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Diffstat (limited to 'vendor/rustix/src/imp/libc/weak.rs')
| -rw-r--r-- | vendor/rustix/src/imp/libc/weak.rs | 226 |
1 files changed, 226 insertions, 0 deletions
diff --git a/vendor/rustix/src/imp/libc/weak.rs b/vendor/rustix/src/imp/libc/weak.rs new file mode 100644 index 000000000..893403b1c --- /dev/null +++ b/vendor/rustix/src/imp/libc/weak.rs @@ -0,0 +1,226 @@ +// Implementation derived from `weak` in Rust's +// library/std/src/sys/unix/weak.rs at revision +// fd0cb0cdc21dd9c06025277d772108f8d42cb25f. + +//! 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: $crate::imp::weak::Weak<unsafe extern fn($($t),*) -> $ret> = + $crate::imp::weak::Weak::new(concat!(stringify!($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().cast()) +} + +#[cfg(not(any(target_os = "android", target_os = "linux")))] +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(any(target_os = "android", target_os = "linux"))] +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. + /* + 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),*) + } + } + ) +} |