std/sys/pal/unix/weak.rs
1//! Support for "weak linkage" to symbols on Unix
2//!
3//! Some I/O operations we do in std require newer versions of OSes but we need
4//! to maintain binary compatibility with older releases for now. In order to
5//! use the new functionality when available we use this module for detection.
6//!
7//! One option to use here is weak linkage, but that is unfortunately only
8//! really workable with ELF. Otherwise, use dlsym to get the symbol value at
9//! runtime. This is also done for compatibility with older versions of glibc,
10//! and to avoid creating dependencies on GLIBC_PRIVATE symbols. It assumes that
11//! we've been dynamically linked to the library the symbol comes from, but that
12//! is currently always the case for things like libpthread/libc.
13//!
14//! A long time ago this used weak linkage for the __pthread_get_minstack
15//! symbol, but that caused Debian to detect an unnecessarily strict versioned
16//! dependency on libc6 (#23628) because it is GLIBC_PRIVATE. We now use `dlsym`
17//! for a runtime lookup of that symbol to avoid the ELF versioned dependency.
18
19// There are a variety of `#[cfg]`s controlling which targets are involved in
20// each instance of `weak!` and `syscall!`. Rather than trying to unify all of
21// that, we'll just allow that some unix targets don't use this module at all.
22#![allow(dead_code, unused_macros)]
23
24use crate::ffi::CStr;
25use crate::marker::PhantomData;
26use crate::sync::atomic::{self, AtomicPtr, Ordering};
27use crate::{mem, ptr};
28
29// We can use true weak linkage on ELF targets.
30#[cfg(all(unix, not(target_vendor = "apple")))]
31pub(crate) macro weak {
32 (fn $name:ident($($param:ident : $t:ty),* $(,)?) -> $ret:ty;) => (
33 let ref $name: ExternWeak<unsafe extern "C" fn($($t),*) -> $ret> = {
34 unsafe extern "C" {
35 #[linkage = "extern_weak"]
36 static $name: Option<unsafe extern "C" fn($($t),*) -> $ret>;
37 }
38 #[allow(unused_unsafe)]
39 ExternWeak::new(unsafe { $name })
40 };
41 )
42}
43
44// On non-ELF targets, use the dlsym approximation of weak linkage.
45#[cfg(target_vendor = "apple")]
46pub(crate) use self::dlsym as weak;
47
48pub(crate) struct ExternWeak<F: Copy> {
49 weak_ptr: Option<F>,
50}
51
52impl<F: Copy> ExternWeak<F> {
53 #[inline]
54 pub(crate) fn new(weak_ptr: Option<F>) -> Self {
55 ExternWeak { weak_ptr }
56 }
57
58 #[inline]
59 pub(crate) fn get(&self) -> Option<F> {
60 self.weak_ptr
61 }
62}
63
64pub(crate) macro dlsym {
65 (fn $name:ident($($param:ident : $t:ty),* $(,)?) -> $ret:ty;) => (
66 dlsym!(
67 #[link_name = stringify!($name)]
68 fn $name($($param : $t),*) -> $ret;
69 );
70 ),
71 (
72 #[link_name = $sym:expr]
73 fn $name:ident($($param:ident : $t:ty),* $(,)?) -> $ret:ty;
74 ) => (
75 static DLSYM: DlsymWeak<unsafe extern "C" fn($($t),*) -> $ret> =
76 DlsymWeak::new(concat!($sym, '\0'));
77 let $name = &DLSYM;
78 )
79}
80pub(crate) struct DlsymWeak<F> {
81 name: &'static str,
82 func: AtomicPtr<libc::c_void>,
83 _marker: PhantomData<F>,
84}
85
86impl<F> DlsymWeak<F> {
87 pub(crate) const fn new(name: &'static str) -> Self {
88 DlsymWeak {
89 name,
90 func: AtomicPtr::new(ptr::without_provenance_mut(1)),
91 _marker: PhantomData,
92 }
93 }
94
95 #[inline]
96 pub(crate) fn get(&self) -> Option<F> {
97 unsafe {
98 // Relaxed is fine here because we fence before reading through the
99 // pointer (see the comment below).
100 match self.func.load(Ordering::Relaxed) {
101 func if func.addr() == 1 => self.initialize(),
102 func if func.is_null() => None,
103 func => {
104 let func = mem::transmute_copy::<*mut libc::c_void, F>(&func);
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).
112 //
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`).
120 //
121 // That said, none of that's *guaranteed*, and so we fence.
122 atomic::fence(Ordering::Acquire);
123 Some(func)
124 }
125 }
126 }
127 }
128
129 // Cold because it should only happen during first-time initialization.
130 #[cold]
131 unsafe fn initialize(&self) -> Option<F> {
132 assert_eq!(size_of::<F>(), size_of::<*mut libc::c_void>());
133
134 let val = fetch(self.name);
135 // This synchronizes with the acquire fence in `get`.
136 self.func.store(val, Ordering::Release);
137
138 if val.is_null() { None } else { Some(mem::transmute_copy::<*mut libc::c_void, F>(&val)) }
139 }
140}
141
142unsafe fn fetch(name: &str) -> *mut libc::c_void {
143 let name = match CStr::from_bytes_with_nul(name.as_bytes()) {
144 Ok(cstr) => cstr,
145 Err(..) => return ptr::null_mut(),
146 };
147 libc::dlsym(libc::RTLD_DEFAULT, name.as_ptr())
148}
149
150#[cfg(not(any(target_os = "linux", target_os = "android")))]
151pub(crate) macro syscall {
152 (fn $name:ident($($param:ident : $t:ty),* $(,)?) -> $ret:ty;) => (
153 // FIXME(#115199): Rust currently omits weak function definitions
154 // and its metadata from LLVM IR.
155 #[no_sanitize(cfi)]
156 unsafe fn $name($($param: $t),*) -> $ret {
157 weak!(fn $name($($param: $t),*) -> $ret;);
158
159 if let Some(fun) = $name.get() {
160 fun($($param),*)
161 } else {
162 super::os::set_errno(libc::ENOSYS);
163 -1
164 }
165 }
166 )
167}
168
169#[cfg(any(target_os = "linux", target_os = "android"))]
170pub(crate) macro syscall {
171 (
172 fn $name:ident($($param:ident : $t:ty),* $(,)?) -> $ret:ty;
173 ) => (
174 unsafe fn $name($($param: $t),*) -> $ret {
175 weak!(fn $name($($param: $t),*) -> $ret;);
176
177 // Use a weak symbol from libc when possible, allowing `LD_PRELOAD`
178 // interposition, but if it's not found just use a raw syscall.
179 if let Some(fun) = $name.get() {
180 fun($($param),*)
181 } else {
182 libc::syscall(libc::${concat(SYS_, $name)}, $($param),*) as $ret
183 }
184 }
185 )
186}
187
188#[cfg(any(target_os = "linux", target_os = "android"))]
189pub(crate) macro raw_syscall {
190 (fn $name:ident($($param:ident : $t:ty),* $(,)?) -> $ret:ty;) => (
191 unsafe fn $name($($param: $t),*) -> $ret {
192 libc::syscall(libc::${concat(SYS_, $name)}, $($param),*) as $ret
193 }
194 )
195}