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#![forbid(unsafe_op_in_unsafe_fn)]
24
25use crate::ffi::CStr;
26use crate::marker::PhantomData;
27use crate::sync::atomic::{self, Atomic, AtomicPtr, Ordering};
28use crate::{mem, ptr};
29
30// We can use true weak linkage on ELF targets.
31#[cfg(all(unix, not(target_vendor = "apple")))]
32pub(crate) macro weak {
33 (fn $name:ident($($param:ident : $t:ty),* $(,)?) -> $ret:ty;) => (
34 let ref $name: ExternWeak<unsafe extern "C" fn($($t),*) -> $ret> = {
35 unsafe 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(target_vendor = "apple")]
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 $name:ident($($param:ident : $t:ty),* $(,)?) -> $ret:ty;) => (
67 dlsym!(
68 #[link_name = stringify!($name)]
69 fn $name($($param : $t),*) -> $ret;
70 );
71 ),
72 (
73 #[link_name = $sym:expr]
74 fn $name:ident($($param:ident : $t:ty),* $(,)?) -> $ret:ty;
75 ) => (
76 static DLSYM: DlsymWeak<unsafe extern "C" fn($($t),*) -> $ret> =
77 DlsymWeak::new(concat!($sym, '\0'));
78 let $name = &DLSYM;
79 )
80}
81pub(crate) struct DlsymWeak<F> {
82 name: &'static str,
83 func: Atomic<*mut libc::c_void>,
84 _marker: PhantomData<F>,
85}
86
87impl<F> DlsymWeak<F> {
88 pub(crate) const fn new(name: &'static str) -> Self {
89 DlsymWeak {
90 name,
91 func: AtomicPtr::new(ptr::without_provenance_mut(1)),
92 _marker: PhantomData,
93 }
94 }
95
96 #[inline]
97 pub(crate) fn get(&self) -> Option<F> {
98 unsafe {
99 // Relaxed is fine here because we fence before reading through the
100 // pointer (see the comment below).
101 match self.func.load(Ordering::Relaxed) {
102 func if func.addr() == 1 => self.initialize(),
103 func if func.is_null() => None,
104 func => {
105 let func = mem::transmute_copy::<*mut libc::c_void, F>(&func);
106 // The caller is presumably going to read through this value
107 // (by calling the function we've dlsymed). This means we'd
108 // need to have loaded it with at least C11's consume
109 // ordering in order to be guaranteed that the data we read
110 // from the pointer isn't from before the pointer was
111 // stored. Rust has no equivalent to memory_order_consume,
112 // so we use an acquire fence (sorry, ARM).
113 //
114 // Now, in practice this likely isn't needed even on CPUs
115 // where relaxed and consume mean different things. The
116 // symbols we're loading are probably present (or not) at
117 // init, and even if they aren't the runtime dynamic loader
118 // is extremely likely have sufficient barriers internally
119 // (possibly implicitly, for example the ones provided by
120 // invoking `mprotect`).
121 //
122 // That said, none of that's *guaranteed*, and so we fence.
123 atomic::fence(Ordering::Acquire);
124 Some(func)
125 }
126 }
127 }
128 }
129
130 // Cold because it should only happen during first-time initialization.
131 #[cold]
132 unsafe fn initialize(&self) -> Option<F> {
133 assert_eq!(size_of::<F>(), size_of::<*mut libc::c_void>());
134
135 let val = unsafe { fetch(self.name) };
136 // This synchronizes with the acquire fence in `get`.
137 self.func.store(val, Ordering::Release);
138
139 if val.is_null() {
140 None
141 } else {
142 Some(unsafe { mem::transmute_copy::<*mut libc::c_void, F>(&val) })
143 }
144 }
145}
146
147unsafe fn fetch(name: &str) -> *mut libc::c_void {
148 let name: &CStr = match CStr::from_bytes_with_nul(name.as_bytes()) {
149 Ok(cstr: &CStr) => cstr,
150 Err(..) => return ptr::null_mut(),
151 };
152 unsafe { libc::dlsym(handle:libc::RTLD_DEFAULT, symbol:name.as_ptr()) }
153}
154
155#[cfg(not(any(target_os = "linux", target_os = "android")))]
156pub(crate) macro syscall {
157 (fn $name:ident($($param:ident : $t:ty),* $(,)?) -> $ret:ty;) => (
158 unsafe fn $name($($param: $t),*) -> $ret {
159 weak!(fn $name($($param: $t),*) -> $ret;);
160
161 if let Some(fun) = $name.get() {
162 unsafe { fun($($param),*) }
163 } else {
164 super::os::set_errno(libc::ENOSYS);
165 -1
166 }
167 }
168 )
169}
170
171#[cfg(any(target_os = "linux", target_os = "android"))]
172pub(crate) macro syscall {
173 (
174 fn $name:ident($($param:ident : $t:ty),* $(,)?) -> $ret:ty;
175 ) => (
176 unsafe fn $name($($param: $t),*) -> $ret {
177 weak!(fn $name($($param: $t),*) -> $ret;);
178
179 // Use a weak symbol from libc when possible, allowing `LD_PRELOAD`
180 // interposition, but if it's not found just use a raw syscall.
181 if let Some(fun) = $name.get() {
182 unsafe { fun($($param),*) }
183 } else {
184 unsafe { libc::syscall(libc::${concat(SYS_, $name)}, $($param),*) as $ret }
185 }
186 }
187 )
188}
189
190#[cfg(any(target_os = "linux", target_os = "android"))]
191pub(crate) macro raw_syscall {
192 (fn $name:ident($($param:ident : $t:ty),* $(,)?) -> $ret:ty;) => (
193 unsafe fn $name($($param: $t),*) -> $ret {
194 unsafe { libc::syscall(libc::${concat(SYS_, $name)}, $($param),*) as $ret }
195 }
196 )
197}
198

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