(), &mut bytes, this),
             Some(offset) => {
                 let Ok(offset) = u64::try_from(offset) else {
                     let einval = this.eval_libc("EINVAL");
                     this.set_last_error(einval)?;
                     return Ok(Scalar::from_target_isize(-1, this));
                 };
-                fd.borrow_mut().pread(communicate, &mut bytes, offset, this)
+                fd.pread(communicate, &mut bytes, offset, this)
             }
         };
 
@@ -629,19 +620,19 @@ pub trait EvalContextExt<'tcx>: crate::MiriInterpCxExt<'tcx> {
 
         let bytes = this.read_bytes_ptr_strip_provenance(buf, Size::from_bytes(count))?.to_owned();
         // We temporarily dup the FD to be able to retain mutable access to `this`.
-        let Some(fd) = this.machine.fds.get_ref(fd) else {
+        let Some(fd) = this.machine.fds.get(fd) else {
             return Ok(Scalar::from_target_isize(this.fd_not_found()?, this));
         };
 
         let result = match offset {
-            None => fd.borrow_mut().write(communicate, fd.get_id(), &bytes, this),
+            None => fd.write(communicate, fd.get_id(), &bytes, this),
             Some(offset) => {
                 let Ok(offset) = u64::try_from(offset) else {
                     let einval = this.eval_libc("EINVAL");
                     this.set_last_error(einval)?;
                     return Ok(Scalar::from_target_isize(-1, this));
                 };
-                fd.borrow_mut().pwrite(communicate, &bytes, offset, this)
+                fd.pwrite(communicate, &bytes, offset, this)
             }
         };
 
diff --git a/src/tools/miri/src/shims/unix/fs.rs b/src/tools/miri/src/shims/unix/fs.rs
index 9da36e64a0f..e6076abedbd 100644
--- a/src/tools/miri/src/shims/unix/fs.rs
+++ b/src/tools/miri/src/shims/unix/fs.rs
@@ -31,29 +31,29 @@ impl FileDescription for FileHandle {
     }
 
     fn read<'tcx>(
-        &mut self,
+        &self,
         communicate_allowed: bool,
         _fd_id: FdId,
         bytes: &mut [u8],
         _ecx: &mut MiriInterpCx<'tcx>,
     ) -> InterpResult<'tcx, io::Result<usize>> {
         assert!(communicate_allowed, "isolation should have prevented even opening a file");
-        Ok(self.file.read(bytes))
+        Ok((&mut &self.file).read(bytes))
     }
 
     fn write<'tcx>(
-        &mut self,
+        &self,
         communicate_allowed: bool,
         _fd_id: FdId,
         bytes: &[u8],
         _ecx: &mut MiriInterpCx<'tcx>,
     ) -> InterpResult<'tcx, io::Result<usize>> {
         assert!(communicate_allowed, "isolation should have prevented even opening a file");
-        Ok(self.file.write(bytes))
+        Ok((&mut &self.file).write(bytes))
     }
 
     fn pread<'tcx>(
-        &mut self,
+        &self,
         communicate_allowed: bool,
         bytes: &mut [u8],
         offset: u64,
@@ -63,13 +63,13 @@ impl FileDescription for FileHandle {
         // Emulates pread using seek + read + seek to restore cursor position.
         // Correctness of this emulation relies on sequential nature of Miri execution.
         // The closure is used to emulate `try` block, since we "bubble" `io::Error` using `?`.
+        let file = &mut &self.file;
         let mut f = || {
-            let cursor_pos = self.file.stream_position()?;
-            self.file.seek(SeekFrom::Start(offset))?;
-            let res = self.file.read(bytes);
+            let cursor_pos = file.stream_position()?;
+            file.seek(SeekFrom::Start(offset))?;
+            let res = file.read(bytes);
             // Attempt to restore cursor position even if the read has failed
-            self.file
-                .seek(SeekFrom::Start(cursor_pos))
+            file.seek(SeekFrom::Start(cursor_pos))
                 .expect("failed to restore file position, this shouldn't be possible");
             res
         };
@@ -77,7 +77,7 @@ impl FileDescription for FileHandle {
     }
 
     fn pwrite<'tcx>(
-        &mut self,
+        &self,
         communicate_allowed: bool,
         bytes: &[u8],
         offset: u64,
@@ -87,13 +87,13 @@ impl FileDescription for FileHandle {
         // Emulates pwrite using seek + write + seek to restore cursor position.
         // Correctness of this emulation relies on sequential nature of Miri execution.
         // The closure is used to emulate `try` block, since we "bubble" `io::Error` using `?`.
+        let file = &mut &self.file;
         let mut f = || {
-            let cursor_pos = self.file.stream_position()?;
-            self.file.seek(SeekFrom::Start(offset))?;
-            let res = self.file.write(bytes);
+            let cursor_pos = file.stream_position()?;
+            file.seek(SeekFrom::Start(offset))?;
+            let res = file.write(bytes);
             // Attempt to restore cursor position even if the write has failed
-            self.file
-                .seek(SeekFrom::Start(cursor_pos))
+            file.seek(SeekFrom::Start(cursor_pos))
                 .expect("failed to restore file position, this shouldn't be possible");
             res
         };
@@ -101,12 +101,12 @@ impl FileDescription for FileHandle {
     }
 
     fn seek<'tcx>(
-        &mut self,
+        &self,
         communicate_allowed: bool,
         offset: SeekFrom,
     ) -> InterpResult<'tcx, io::Result<u64>> {
         assert!(communicate_allowed, "isolation should have prevented even opening a file");
-        Ok(self.file.seek(offset))
+        Ok((&mut &self.file).seek(offset))
     }
 
     fn close<'tcx>(
@@ -580,7 +580,7 @@ pub trait EvalContextExt<'tcx>: crate::MiriInterpCxExt<'tcx> {
 
         let communicate = this.machine.communicate();
 
-        let Some(mut file_description) = this.machine.fds.get_mut(fd) else {
+        let Some(file_description) = this.machine.fds.get(fd) else {
             return Ok(Scalar::from_i64(this.fd_not_found()?));
         };
         let result = file_description
@@ -1276,7 +1276,7 @@ pub trait EvalContextExt<'tcx>: crate::MiriInterpCxExt<'tcx> {
 
         // FIXME: Support ftruncate64 for all FDs
         let FileHandle { file, writable } =
-            file_description.downcast_ref::<FileHandle>().ok_or_else(|| {
+            file_description.downcast::<FileHandle>().ok_or_else(|| {
                 err_unsup_format!("`ftruncate64` is only supported on file-backed file descriptors")
             })?;
 
@@ -1328,7 +1328,7 @@ pub trait EvalContextExt<'tcx>: crate::MiriInterpCxExt<'tcx> {
         };
         // Only regular files support synchronization.
         let FileHandle { file, writable } =
-            file_description.downcast_ref::<FileHandle>().ok_or_else(|| {
+            file_description.downcast::<FileHandle>().ok_or_else(|| {
                 err_unsup_format!("`fsync` is only supported on file-backed file descriptors")
             })?;
         let io_result = maybe_sync_file(file, *writable, File::sync_all);
@@ -1353,7 +1353,7 @@ pub trait EvalContextExt<'tcx>: crate::MiriInterpCxExt<'tcx> {
         };
         // Only regular files support synchronization.
         let FileHandle { file, writable } =
-            file_description.downcast_ref::<FileHandle>().ok_or_else(|| {
+            file_description.downcast::<FileHandle>().ok_or_else(|| {
                 err_unsup_format!("`fdatasync` is only supported on file-backed file descriptors")
             })?;
         let io_result = maybe_sync_file(file, *writable, File::sync_data);
@@ -1401,7 +1401,7 @@ pub trait EvalContextExt<'tcx>: crate::MiriInterpCxExt<'tcx> {
         };
         // Only regular files support synchronization.
         let FileHandle { file, writable } =
-            file_description.downcast_ref::<FileHandle>().ok_or_else(|| {
+            file_description.downcast::<FileHandle>().ok_or_else(|| {
                 err_unsup_format!(
                     "`sync_data_range` is only supported on file-backed file descriptors"
                 )
@@ -1708,7 +1708,7 @@ impl FileMetadata {
         };
 
         let file = &file_description
-            .downcast_ref::<FileHandle>()
+            .downcast::<FileHandle>()
             .ok_or_else(|| {
                 err_unsup_format!(
                     "obtaining metadata is only supported on file-backed file descriptors"
diff --git a/src/tools/miri/src/shims/unix/linux/epoll.rs b/src/tools/miri/src/shims/unix/linux/epoll.rs
index 89616bd0d07..a1b943f2e82 100644
--- a/src/tools/miri/src/shims/unix/linux/epoll.rs
+++ b/src/tools/miri/src/shims/unix/linux/epoll.rs
@@ -12,7 +12,7 @@ use crate::*;
 struct Epoll {
     /// A map of EpollEventInterests registered under this epoll instance.
     /// Each entry is differentiated using FdId and file descriptor value.
-    interest_list: BTreeMap<(FdId, i32), Rc<RefCell<EpollEventInterest>>>,
+    interest_list: RefCell<BTreeMap<(FdId, i32), Rc<RefCell<EpollEventInterest>>>>,
     /// A map of EpollEventInstance that will be returned when `epoll_wait` is called.
     /// Similar to interest_list, the entry is also differentiated using FdId
     /// and file descriptor value.
@@ -226,18 +226,17 @@ pub trait EvalContextExt<'tcx>: crate::MiriInterpCxExt<'tcx> {
         }
 
         // Check if epfd is a valid epoll file descriptor.
-        let Some(epfd) = this.machine.fds.get_ref(epfd_value) else {
+        let Some(epfd) = this.machine.fds.get(epfd_value) else {
             return Ok(Scalar::from_i32(this.fd_not_found()?));
         };
-        let mut binding = epfd.borrow_mut();
-        let epoll_file_description = &mut binding
-            .downcast_mut::<Epoll>()
+        let epoll_file_description = epfd
+            .downcast::<Epoll>()
             .ok_or_else(|| err_unsup_format!("non-epoll FD passed to `epoll_ctl`"))?;
 
-        let interest_list = &mut epoll_file_description.interest_list;
+        let mut interest_list = epoll_file_description.interest_list.borrow_mut();
         let ready_list = &epoll_file_description.ready_list;
 
-        let Some(file_descriptor) = this.machine.fds.get_ref(fd) else {
+        let Some(file_descriptor) = this.machine.fds.get(fd) else {
             return Ok(Scalar::from_i32(this.fd_not_found()?));
         };
         let id = file_descriptor.get_id();
@@ -399,16 +398,15 @@ pub trait EvalContextExt<'tcx>: crate::MiriInterpCxExt<'tcx> {
             throw_unsup_format!("epoll_wait: timeout value can only be 0");
         }
 
-        let Some(epfd) = this.machine.fds.get_ref(epfd) else {
+        let Some(epfd) = this.machine.fds.get(epfd) else {
             return Ok(Scalar::from_i32(this.fd_not_found()?));
         };
-        let mut binding = epfd.borrow_mut();
-        let epoll_file_description = &mut binding
-            .downcast_mut::<Epoll>()
+        let epoll_file_description = epfd
+            .downcast::<Epoll>()
             .ok_or_else(|| err_unsup_format!("non-epoll FD passed to `epoll_wait`"))?;
 
-        let binding = epoll_file_description.get_ready_list();
-        let mut ready_list = binding.borrow_mut();
+        let ready_list = epoll_file_description.get_ready_list();
+        let mut ready_list = ready_list.borrow_mut();
         let mut num_of_events: i32 = 0;
         let mut array_iter = this.project_array_fields(&event)?;
 
diff --git a/src/tools/miri/src/shims/unix/linux/eventfd.rs b/src/tools/miri/src/shims/unix/linux/eventfd.rs
index 8a11f225b22..cede3967bc8 100644
--- a/src/tools/miri/src/shims/unix/linux/eventfd.rs
+++ b/src/tools/miri/src/shims/unix/linux/eventfd.rs
@@ -1,4 +1,5 @@
 //! Linux `eventfd` implementation.
+use std::cell::{Cell, RefCell};
 use std::io;
 use std::io::{Error, ErrorKind};
 use std::mem;
@@ -27,9 +28,9 @@ const MAX_COUNTER: u64 = u64::MAX - 1;
 struct Event {
     /// The object contains an unsigned 64-bit integer (uint64_t) counter that is maintained by the
     /// kernel. This counter is initialized with the value specified in the argument initval.
-    counter: u64,
+    counter: Cell<u64>,
     is_nonblock: bool,
-    clock: VClock,
+    clock: RefCell<VClock>,
 }
 
 impl FileDescription for Event {
@@ -42,8 +43,8 @@ impl FileDescription for Event {
         // need to be supported in the future, the check should be added here.
 
         Ok(EpollReadyEvents {
-            epollin: self.counter != 0,
-            epollout: self.counter != MAX_COUNTER,
+            epollin: self.counter.get() != 0,
+            epollout: self.counter.get() != MAX_COUNTER,
             ..EpollReadyEvents::new()
         })
     }
@@ -58,7 +59,7 @@ impl FileDescription for Event {
 
     /// Read the counter in the buffer and return the counter if succeeded.
     fn read<'tcx>(
-        &mut self,
+        &self,
         _communicate_allowed: bool,
         fd_id: FdId,
         bytes: &mut [u8],
@@ -69,7 +70,8 @@ impl FileDescription for Event {
             return Ok(Err(Error::from(ErrorKind::InvalidInput)));
         };
         // Block when counter == 0.
-        if self.counter == 0 {
+        let counter = self.counter.get();
+        if counter == 0 {
             if self.is_nonblock {
                 return Ok(Err(Error::from(ErrorKind::WouldBlock)));
             } else {
@@ -78,13 +80,13 @@ impl FileDescription for Event {
             }
         } else {
             // Synchronize with all prior `write` calls to this FD.
-            ecx.acquire_clock(&self.clock);
+            ecx.acquire_clock(&self.clock.borrow());
             // Return the counter in the host endianness using the buffer provided by caller.
             *bytes = match ecx.tcx.sess.target.endian {
-                Endian::Little => self.counter.to_le_bytes(),
-                Endian::Big => self.counter.to_be_bytes(),
+                Endian::Little => counter.to_le_bytes(),
+                Endian::Big => counter.to_be_bytes(),
             };
-            self.counter = 0;
+            self.counter.set(0);
             // When any of the event happened, we check and update the status of all supported event
             // types for current file description.
 
@@ -114,7 +116,7 @@ impl FileDescription for Event {
     /// supplied buffer is less than 8 bytes, or if an attempt is
     /// made to write the value 0xffffffffffffffff.
     fn write<'tcx>(
-        &mut self,
+        &self,
         _communicate_allowed: bool,
         fd_id: FdId,
         bytes: &[u8],
@@ -135,13 +137,13 @@ impl FileDescription for Event {
         }
         // If the addition does not let the counter to exceed the maximum value, update the counter.
         // Else, block.
-        match self.counter.checked_add(num) {
+        match self.counter.get().checked_add(num) {
             Some(new_count @ 0..=MAX_COUNTER) => {
                 // Future `read` calls will synchronize with this write, so update the FD clock.
                 if let Some(clock) = &ecx.release_clock() {
-                    self.clock.join(clock);
+                    self.clock.borrow_mut().join(clock);
                 }
-                self.counter = new_count;
+                self.counter.set(new_count);
             }
             None | Some(u64::MAX) => {
                 if self.is_nonblock {
@@ -219,8 +221,11 @@ pub trait EvalContextExt<'tcx>: crate::MiriInterpCxExt<'tcx> {
 
         let fds = &mut this.machine.fds;
 
-        let fd_value =
-            fds.insert_new(Event { counter: val.into(), is_nonblock, clock: VClock::default() });
+        let fd_value = fds.insert_new(Event {
+            counter: Cell::new(val.into()),
+            is_nonblock,
+            clock: RefCell::new(VClock::default()),
+        });
 
         Ok(Scalar::from_i32(fd_value))
     }
diff --git a/src/tools/miri/src/shims/unix/socket.rs b/src/tools/miri/src/shims/unix/socket.rs
index 0f40d9776bb..2694391dfbb 100644
--- a/src/tools/miri/src/shims/unix/socket.rs
+++ b/src/tools/miri/src/shims/unix/socket.rs
@@ -1,8 +1,7 @@
-use std::cell::RefCell;
+use std::cell::{OnceCell, RefCell};
 use std::collections::VecDeque;
 use std::io;
 use std::io::{Error, ErrorKind, Read};
-use std::rc::{Rc, Weak};
 
 use crate::shims::unix::fd::{FdId, WeakFileDescriptionRef};
 use crate::shims::unix::linux::epoll::EpollReadyEvents;
@@ -17,15 +16,12 @@ const MAX_SOCKETPAIR_BUFFER_CAPACITY: usize = 212992;
 /// Pair of connected sockets.
 #[derive(Debug)]
 struct SocketPair {
-    // By making the write link weak, a `write` can detect when all readers are
-    // gone, and trigger EPIPE as appropriate.
-    writebuf: Weak<RefCell<Buffer>>,
-    readbuf: Rc<RefCell<Buffer>>,
-    /// When a socketpair instance is created, two socketpair file descriptions are generated.
-    /// The peer_fd field holds a weak reference to the file description of peer socketpair.
-    // TODO: It might be possible to retrieve writebuf from peer_fd and remove the writebuf
-    // field above.
-    peer_fd: WeakFileDescriptionRef,
+    /// The buffer we are reading from.
+    readbuf: RefCell<Buffer>,
+    /// The `SocketPair` file descriptor that is our "peer", and that holds the buffer we are
+    /// writing to. This is a weak reference because the other side may be closed before us; all
+    /// future writes will then trigger EPIPE.
+    peer_fd: OnceCell<WeakFileDescriptionRef>,
     is_nonblock: bool,
 }
 
@@ -39,6 +35,18 @@ struct Buffer {
     buf_has_writer: bool,
 }
 
+impl Buffer {
+    fn new() -> Self {
+        Buffer { buf: VecDeque::new(), clock: VClock::default(), buf_has_writer: true }
+    }
+}
+
+impl SocketPair {
+    fn peer_fd(&self) -> &WeakFileDescriptionRef {
+        self.peer_fd.get().unwrap()
+    }
+}
+
 impl FileDescription for SocketPair {
     fn name(&self) -> &'static str {
         "socketpair"
@@ -49,29 +57,29 @@ impl FileDescription for SocketPair {
         // need to be supported in the future, the check should be added here.
 
         let mut epoll_ready_events = EpollReadyEvents::new();
-        let readbuf = self.readbuf.borrow();
 
         // Check if it is readable.
+        let readbuf = self.readbuf.borrow();
         if !readbuf.buf.is_empty() {
             epoll_ready_events.epollin = true;
         }
 
         // Check if is writable.
-        if let Some(writebuf) = self.writebuf.upgrade() {
-            let writebuf = writebuf.borrow();
+        if let Some(peer_fd) = self.peer_fd().upgrade() {
+            let writebuf = &peer_fd.downcast::<SocketPair>().unwrap().readbuf.borrow();
             let data_size = writebuf.buf.len();
             let available_space = MAX_SOCKETPAIR_BUFFER_CAPACITY.strict_sub(data_size);
             if available_space != 0 {
                 epoll_ready_events.epollout = true;
             }
-        }
-
-        // Check if the peer_fd closed
-        if self.peer_fd.upgrade().is_none() {
+        } else {
+            // Peer FD has been closed.
             epoll_ready_events.epollrdhup = true;
-            // This is an edge case. Whenever epollrdhup is triggered, epollin will be added
-            // even though there is no data in the buffer.
+            // This is an edge case. Whenever epollrdhup is triggered, epollin and epollout will be
+            // added even though there is no data in the buffer.
+            // FIXME: Figure out why. This looks like a bug.
             epoll_ready_events.epollin = true;
+            epoll_ready_events.epollout = true;
         }
         Ok(epoll_ready_events)
     }
@@ -81,15 +89,13 @@ impl FileDescription for SocketPair {
         _communicate_allowed: bool,
         ecx: &mut MiriInterpCx<'tcx>,
     ) -> InterpResult<'tcx, io::Result<()>> {
-        // This is used to signal socketfd of other side that there is no writer to its readbuf.
-        // If the upgrade fails, there is no need to update as all read ends have been dropped.
-        if let Some(writebuf) = self.writebuf.upgrade() {
-            writebuf.borrow_mut().buf_has_writer = false;
-        };
+        if let Some(peer_fd) = self.peer_fd().upgrade() {
+            // This is used to signal socketfd of other side that there is no writer to its readbuf.
+            // If the upgrade fails, there is no need to update as all read ends have been dropped.
+            peer_fd.downcast::<SocketPair>().unwrap().readbuf.borrow_mut().buf_has_writer = false;
 
-        // Notify peer fd that closed has happened.
-        if let Some(peer_fd) = self.peer_fd.upgrade() {
-            // When any of the event happened, we check and update the status of all supported events
+            // Notify peer fd that closed has happened.
+            // When any of the events happened, we check and update the status of all supported events
             // types of peer fd.
             peer_fd.check_and_update_readiness(ecx)?;
         }
@@ -97,20 +103,20 @@ impl FileDescription for SocketPair {
     }
 
     fn read<'tcx>(
-        &mut self,
+        &self,
         _communicate_allowed: bool,
         _fd_id: FdId,
         bytes: &mut [u8],
         ecx: &mut MiriInterpCx<'tcx>,
     ) -> InterpResult<'tcx, io::Result<usize>> {
         let request_byte_size = bytes.len();
-        let mut readbuf = self.readbuf.borrow_mut();
 
         // Always succeed on read size 0.
         if request_byte_size == 0 {
             return Ok(Ok(0));
         }
 
+        let mut readbuf = self.readbuf.borrow_mut();
         if readbuf.buf.is_empty() {
             if !readbuf.buf_has_writer {
                 // Socketpair with no writer and empty buffer.
@@ -141,8 +147,7 @@ impl FileDescription for SocketPair {
         // Conveniently, `read` exists on `VecDeque` and has exactly the desired behavior.
         let actual_read_size = readbuf.buf.read(bytes).unwrap();
 
-        // The readbuf needs to be explicitly dropped because it will cause panic when
-        // check_and_update_readiness borrows it again.
+        // Need to drop before others can access the readbuf again.
         drop(readbuf);
 
         // A notification should be provided for the peer file description even when it can
@@ -152,7 +157,7 @@ impl FileDescription for SocketPair {
         // don't know what that *certain number* is, we will provide a notification every time
         // a read is successful. This might result in our epoll emulation providing more
         // notifications than the real system.
-        if let Some(peer_fd) = self.peer_fd.upgrade() {
+        if let Some(peer_fd) = self.peer_fd().upgrade() {
             peer_fd.check_and_update_readiness(ecx)?;
         }
 
@@ -160,7 +165,7 @@ impl FileDescription for SocketPair {
     }
 
     fn write<'tcx>(
-        &mut self,
+        &self,
         _communicate_allowed: bool,
         _fd_id: FdId,
         bytes: &[u8],
@@ -173,12 +178,13 @@ impl FileDescription for SocketPair {
             return Ok(Ok(0));
         }
 
-        let Some(writebuf) = self.writebuf.upgrade() else {
+        // We are writing to our peer's readbuf.
+        let Some(peer_fd) = self.peer_fd().upgrade() else {
             // If the upgrade from Weak to Rc fails, it indicates that all read ends have been
             // closed.
             return Ok(Err(Error::from(ErrorKind::BrokenPipe)));
         };
-        let mut writebuf = writebuf.borrow_mut();
+        let mut writebuf = peer_fd.downcast::<SocketPair>().unwrap().readbuf.borrow_mut();
         let data_size = writebuf.buf.len();
         let available_space = MAX_SOCKETPAIR_BUFFER_CAPACITY.strict_sub(data_size);
         if available_space == 0 {
@@ -198,13 +204,12 @@ impl FileDescription for SocketPair {
         let actual_write_size = write_size.min(available_space);
         writebuf.buf.extend(&bytes[..actual_write_size]);
 
-        // The writebuf needs to be explicitly dropped because it will cause panic when
-        // check_and_update_readiness borrows it again.
+        // Need to stop accessing peer_fd so that it can be notified.
         drop(writebuf);
+
         // Notification should be provided for peer fd as it became readable.
-        if let Some(peer_fd) = self.peer_fd.upgrade() {
-            peer_fd.check_and_update_readiness(ecx)?;
-        }
+        peer_fd.check_and_update_readiness(ecx)?;
+
         return Ok(Ok(actual_write_size));
     }
 }
@@ -268,51 +273,30 @@ pub trait EvalContextExt<'tcx>: crate::MiriInterpCxExt<'tcx> {
             );
         }
 
-        let buffer1 = Rc::new(RefCell::new(Buffer {
-            buf: VecDeque::new(),
-            clock: VClock::default(),
-            buf_has_writer: true,
-        }));
-
-        let buffer2 = Rc::new(RefCell::new(Buffer {
-            buf: VecDeque::new(),
-            clock: VClock::default(),
-            buf_has_writer: true,
-        }));
-
-        let socketpair_0 = SocketPair {
-            writebuf: Rc::downgrade(&buffer1),
-            readbuf: Rc::clone(&buffer2),
-            peer_fd: WeakFileDescriptionRef::default(),
+        // Generate file descriptions.
+        let fds = &mut this.machine.fds;
+        let fd0 = fds.new_ref(SocketPair {
+            readbuf: RefCell::new(Buffer::new()),
+            peer_fd: OnceCell::new(),
             is_nonblock: is_sock_nonblock,
-        };
-        let socketpair_1 = SocketPair {
-            writebuf: Rc::downgrade(&buffer2),
-            readbuf: Rc::clone(&buffer1),
-            peer_fd: WeakFileDescriptionRef::default(),
+        });
+        let fd1 = fds.new_ref(SocketPair {
+            readbuf: RefCell::new(Buffer::new()),
+            peer_fd: OnceCell::new(),
             is_nonblock: is_sock_nonblock,
-        };
-
-        // Insert the file description to the fd table.
-        let fds = &mut this.machine.fds;
-        let sv0 = fds.insert_new(socketpair_0);
-        let sv1 = fds.insert_new(socketpair_1);
-
-        // Get weak file descriptor and file description id value.
-        let fd_ref0 = fds.get_ref(sv0).unwrap();
-        let fd_ref1 = fds.get_ref(sv1).unwrap();
-        let weak_fd_ref0 = fd_ref0.downgrade();
-        let weak_fd_ref1 = fd_ref1.downgrade();
+        });
 
-        // Update peer_fd and id field.
-        fd_ref1.borrow_mut().downcast_mut::<SocketPair>().unwrap().peer_fd = weak_fd_ref0;
+        // Make the file descriptions point to each other.
+        fd0.downcast::<SocketPair>().unwrap().peer_fd.set(fd1.downgrade()).unwrap();
+        fd1.downcast::<SocketPair>().unwrap().peer_fd.set(fd0.downgrade()).unwrap();
 
-        fd_ref0.borrow_mut().downcast_mut::<SocketPair>().unwrap().peer_fd = weak_fd_ref1;
+        // Insert the file description to the fd table, generating the file descriptors.
+        let sv0 = fds.insert(fd0);
+        let sv1 = fds.insert(fd1);
 
-        // Return socketpair file description value to the caller.
+        // Return socketpair file descriptors to the caller.
         let sv0 = Scalar::from_int(sv0, sv.layout.size);
         let sv1 = Scalar::from_int(sv1, sv.layout.size);
-
         this.write_scalar(sv0, &sv)?;
         this.write_scalar(sv1, &sv.offset(sv.layout.size, sv.layout, this)?)?;
 
diff --git a/src/tools/miri/tests/pass-dep/libc/libc-epoll.rs b/src/tools/miri/tests/pass-dep/libc/libc-epoll.rs
index 11a0257dc4e..95fdf2f6035 100644
--- a/src/tools/miri/tests/pass-dep/libc/libc-epoll.rs
+++ b/src/tools/miri/tests/pass-dep/libc/libc-epoll.rs
@@ -19,6 +19,7 @@ fn main() {
     test_socketpair_read();
 }
 
+#[track_caller]
 fn check_epoll_wait<const N: usize>(
     epfd: i32,
     mut expected_notifications: Vec<(u32, u64)>,
@@ -28,6 +29,9 @@ fn check_epoll_wait<const N: usize>(
     let maxsize = N;
     let array_ptr = array.as_mut_ptr();
     let res = unsafe { libc::epoll_wait(epfd, array_ptr, maxsize.try_into().unwrap(), 0) };
+    if res < 0 {
+        panic!("epoll_wait failed: {}", std::io::Error::last_os_error());
+    }
     assert_eq!(res, expected_notifications.len().try_into().unwrap());
     let slice = unsafe { std::slice::from_raw_parts(array_ptr, res.try_into().unwrap()) };
     let mut return_events = slice.iter();