vendor/rustix-0.36.5/src/backend/libc/weak.rs - toolchain/rustc - Git at Google

 // 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: $crate::backend::weak::Weak<unsafe extern fn($($t),*) -> $ret> =
             $crate::backend::weak::Weak::new(concat!(stringify!($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),*)
             }
         }
     )
 }
	// 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: $crate::backend::weak::Weak<unsafe extern fn($($t),*) -> $ret> =
	$crate::backend::weak::Weak::new(concat!(stringify!($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),*)
	}
	}
	)
	}