1cfg_if::cfg_if! {
2 if #[cfg(target_os = "linux")] {
3 /// Mitigation for <https://github.com/rust-lang/rust/issues/126600>
4 ///
5 /// On glibc, `libc::exit` has been observed to not always be thread-safe.
6 /// It is currently unclear whether that is a glibc bug or allowed by the standard.
7 /// To mitigate this problem, we ensure that only one
8 /// Rust thread calls `libc::exit` (or returns from `main`) by calling this function before
9 /// calling `libc::exit` (or returning from `main`).
10 ///
11 /// Technically, this is not enough to ensure soundness, since other code directly calling
12 /// `libc::exit` will still race with this.
13 ///
14 /// *This function does not itself call `libc::exit`.* This is so it can also be used
15 /// to guard returning from `main`.
16 ///
17 /// This function will return only the first time it is called in a process.
18 ///
19 /// * If it is called again on the same thread as the first call, it will abort.
20 /// * If it is called again on a different thread, it will wait in a loop
21 /// (waiting for the process to exit).
22 #[cfg_attr(any(test, doctest), allow(dead_code))]
23 pub(crate) fn unique_thread_exit() {
24 use crate::ffi::c_int;
25 use crate::ptr;
26 use crate::sync::atomic::AtomicPtr;
27 use crate::sync::atomic::Ordering::{Acquire, Relaxed};
28
29 static EXITING_THREAD_ID: AtomicPtr<c_int> = AtomicPtr::new(ptr::null_mut());
30
31 // We use the address of `errno` as a cheap and safe way to identify
32 // threads. As the C standard mandates that `errno` must have thread
33 // storage duration, we can rely on its address not changing over the
34 // lifetime of the thread. Additionally, accesses to `errno` are
35 // async-signal-safe, so this function is available in all imaginable
36 // circumstances.
37 let this_thread_id = crate::sys::os::errno_location();
38 match EXITING_THREAD_ID.compare_exchange(ptr::null_mut(), this_thread_id, Acquire, Relaxed) {
39 Ok(_) => {
40 // This is the first thread to call `unique_thread_exit`,
41 // and this is the first time it is called. Continue exiting.
42 }
43 Err(exiting_thread_id) if exiting_thread_id == this_thread_id => {
44 // This is the first thread to call `unique_thread_exit`,
45 // but this is the second time it is called.
46 // Abort the process.
47 core::panicking::panic_nounwind("std::process::exit called re-entrantly")
48 }
49 Err(_) => {
50 // This is not the first thread to call `unique_thread_exit`.
51 // Pause until the process exits.
52 loop {
53 // Safety: libc::pause is safe to call.
54 unsafe { libc::pause(); }
55 }
56 }
57 }
58 }
59 } else {
60 /// Mitigation for <https://github.com/rust-lang/rust/issues/126600>
61 ///
62 /// Mitigation is ***NOT*** implemented on this platform, either because this platform
63 /// is not affected, or because mitigation is not yet implemented for this platform.
64 #[cfg_attr(any(test, doctest), allow(dead_code))]
65 pub(crate) fn unique_thread_exit() {
66 // Mitigation not required on platforms where `exit` is thread-safe.
67 }
68 }
69}
70