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// Implementation derived from `weak` in Rust's
// library/std/src/sys/unix/weak.rs at revision
// fd0cb0cdc21dd9c06025277d772108f8d42cb25f.
//
// Ideally we should update to a newer version which doesn't need `dlsym`,
// however that depends on the `extern_weak` feature which is currrently
// unstable.
#![cfg_attr(linux_raw, allow(unsafe_code))]
//! Support for "weak linkage" to symbols on Unix
//!
//! Some I/O operations we do in libstd 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 on Linux. Hence, 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).
// 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)]
#![allow(clippy::doc_markdown)]
use crate::ffi::CStr;
use core::ffi::c_void;
use core::ptr::null_mut;
use core::sync::atomic::{self, AtomicPtr, Ordering};
use core::{marker, mem};
const NULL: *mut c_void = null_mut();
const INVALID: *mut c_void = 1 as *mut c_void;
macro_rules! weak {
($vis:vis fn $name:ident($($t:ty),*) -> $ret:ty) => (
#[allow(non_upper_case_globals)]
$vis static $name: $crate::weak::Weak<unsafe extern fn($($t),*) -> $ret> =
$crate::weak::Weak::new(concat!(stringify!($name), '\0'));
)
}
pub(crate) struct Weak<F> {
name: &'static str,
addr: AtomicPtr<c_void>,
_marker: marker::PhantomData<F>,
}
impl<F> Weak<F> {
pub(crate) const fn new(name: &'static str) -> Self {
Self {
name,
addr: AtomicPtr::new(INVALID),
_marker: marker::PhantomData,
}
}
pub(crate) fn get(&self) -> Option<F> {
assert_eq!(mem::size_of::<F>(), mem::size_of::<usize>());
unsafe {
// Relaxed is fine here because we fence before reading through the
// pointer (see the comment below).
match self.addr.load(Ordering::Relaxed) {
INVALID => self.initialize(),
NULL => None,
addr => {
let func = mem::transmute_copy::<*mut c_void, F>(&addr);
// 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> {
let val = fetch(self.name);
// This synchronizes with the acquire fence in `get`.
self.addr.store(val, Ordering::Release);
match val {
NULL => None,
addr => Some(mem::transmute_copy::<*mut c_void, F>(&addr)),
}
}
}
unsafe fn fetch(name: &str) -> *mut c_void {
let name = match CStr::from_bytes_with_nul(name.as_bytes()) {
Ok(c_str) => c_str,
Err(..) => return null_mut(),
};
libc::dlsym(libc::RTLD_DEFAULT, name.as_ptr())
}
#[cfg(not(linux_kernel))]
macro_rules! syscall {
(fn $name:ident($($arg_name:ident: $t:ty),*) via $_sys_name:ident -> $ret:ty) => (
unsafe fn $name($($arg_name: $t),*) -> $ret {
weak! { fn $name($($t),*) -> $ret }
if let Some(fun) = $name.get() {
fun($($arg_name),*)
} else {
libc_errno::set_errno(libc_errno::Errno(libc::ENOSYS));
-1
}
}
)
}
#[cfg(linux_kernel)]
macro_rules! syscall {
(fn $name:ident($($arg_name:ident: $t:ty),*) via $sys_name:ident -> $ret:ty) => (
unsafe fn $name($($arg_name:$t),*) -> $ret {
// This looks like a hack, but `concat_idents` only accepts idents
// (not paths).
use libc::*;
trait AsSyscallArg {
type SyscallArgType;
fn into_syscall_arg(self) -> Self::SyscallArgType;
}
// Pass pointer types as pointers, to preserve provenance.
impl<T> AsSyscallArg for *mut T {
type SyscallArgType = *mut T;
fn into_syscall_arg(self) -> Self::SyscallArgType { self }
}
impl<T> AsSyscallArg for *const T {
type SyscallArgType = *const T;
fn into_syscall_arg(self) -> Self::SyscallArgType { self }
}
// Pass `BorrowedFd` values as the integer value.
impl AsSyscallArg for $crate::fd::BorrowedFd<'_> {
type SyscallArgType = c::c_long;
fn into_syscall_arg(self) -> Self::SyscallArgType {
$crate::fd::AsRawFd::as_raw_fd(&self) as _
}
}
// Coerce integer values into `c_long`.
impl AsSyscallArg for i32 {
type SyscallArgType = c::c_long;
fn into_syscall_arg(self) -> Self::SyscallArgType { self as _ }
}
impl AsSyscallArg for u32 {
type SyscallArgType = c::c_long;
fn into_syscall_arg(self) -> Self::SyscallArgType { self as _ }
}
impl AsSyscallArg for usize {
type SyscallArgType = c::c_long;
fn into_syscall_arg(self) -> Self::SyscallArgType { self as _ }
}
// `concat_idents` is [unstable], so we take an extra `sys_name`
// parameter and have our users do the concat for us for now.
//
// [unstable]: https://github.com/rust-lang/rust/issues/29599
/*
syscall(
concat_idents!(SYS_, $name),
$($arg_name.into_syscall_arg()),*
) as $ret
*/
syscall($sys_name, $($arg_name.into_syscall_arg()),*) as $ret
}
)
}
macro_rules! weakcall {
($vis:vis fn $name:ident($($arg_name:ident: $t:ty),*) -> $ret:ty) => (
$vis 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 fail.
if let Some(fun) = $name.get() {
fun($($arg_name),*)
} else {
libc_errno::set_errno(libc_errno::Errno(libc::ENOSYS));
-1
}
}
)
}
/// A combination of `weakcall` and `syscall`. Use the libc function if it's
/// available, and fall back to `libc::syscall` otherwise.
macro_rules! weak_or_syscall {
($vis:vis fn $name:ident($($arg_name:ident: $t:ty),*) via $sys_name:ident -> $ret:ty) => (
$vis 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 fail.
if let Some(fun) = $name.get() {
fun($($arg_name),*)
} else {
syscall! { fn $name($($arg_name: $t),*) via $sys_name -> $ret }
$name($($arg_name),*)
}
}
)
}
|
