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::mem;
27use crate::ptr;
28use crate::sync::atomic::{self, AtomicPtr, Ordering};
29
30// We can use true weak linkage on ELF targets.
31#[cfg(not(any(target_os = "macos", target_os = "ios", target_os = "tvos")))]
32pub(crate) macro weak {
33 (fn $name:ident($($t:ty),*) -> $ret:ty) => (
34 let ref $name: ExternWeak<unsafe extern "C" fn($($t),*) -> $ret> = {
35 extern "C" {
36 #[linkage = "extern_weak"]
37 static $name: Option<unsafe extern "C" fn($($t),*) -> $ret>;
38 }
39 #[allow(unused_unsafe)]
40 ExternWeak::new(unsafe { $name })
41 };
42 )
43}
44
45// On non-ELF targets, use the dlsym approximation of weak linkage.
46#[cfg(any(target_os = "macos", target_os = "ios", target_os = "tvos"))]
47pub(crate) use self::dlsym as weak;
48
49pub(crate) struct ExternWeak<F: Copy> {
50 weak_ptr: Option<F>,
51}
52
53impl<F: Copy> ExternWeak<F> {
54 #[inline]
55 pub(crate) fn new(weak_ptr: Option<F>) -> Self {
56 ExternWeak { weak_ptr }
57 }
58
59 #[inline]
60 pub(crate) fn get(&self) -> Option<F> {
61 self.weak_ptr
62 }
63}
64
65pub(crate) macro dlsym {
66 (fn $name:ident($($t:ty),*) -> $ret:ty) => (
67 dlsym!(fn $name($($t),*) -> $ret, stringify!($name));
68 ),
69 (fn $name:ident($($t:ty),*) -> $ret:ty, $sym:expr) => (
70 static DLSYM: DlsymWeak<unsafe extern "C" fn($($t),*) -> $ret> =
71 DlsymWeak::new(concat!($sym, '\0'));
72 let $name = &DLSYM;
73 )
74}
75pub(crate) struct DlsymWeak<F> {
76 name: &'static str,
77 func: AtomicPtr<libc::c_void>,
78 _marker: PhantomData<F>,
79}
80
81impl<F> DlsymWeak<F> {
82 pub(crate) const fn new(name: &'static str) -> Self {
83 DlsymWeak { name, func: AtomicPtr::new(ptr::invalid_mut(1)), _marker: PhantomData }
84 }
85
86 #[inline]
87 pub(crate) fn get(&self) -> Option<F> {
88 unsafe {
89 // Relaxed is fine here because we fence before reading through the
90 // pointer (see the comment below).
91 match self.func.load(Ordering::Relaxed) {
92 func if func.addr() == 1 => self.initialize(),
93 func if func.is_null() => None,
94 func => {
95 let func = mem::transmute_copy::<*mut libc::c_void, F>(&func);
96 // The caller is presumably going to read through this value
97 // (by calling the function we've dlsymed). This means we'd
98 // need to have loaded it with at least C11's consume
99 // ordering in order to be guaranteed that the data we read
100 // from the pointer isn't from before the pointer was
101 // stored. Rust has no equivalent to memory_order_consume,
102 // so we use an acquire fence (sorry, ARM).
103 //
104 // Now, in practice this likely isn't needed even on CPUs
105 // where relaxed and consume mean different things. The
106 // symbols we're loading are probably present (or not) at
107 // init, and even if they aren't the runtime dynamic loader
108 // is extremely likely have sufficient barriers internally
109 // (possibly implicitly, for example the ones provided by
110 // invoking `mprotect`).
111 //
112 // That said, none of that's *guaranteed*, and so we fence.
113 atomic::fence(Ordering::Acquire);
114 Some(func)
115 }
116 }
117 }
118 }
119
120 // Cold because it should only happen during first-time initialization.
121 #[cold]
122 unsafe fn initialize(&self) -> Option<F> {
123 assert_eq!(mem::size_of::<F>(), mem::size_of::<*mut libc::c_void>());
124
125 let val = fetch(self.name);
126 // This synchronizes with the acquire fence in `get`.
127 self.func.store(val, Ordering::Release);
128
129 if val.is_null() { None } else { Some(mem::transmute_copy::<*mut libc::c_void, F>(&val)) }
130 }
131}
132
133unsafe fn fetch(name: &str) -> *mut libc::c_void {
134 let name: &CStr = match CStr::from_bytes_with_nul(name.as_bytes()) {
135 Ok(cstr: &CStr) => cstr,
136 Err(..) => return ptr::null_mut(),
137 };
138 libc::dlsym(libc::RTLD_DEFAULT, name.as_ptr())
139}
140
141#[cfg(not(any(target_os = "linux", target_os = "android")))]
142pub(crate) macro syscall {
143 (fn $name:ident($($arg_name:ident: $t:ty),*) -> $ret:ty) => (
144 unsafe fn $name($($arg_name: $t),*) -> $ret {
145 weak! { fn $name($($t),*) -> $ret }
146
147 if let Some(fun) = $name.get() {
148 fun($($arg_name),*)
149 } else {
150 super::os::set_errno(libc::ENOSYS);
151 -1
152 }
153 }
154 )
155}
156
157#[cfg(any(target_os = "linux", target_os = "android"))]
158pub(crate) macro syscall {
159 (fn $name:ident($($arg_name:ident: $t:ty),*) -> $ret:ty) => (
160 unsafe fn $name($($arg_name:$t),*) -> $ret {
161 weak! { fn $name($($t),*) -> $ret }
162
163 // Use a weak symbol from libc when possible, allowing `LD_PRELOAD`
164 // interposition, but if it's not found just use a raw syscall.
165 if let Some(fun) = $name.get() {
166 fun($($arg_name),*)
167 } else {
168 // This looks like a hack, but concat_idents only accepts idents
169 // (not paths).
170 use libc::*;
171
172 syscall(
173 concat_idents!(SYS_, $name),
174 $($arg_name),*
175 ) as $ret
176 }
177 }
178 )
179}
180
181#[cfg(any(target_os = "linux", target_os = "android"))]
182pub(crate) macro raw_syscall {
183 (fn $name:ident($($arg_name:ident: $t:ty),*) -> $ret:ty) => (
184 unsafe fn $name($($arg_name:$t),*) -> $ret {
185 // This looks like a hack, but concat_idents only accepts idents
186 // (not paths).
187 use libc::*;
188
189 syscall(
190 concat_idents!(SYS_, $name),
191 $($arg_name),*
192 ) as $ret
193 }
194 )
195}
196

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