1 //! Support for "weak linkage" to symbols on Unix
3 //! Some I/O operations we do in libstd require newer versions of OSes but we
4 //! need to maintain binary compatibility with older releases for now. In order
5 //! to use the new functionality when available we use this module for
8 //! One option to use here is weak linkage, but that is unfortunately only
9 //! really workable with ELF. Otherwise, use dlsym to get the symbol value at
10 //! runtime. This is also done for compatibility with older versions of glibc,
11 //! and to avoid creating dependencies on GLIBC_PRIVATE symbols. It assumes that
12 //! we've been dynamically linked to the library the symbol comes from, but that
13 //! is currently always the case for things like libpthread/libc.
15 //! A long time ago this used weak linkage for the __pthread_get_minstack
16 //! symbol, but that caused Debian to detect an unnecessarily strict versioned
17 //! dependency on libc6 (#23628) because it is GLIBC_PRIVATE. We now use `dlsym`
18 //! for a runtime lookup of that symbol to avoid the ELF versioned dependency.
20 // There are a variety of `#[cfg]`s controlling which targets are involved in
21 // each instance of `weak!` and `syscall!`. Rather than trying to unify all of
22 // that, we'll just allow that some unix targets don't use this module at all.
23 #![allow(dead_code, unused_macros)]
26 use crate::marker::PhantomData;
28 use crate::sync::atomic::{self, AtomicUsize, Ordering};
30 // We can use true weak linkage on ELF targets.
31 #[cfg(not(any(target_os = "macos", target_os = "ios")))]
32 pub(crate) macro weak {
33 (fn $name:ident($($t:ty),*) -> $ret:ty) => (
34 let ref $name: ExternWeak<unsafe extern "C" fn($($t),*) -> $ret> = {
36 #[linkage = "extern_weak"]
37 static $name: *const libc::c_void;
39 #[allow(unused_unsafe)]
40 ExternWeak::new(unsafe { $name })
45 // On non-ELF targets, use the dlsym approximation of weak linkage.
46 #[cfg(any(target_os = "macos", target_os = "ios"))]
47 pub(crate) use self::dlsym as weak;
49 pub(crate) struct ExternWeak<F> {
50 weak_ptr: *const libc::c_void,
51 _marker: PhantomData<F>,
54 impl<F> ExternWeak<F> {
56 pub(crate) fn new(weak_ptr: *const libc::c_void) -> Self {
57 ExternWeak { weak_ptr, _marker: PhantomData }
61 impl<F> ExternWeak<F> {
63 pub(crate) fn get(&self) -> Option<F> {
65 if self.weak_ptr.is_null() {
68 Some(mem::transmute_copy::<*const libc::c_void, F>(&self.weak_ptr))
74 pub(crate) macro dlsym {
75 (fn $name:ident($($t:ty),*) -> $ret:ty) => (
76 dlsym!(fn $name($($t),*) -> $ret, stringify!($name));
78 (fn $name:ident($($t:ty),*) -> $ret:ty, $sym:expr) => (
79 static DLSYM: DlsymWeak<unsafe extern "C" fn($($t),*) -> $ret> =
80 DlsymWeak::new(concat!($sym, '\0'));
84 pub(crate) struct DlsymWeak<F> {
87 _marker: PhantomData<F>,
90 impl<F> DlsymWeak<F> {
91 pub(crate) const fn new(name: &'static str) -> Self {
92 DlsymWeak { name, addr: AtomicUsize::new(1), _marker: PhantomData }
96 pub(crate) fn get(&self) -> Option<F> {
98 // Relaxed is fine here because we fence before reading through the
99 // pointer (see the comment below).
100 match self.addr.load(Ordering::Relaxed) {
101 1 => self.initialize(),
104 let func = mem::transmute_copy::<usize, F>(&addr);
105 // The caller is presumably going to read through this value
106 // (by calling the function we've dlsymed). This means we'd
107 // need to have loaded it with at least C11's consume
108 // ordering in order to be guaranteed that the data we read
109 // from the pointer isn't from before the pointer was
110 // stored. Rust has no equivalent to memory_order_consume,
111 // so we use an acquire fence (sorry, ARM).
113 // Now, in practice this likely isn't needed even on CPUs
114 // where relaxed and consume mean different things. The
115 // symbols we're loading are probably present (or not) at
116 // init, and even if they aren't the runtime dynamic loader
117 // is extremely likely have sufficient barriers internally
118 // (possibly implicitly, for example the ones provided by
119 // invoking `mprotect`).
121 // That said, none of that's *guaranteed*, and so we fence.
122 atomic::fence(Ordering::Acquire);
129 // Cold because it should only happen during first-time initialization.
131 unsafe fn initialize(&self) -> Option<F> {
132 assert_eq!(mem::size_of::<F>(), mem::size_of::<usize>());
134 let val = fetch(self.name);
135 // This synchronizes with the acquire fence in `get`.
136 self.addr.store(val, Ordering::Release);
140 addr => Some(mem::transmute_copy::<usize, F>(&addr)),
145 unsafe fn fetch(name: &str) -> usize {
146 let name = match CStr::from_bytes_with_nul(name.as_bytes()) {
150 libc::dlsym(libc::RTLD_DEFAULT, name.as_ptr()) as usize
153 #[cfg(not(any(target_os = "linux", target_os = "android")))]
154 pub(crate) macro syscall {
155 (fn $name:ident($($arg_name:ident: $t:ty),*) -> $ret:ty) => (
156 unsafe fn $name($($arg_name: $t),*) -> $ret {
157 weak! { fn $name($($t),*) -> $ret }
159 if let Some(fun) = $name.get() {
162 super::os::set_errno(libc::ENOSYS);
169 #[cfg(any(target_os = "linux", target_os = "android"))]
170 pub(crate) macro syscall {
171 (fn $name:ident($($arg_name:ident: $t:ty),*) -> $ret:ty) => (
172 unsafe fn $name($($arg_name:$t),*) -> $ret {
173 weak! { fn $name($($t),*) -> $ret }
175 // Use a weak symbol from libc when possible, allowing `LD_PRELOAD`
176 // interposition, but if it's not found just use a raw syscall.
177 if let Some(fun) = $name.get() {
180 // This looks like a hack, but concat_idents only accepts idents
185 concat_idents!(SYS_, $name),
193 #[cfg(any(target_os = "linux", target_os = "android"))]
194 pub(crate) macro raw_syscall {
195 (fn $name:ident($($arg_name:ident: $t:ty),*) -> $ret:ty) => (
196 unsafe fn $name($($arg_name:$t),*) -> $ret {
197 // This looks like a hack, but concat_idents only accepts idents
202 concat_idents!(SYS_, $name),