287 lines
11 KiB
Rust
287 lines
11 KiB
Rust
// Implementation derived from `weak` in Rust's
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// library/std/src/sys/unix/weak.rs at revision
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// fd0cb0cdc21dd9c06025277d772108f8d42cb25f.
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//
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// Ideally we should update to a newer version which doesn't need `dlsym`,
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// however that depends on the `extern_weak` feature which is currently
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// unstable.
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#![cfg_attr(linux_raw, allow(unsafe_code))]
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//! Support for "weak linkage" to symbols on Unix
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//!
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//! Some I/O operations we do in libstd require newer versions of OSes but we
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//! need to maintain binary compatibility with older releases for now. In order
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//! to use the new functionality when available we use this module for
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//! detection.
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//!
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//! One option to use here is weak linkage, but that is unfortunately only
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//! really workable on Linux. Hence, use dlsym to get the symbol value at
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//! runtime. This is also done for compatibility with older versions of glibc,
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//! and to avoid creating dependencies on `GLIBC_PRIVATE` symbols. It assumes
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//! that we've been dynamically linked to the library the symbol comes from,
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//! but that is currently always the case for things like libpthread/libc.
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//!
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//! A long time ago this used weak linkage for the `__pthread_get_minstack`
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//! symbol, but that caused Debian to detect an unnecessarily strict versioned
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//! dependency on libc6 (#23628).
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// There are a variety of `#[cfg]`s controlling which targets are involved in
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// each instance of `weak!` and `syscall!`. Rather than trying to unify all of
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// that, we'll just allow that some unix targets don't use this module at all.
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#![allow(dead_code, unused_macros)]
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#![allow(clippy::doc_markdown)]
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use crate::ffi::CStr;
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use core::ffi::c_void;
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use core::ptr::null_mut;
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use core::sync::atomic::{self, AtomicPtr, Ordering};
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use core::{marker, mem};
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const NULL: *mut c_void = null_mut();
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const INVALID: *mut c_void = 1 as *mut c_void;
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macro_rules! weak {
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($vis:vis fn $name:ident($($t:ty),*) -> $ret:ty) => (
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#[allow(non_upper_case_globals)]
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$vis static $name: $crate::weak::Weak<unsafe extern fn($($t),*) -> $ret> =
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$crate::weak::Weak::new(concat!(stringify!($name), '\0'));
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)
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}
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pub(crate) struct Weak<F> {
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name: &'static str,
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addr: AtomicPtr<c_void>,
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_marker: marker::PhantomData<F>,
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}
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impl<F> Weak<F> {
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pub(crate) const fn new(name: &'static str) -> Self {
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Self {
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name,
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addr: AtomicPtr::new(INVALID),
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_marker: marker::PhantomData,
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}
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}
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pub(crate) fn get(&self) -> Option<F> {
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assert_eq!(mem::size_of::<F>(), mem::size_of::<usize>());
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unsafe {
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// Relaxed is fine here because we fence before reading through the
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// pointer (see the comment below).
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match self.addr.load(Ordering::Relaxed) {
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INVALID => self.initialize(),
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NULL => None,
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addr => {
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let func = mem::transmute_copy::<*mut c_void, F>(&addr);
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// The caller is presumably going to read through this value
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// (by calling the function we've dlsymed). This means we'd
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// need to have loaded it with at least C11's consume
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// ordering in order to be guaranteed that the data we read
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// from the pointer isn't from before the pointer was
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// stored. Rust has no equivalent to memory_order_consume,
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// so we use an acquire fence (sorry, ARM).
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//
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// Now, in practice this likely isn't needed even on CPUs
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// where relaxed and consume mean different things. The
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// symbols we're loading are probably present (or not) at
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// init, and even if they aren't the runtime dynamic loader
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// is extremely likely have sufficient barriers internally
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// (possibly implicitly, for example the ones provided by
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// invoking `mprotect`).
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//
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// That said, none of that's *guaranteed*, and so we fence.
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atomic::fence(Ordering::Acquire);
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Some(func)
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}
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}
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}
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}
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// Cold because it should only happen during first-time initialization.
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#[cold]
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unsafe fn initialize(&self) -> Option<F> {
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let val = fetch(self.name);
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// This synchronizes with the acquire fence in `get`.
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self.addr.store(val, Ordering::Release);
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match val {
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NULL => None,
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addr => Some(mem::transmute_copy::<*mut c_void, F>(&addr)),
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}
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}
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}
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// To avoid having the `linux_raw` backend depend on the libc crate, just
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// declare the few things we need in a module called `libc` so that `fetch`
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// uses it.
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#[cfg(linux_raw)]
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mod libc {
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use core::ptr;
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use linux_raw_sys::ctypes::{c_char, c_void};
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#[cfg(all(target_os = "android", target_pointer_width = "32"))]
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pub(super) const RTLD_DEFAULT: *mut c_void = -1isize as *mut c_void;
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#[cfg(not(all(target_os = "android", target_pointer_width = "32")))]
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pub(super) const RTLD_DEFAULT: *mut c_void = ptr::null_mut();
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extern "C" {
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pub(super) fn dlsym(handle: *mut c_void, symbol: *const c_char) -> *mut c_void;
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}
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#[test]
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fn test_abi() {
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assert_eq!(self::RTLD_DEFAULT, ::libc::RTLD_DEFAULT);
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}
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}
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unsafe fn fetch(name: &str) -> *mut c_void {
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let name = match CStr::from_bytes_with_nul(name.as_bytes()) {
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Ok(c_str) => c_str,
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Err(..) => return null_mut(),
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};
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libc::dlsym(libc::RTLD_DEFAULT, name.as_ptr().cast())
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}
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#[cfg(not(linux_kernel))]
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macro_rules! syscall {
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(fn $name:ident($($arg_name:ident: $t:ty),*) via $_sys_name:ident -> $ret:ty) => (
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unsafe fn $name($($arg_name: $t),*) -> $ret {
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weak! { fn $name($($t),*) -> $ret }
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if let Some(fun) = $name.get() {
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fun($($arg_name),*)
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} else {
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libc_errno::set_errno(libc_errno::Errno(libc::ENOSYS));
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-1
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}
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}
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)
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}
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#[cfg(linux_kernel)]
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macro_rules! syscall {
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(fn $name:ident($($arg_name:ident: $t:ty),*) via $sys_name:ident -> $ret:ty) => (
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unsafe fn $name($($arg_name:$t),*) -> $ret {
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// This looks like a hack, but `concat_idents` only accepts idents
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// (not paths).
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use libc::*;
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#[allow(dead_code)]
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trait AsSyscallArg {
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type SyscallArgType;
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fn into_syscall_arg(self) -> Self::SyscallArgType;
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}
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// Pass pointer types as pointers, to preserve provenance.
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impl<T> AsSyscallArg for *mut T {
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type SyscallArgType = *mut T;
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fn into_syscall_arg(self) -> Self::SyscallArgType { self }
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}
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impl<T> AsSyscallArg for *const T {
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type SyscallArgType = *const T;
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fn into_syscall_arg(self) -> Self::SyscallArgType { self }
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}
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// Pass `BorrowedFd` values as the integer value.
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impl AsSyscallArg for $crate::fd::BorrowedFd<'_> {
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type SyscallArgType = ::libc::c_int;
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fn into_syscall_arg(self) -> Self::SyscallArgType {
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$crate::fd::AsRawFd::as_raw_fd(&self) as _
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}
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}
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// Coerce integer values into `c_long`.
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impl AsSyscallArg for i8 {
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type SyscallArgType = ::libc::c_int;
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fn into_syscall_arg(self) -> Self::SyscallArgType { self.into() }
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}
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impl AsSyscallArg for u8 {
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type SyscallArgType = ::libc::c_int;
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fn into_syscall_arg(self) -> Self::SyscallArgType { self.into() }
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}
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impl AsSyscallArg for i16 {
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type SyscallArgType = ::libc::c_int;
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fn into_syscall_arg(self) -> Self::SyscallArgType { self.into() }
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}
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impl AsSyscallArg for u16 {
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type SyscallArgType = ::libc::c_int;
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fn into_syscall_arg(self) -> Self::SyscallArgType { self.into() }
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}
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impl AsSyscallArg for i32 {
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type SyscallArgType = ::libc::c_int;
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fn into_syscall_arg(self) -> Self::SyscallArgType { self }
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}
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impl AsSyscallArg for u32 {
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type SyscallArgType = ::libc::c_uint;
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fn into_syscall_arg(self) -> Self::SyscallArgType { self }
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}
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impl AsSyscallArg for usize {
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type SyscallArgType = ::libc::c_ulong;
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fn into_syscall_arg(self) -> Self::SyscallArgType { self as _ }
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}
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// On 64-bit platforms, also coerce `i64` and `u64` since `c_long`
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// is 64-bit and can hold those values.
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#[cfg(target_pointer_width = "64")]
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impl AsSyscallArg for i64 {
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type SyscallArgType = ::libc::c_long;
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fn into_syscall_arg(self) -> Self::SyscallArgType { self }
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}
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#[cfg(target_pointer_width = "64")]
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impl AsSyscallArg for u64 {
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type SyscallArgType = ::libc::c_ulong;
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fn into_syscall_arg(self) -> Self::SyscallArgType { self }
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}
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// `concat_idents` is [unstable], so we take an extra `sys_name`
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// parameter and have our users do the concat for us for now.
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//
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// [unstable]: https://github.com/rust-lang/rust/issues/29599
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/*
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syscall(
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concat_idents!(SYS_, $name),
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$($arg_name.into_syscall_arg()),*
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) as $ret
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*/
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syscall($sys_name, $($arg_name.into_syscall_arg()),*) as $ret
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}
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)
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}
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macro_rules! weakcall {
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($vis:vis fn $name:ident($($arg_name:ident: $t:ty),*) -> $ret:ty) => (
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$vis unsafe fn $name($($arg_name: $t),*) -> $ret {
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weak! { fn $name($($t),*) -> $ret }
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// Use a weak symbol from libc when possible, allowing `LD_PRELOAD`
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// interposition, but if it's not found just fail.
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if let Some(fun) = $name.get() {
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fun($($arg_name),*)
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} else {
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libc_errno::set_errno(libc_errno::Errno(libc::ENOSYS));
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-1
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}
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}
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)
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}
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/// A combination of `weakcall` and `syscall`. Use the libc function if it's
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/// available, and fall back to `libc::syscall` otherwise.
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macro_rules! weak_or_syscall {
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($vis:vis fn $name:ident($($arg_name:ident: $t:ty),*) via $sys_name:ident -> $ret:ty) => (
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$vis unsafe fn $name($($arg_name: $t),*) -> $ret {
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weak! { fn $name($($t),*) -> $ret }
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// Use a weak symbol from libc when possible, allowing `LD_PRELOAD`
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// interposition, but if it's not found just fail.
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if let Some(fun) = $name.get() {
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fun($($arg_name),*)
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} else {
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syscall! { fn $name($($arg_name: $t),*) via $sys_name -> $ret }
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$name($($arg_name),*)
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}
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}
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)
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}
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