1 files changed, 195 insertions, 0 deletions
diff --git a/library/std/src/sys/pal/unix/weak.rs b/library/std/src/sys/pal/unix/weak.rs
new file mode 100644
index 00000000000..61088ff16ed
--- /dev/null
+++ b/library/std/src/sys/pal/unix/weak.rs
@@ -0,0 +1,195 @@
+//! Support for "weak linkage" to symbols on Unix
+//!
+//! Some I/O operations we do in std require newer versions of OSes but we need
+//! to maintain binary compatibility with older releases for now. In order to
+//! use the new functionality when available we use this module for detection.
+//!
+//! One option to use here is weak linkage, but that is unfortunately only
+//! really workable with ELF. Otherwise, use dlsym to get the symbol value at
+//! runtime. This is also done for compatibility with older versions of glibc,
+//! and to avoid creating dependencies on GLIBC_PRIVATE symbols. It assumes that
+//! we've been dynamically linked to the library the symbol comes from, but that
+//! is currently always the case for things like libpthread/libc.
+//!
+//! A long time ago this used weak linkage for the __pthread_get_minstack
+//! symbol, but that caused Debian to detect an unnecessarily strict versioned
+//! dependency on libc6 (#23628) because it is GLIBC_PRIVATE. We now use `dlsym`
+//! for a runtime lookup of that symbol to avoid the ELF versioned dependency.
+
+// There are a variety of `#[cfg]`s controlling which targets are involved in
+// each instance of `weak!` and `syscall!`. Rather than trying to unify all of
+// that, we'll just allow that some unix targets don't use this module at all.
+#![allow(dead_code, unused_macros)]
+
+use crate::ffi::CStr;
+use crate::marker::PhantomData;
+use crate::mem;
+use crate::ptr;
+use crate::sync::atomic::{self, AtomicPtr, Ordering};
+
+// We can use true weak linkage on ELF targets.
+#[cfg(not(any(target_os = "macos", target_os = "ios", target_os = "tvos")))]
+pub(crate) macro weak {
+    (fn $name:ident($($t:ty),*) -> $ret:ty) => (
+        let ref $name: ExternWeak<unsafe extern "C" fn($($t),*) -> $ret> = {
+            extern "C" {
+                #[linkage = "extern_weak"]
+                static $name: Option<unsafe extern "C" fn($($t),*) -> $ret>;
+            }
+            #[allow(unused_unsafe)]
+            ExternWeak::new(unsafe { $name })
+        };
+    )
+}
+
+// On non-ELF targets, use the dlsym approximation of weak linkage.
+#[cfg(any(target_os = "macos", target_os = "ios", target_os = "tvos"))]
+pub(crate) use self::dlsym as weak;
+
+pub(crate) struct ExternWeak<F: Copy> {
+    weak_ptr: Option<F>,
+}
+
+impl<F: Copy> ExternWeak<F> {
+    #[inline]
+    pub(crate) fn new(weak_ptr: Option<F>) -> Self {
+        ExternWeak { weak_ptr }
+    }
+
+    #[inline]
+    pub(crate) fn get(&self) -> Option<F> {
+        self.weak_ptr
+    }
+}
+
+pub(crate) macro dlsym {
+    (fn $name:ident($($t:ty),*) -> $ret:ty) => (
+         dlsym!(fn $name($($t),*) -> $ret, stringify!($name));
+    ),
+    (fn $name:ident($($t:ty),*) -> $ret:ty, $sym:expr) => (
+        static DLSYM: DlsymWeak<unsafe extern "C" fn($($t),*) -> $ret> =
+            DlsymWeak::new(concat!($sym, '\0'));
+        let $name = &DLSYM;
+    )
+}
+pub(crate) struct DlsymWeak<F> {
+    name: &'static str,
+    func: AtomicPtr<libc::c_void>,
+    _marker: PhantomData<F>,
+}
+
+impl<F> DlsymWeak<F> {
+    pub(crate) const fn new(name: &'static str) -> Self {
+        DlsymWeak { name, func: AtomicPtr::new(ptr::invalid_mut(1)), _marker: PhantomData }
+    }
+
+    #[inline]
+    pub(crate) fn get(&self) -> Option<F> {
+        unsafe {
+            // Relaxed is fine here because we fence before reading through the
+            // pointer (see the comment below).
+            match self.func.load(Ordering::Relaxed) {
+                func if func.addr() == 1 => self.initialize(),
+                func if func.is_null() => None,
+                func => {
+                    let func = mem::transmute_copy::<*mut libc::c_void, F>(&func);
+                    // The caller is presumably going to read through this value
+                    // (by calling the function we've dlsymed). This means we'd
+                    // need to have loaded it with at least C11's consume
+                    // ordering in order to be guaranteed that the data we read
+                    // from the pointer isn't from before the pointer was
+                    // stored. Rust has no equivalent to memory_order_consume,
+                    // so we use an acquire fence (sorry, ARM).
+                    //
+                    // Now, in practice this likely isn't needed even on CPUs
+                    // where relaxed and consume mean different things. The
+                    // symbols we're loading are probably present (or not) at
+                    // init, and even if they aren't the runtime dynamic loader
+                    // is extremely likely have sufficient barriers internally
+                    // (possibly implicitly, for example the ones provided by
+                    // invoking `mprotect`).
+                    //
+                    // That said, none of that's *guaranteed*, and so we fence.
+                    atomic::fence(Ordering::Acquire);
+                    Some(func)
+                }
+            }
+        }
+    }
+
+    // Cold because it should only happen during first-time initialization.
+    #[cold]
+    unsafe fn initialize(&self) -> Option<F> {
+        assert_eq!(mem::size_of::<F>(), mem::size_of::<*mut libc::c_void>());
+
+        let val = fetch(self.name);
+        // This synchronizes with the acquire fence in `get`.
+        self.func.store(val, Ordering::Release);
+
+        if val.is_null() { None } else { Some(mem::transmute_copy::<*mut libc::c_void, F>(&val)) }
+    }
+}
+
+unsafe fn fetch(name: &str) -> *mut libc::c_void {
+    let name = match CStr::from_bytes_with_nul(name.as_bytes()) {
+        Ok(cstr) => cstr,
+        Err(..) => return ptr::null_mut(),
+    };
+    libc::dlsym(libc::RTLD_DEFAULT, name.as_ptr())
+}
+
+#[cfg(not(any(target_os = "linux", target_os = "android")))]
+pub(crate) macro syscall {
+    (fn $name:ident($($arg_name:ident: $t:ty),*) -> $ret:ty) => (
+        unsafe fn $name($($arg_name: $t),*) -> $ret {
+            weak! { fn $name($($t),*) -> $ret }
+
+            if let Some(fun) = $name.get() {
+                fun($($arg_name),*)
+            } else {
+                super::os::set_errno(libc::ENOSYS);
+                -1
+            }
+        }
+    )
+}
+
+#[cfg(any(target_os = "linux", target_os = "android"))]
+pub(crate) macro syscall {
+    (fn $name:ident($($arg_name:ident: $t:ty),*) -> $ret:ty) => (
+        unsafe fn $name($($arg_name:$t),*) -> $ret {
+            weak! { fn $name($($t),*) -> $ret }
+
+            // Use a weak symbol from libc when possible, allowing `LD_PRELOAD`
+            // interposition, but if it's not found just use a raw syscall.
+            if let Some(fun) = $name.get() {
+                fun($($arg_name),*)
+            } else {
+                // This looks like a hack, but concat_idents only accepts idents
+                // (not paths).
+                use libc::*;
+
+                syscall(
+                    concat_idents!(SYS_, $name),
+                    $($arg_name),*
+                ) as $ret
+            }
+        }
+    )
+}
+
+#[cfg(any(target_os = "linux", target_os = "android"))]
+pub(crate) macro raw_syscall {
+    (fn $name:ident($($arg_name:ident: $t:ty),*) -> $ret:ty) => (
+        unsafe fn $name($($arg_name:$t),*) -> $ret {
+            // This looks like a hack, but concat_idents only accepts idents
+            // (not paths).
+            use libc::*;
+
+            syscall(
+                concat_idents!(SYS_, $name),
+                $($arg_name),*
+            ) as $ret
+        }
+    )
+}