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 $name:ident($($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 $name:ident($($t:ty),*) -> $ret:ty) => (
66 dlsym!(fn $name($($t),*) -> $ret, stringify!($name));
67 ),
68 (fn $name:ident($($t:ty),*) -> $ret:ty, $sym:expr) => (
69 static DLSYM: DlsymWeak<unsafe extern "C" fn($($t),*) -> $ret> =
70 DlsymWeak::new(concat!($sym, '\0'));
71 let $name = &DLSYM;
72 )
73}
74pub(crate) struct DlsymWeak<F> {
75 name: &'static str,
76 func: AtomicPtr<libc::c_void>,
77 _marker: PhantomData<F>,
78}
79
80impl<F> DlsymWeak<F> {
81 pub(crate) const fn new(name: &'static str) -> Self {
82 DlsymWeak {
83 name,
84 func: AtomicPtr::new(ptr::without_provenance_mut(1)),
85 _marker: PhantomData,
86 }
87 }
88
89 #[inline]
90 pub(crate) fn get(&self) -> Option<F> {
91 unsafe {
92 // Relaxed is fine here because we fence before reading through the
93 // pointer (see the comment below).
94 match self.func.load(Ordering::Relaxed) {
95 func if func.addr() == 1 => self.initialize(),
96 func if func.is_null() => None,
97 func => {
98 let func = mem::transmute_copy::<*mut libc::c_void, F>(&func);
99 // The caller is presumably going to read through this value
100 // (by calling the function we've dlsymed). This means we'd
101 // need to have loaded it with at least C11's consume
102 // ordering in order to be guaranteed that the data we read
103 // from the pointer isn't from before the pointer was
104 // stored. Rust has no equivalent to memory_order_consume,
105 // so we use an acquire fence (sorry, ARM).
106 //
107 // Now, in practice this likely isn't needed even on CPUs
108 // where relaxed and consume mean different things. The
109 // symbols we're loading are probably present (or not) at
110 // init, and even if they aren't the runtime dynamic loader
111 // is extremely likely have sufficient barriers internally
112 // (possibly implicitly, for example the ones provided by
113 // invoking `mprotect`).
114 //
115 // That said, none of that's *guaranteed*, and so we fence.
116 atomic::fence(Ordering::Acquire);
117 Some(func)
118 }
119 }
120 }
121 }
122
123 // Cold because it should only happen during first-time initialization.
124 #[cold]
125 unsafe fn initialize(&self) -> Option<F> {
126 assert_eq!(size_of::<F>(), size_of::<*mut libc::c_void>());
127
128 let val = fetch(self.name);
129 // This synchronizes with the acquire fence in `get`.
130 self.func.store(val, Ordering::Release);
131
132 if val.is_null() { None } else { Some(mem::transmute_copy::<*mut libc::c_void, F>(&val)) }
133 }
134}
135
136unsafe fn fetch(name: &str) -> *mut libc::c_void {
137 let name: &CStr = match CStr::from_bytes_with_nul(name.as_bytes()) {
138 Ok(cstr: &CStr) => cstr,
139 Err(..) => return ptr::null_mut(),
140 };
141 libc::dlsym(handle:libc::RTLD_DEFAULT, symbol:name.as_ptr())
142}
143
144#[cfg(not(any(target_os = "linux", target_os = "android")))]
145pub(crate) macro syscall {
146 (fn $name:ident($($arg_name:ident: $t:ty),*) -> $ret:ty) => (
147 // FIXME(#115199): Rust currently omits weak function definitions
148 // and its metadata from LLVM IR.
149 #[no_sanitize(cfi)]
150 unsafe fn $name($($arg_name: $t),*) -> $ret {
151 weak! { fn $name($($t),*) -> $ret }
152
153 if let Some(fun) = $name.get() {
154 fun($($arg_name),*)
155 } else {
156 super::os::set_errno(libc::ENOSYS);
157 -1
158 }
159 }
160 )
161}
162
163#[cfg(any(target_os = "linux", target_os = "android"))]
164pub(crate) macro syscall {
165 (fn $name:ident($($arg_name:ident: $t:ty),*) -> $ret:ty) => (
166 unsafe fn $name($($arg_name:$t),*) -> $ret {
167 weak! { fn $name($($t),*) -> $ret }
168
169 // Use a weak symbol from libc when possible, allowing `LD_PRELOAD`
170 // interposition, but if it's not found just use a raw syscall.
171 if let Some(fun) = $name.get() {
172 fun($($arg_name),*)
173 } else {
174 libc::syscall(libc::${concat(SYS_, $name)}, $($arg_name),*) as $ret
175 }
176 }
177 )
178}
179
180#[cfg(any(target_os = "linux", target_os = "android"))]
181pub(crate) macro raw_syscall {
182 (fn $name:ident($($arg_name:ident: $t:ty),*) -> $ret:ty) => (
183 unsafe fn $name($($arg_name:$t),*) -> $ret {
184 libc::syscall(libc::${concat(SYS_, $name)}, $($arg_name),*) as $ret
185 }
186 )
187}
188

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