On target_os = "linux", ensure that only one Rust thread calls libc::exit or returns from main.

library/std/src/rt.rs

@@ -161,5 +161,8 @@ fn lang_start<T: crate::process::Termination + 'static>(
         argv,
         sigpipe,
     );
+    // Guard against multple threads calling `libc::exit` concurrently.
+    // See the documentation for `unique_thread_exit` for more information.
+    crate::sys::common::exit_guard::unique_thread_exit();
     v
 }

library/std/src/sys/pal/common/exit_guard.rs

@@ -0,0 +1,88 @@
+cfg_if::cfg_if! {
+    if #[cfg(target_os = "linux")] {
+        /// Mitigation for https://github.com/rust-lang/rust/issues/126600
+        ///
+        /// On `unix` (where `libc::exit` may not be thread-safe), ensure that only one Rust thread
+        /// calls `libc::exit` (or returns from `main`) by calling this function before calling
+        /// `libc::exit` (or returning from `main`).
+        ///
+        /// Technically not enough to ensure soundness, since other code directly calling
+        /// libc::exit will still race with this.
+        ///
+        /// *This function does not itself call `libc::exit`.* This is so it can also be used
+        /// to guard returning from `main`.
+        ///
+        /// This function will return only the first time it is called in a process.
+        ///
+        /// * If it is called again on the same thread as the first call, it will abort.
+        /// * If it is called again on a different thread, it will `thread::park()` in a loop
+        ///   (waiting for the process to exit).
+        pub(crate) fn unique_thread_exit() {
+            let this_thread_id = unsafe { libc::gettid() };
+            debug_assert_ne!(this_thread_id, 0, "thread ID cannot be zero");
+            #[cfg(target_has_atomic = "32")]
+            {
+                use crate::sync::atomic::{AtomicI32, Ordering};
+                static EXITING_THREAD_ID: AtomicI32 = AtomicI32::new(0);
+                match EXITING_THREAD_ID.compare_exchange(
+                    0,
+                    this_thread_id,
+                    Ordering::Relaxed,
+                    Ordering::Relaxed,
+                ) {
+                    Ok(_zero) => {
+                        // This is the first thread to call `unique_thread_exit`,
+                        // and this is the first time it is called.
+                        // Set EXITING_THREAD_ID to this thread's ID (done by the
+                        // compare_exchange) and return.
+                    }
+                    Err(id) if id == this_thread_id => {
+                        // This is the first thread to call `unique_thread_exit`,
+                        // but this is the second time it is called.
+                        // Abort the process.
+                        core::panicking::panic_nounwind("std::process::exit called re-entrantly")
+                    }
+                    Err(_) => {
+                        // This is not the first thread to call `unique_thread_exit`.
+                        // Park until the process exits.
+                        loop {
+                            crate::thread::park();
+                        }
+                    }
+                }
+            }
+            #[cfg(not(target_has_atomic = "32"))]
+            {
+                use crate::sync::{Mutex, PoisonError};
+                static EXITING_THREAD_ID: Mutex<i32> = Mutex::new(0);
+                let mut exiting_thread_id =
+                    EXITING_THREAD_ID.lock().unwrap_or_else(PoisonError::into_inner);
+                if *exiting_thread_id == 0 {
+                    // This is the first thread to call `unique_thread_exit`,
+                    // and this is the first time it is called.
+                    // Set EXITING_THREAD_ID to this thread's ID and return.
+                    *exiting_thread_id = this_thread_id;
+                } else if *exiting_thread_id == this_thread_id {
+                    // This is the first thread to call `unique_thread_exit`,
+                    // but this is the second time it is called.
+                    // Abort the process.
+                    core::panicking::panic_nounwind("std::process::exit called re-entrantly")
+                } else {
+                    // This is not the first thread to call `unique_thread_exit`.
+                    // Park until the process exits.
+                    drop(exiting_thread_id);
+                    loop {
+                        crate::thread::park();
+                    }
+                }
+            }
+        }
+    } else {
+        /// Mitigation for https://github.com/rust-lang/rust/issues/126600
+        ///
+        /// Mitigation is ***NOT*** implemented on this platform, either because this platform is not affected, or because mitigation is not yet implemented for this platform.
+        pub(crate) fn unique_thread_exit() {
+            // Mitigation not required on platforms where `exit` is thread-safe.
+        }
+    }
+}

library/std/src/sys/pal/common/mod.rs

@@ -11,6 +11,7 @@
 #![allow(dead_code)]
 
 pub mod alloc;
+pub mod exit_guard;
 pub mod small_c_string;
 
 #[cfg(test)]

library/std/src/sys/pal/unix/os.rs

@@ -758,6 +758,7 @@ pub fn home_dir() -> Option<PathBuf> {
 }
 
 pub fn exit(code: i32) -> ! {
+    crate::sys::common::exit_guard::unique_thread_exit();
     unsafe { libc::exit(code as c_int) }
 }