library/std/src/sys/exit.rs

cfg_select! {
    target_os = "linux" => {
        /// Mitigation for <https://github.com/rust-lang/rust/issues/126600>
        ///
        /// On glibc, `libc::exit` has been observed to not always be thread-safe.
        /// It is currently unclear whether that is a glibc bug or allowed by the standard.
        /// To mitigate this problem, we 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, this is 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 wait in a loop
        ///   (waiting for the process to exit).
        pub fn unique_thread_exit() {
            use crate::ffi::c_int;
            use crate::ptr;
            use crate::sync::atomic::AtomicPtr;
            use crate::sync::atomic::Ordering::{Acquire, Relaxed};

            static EXITING_THREAD_ID: AtomicPtr<c_int> = AtomicPtr::new(ptr::null_mut());

            // We use the address of `errno` as a cheap and safe way to identify
            // threads. As the C standard mandates that `errno` must have thread
            // storage duration, we can rely on its address not changing over the
            // lifetime of the thread. Additionally, accesses to `errno` are
            // async-signal-safe, so this function is available in all imaginable
            // circumstances.
            let this_thread_id = crate::sys::io::errno_location();
            match EXITING_THREAD_ID.compare_exchange(ptr::null_mut(), this_thread_id, Acquire, Relaxed) {
                Ok(_) => {
                    // This is the first thread to call `unique_thread_exit`,
                    // and this is the first time it is called. Continue exiting.
                }
                Err(exiting_thread_id) 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")
                }
                Err(_) => {
                    // This is not the first thread to call `unique_thread_exit`.
                    // Pause until the process exits.
                    loop {
                        // Safety: libc::pause is safe to call.
                        unsafe { libc::pause(); }
                    }
                }
            }
        }
    }
    _ => {
        /// 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.
        #[cfg_attr(any(test, doctest), expect(dead_code))]
        pub fn unique_thread_exit() {
            // Mitigation not required on platforms where `exit` is thread-safe.
        }
    }
}

pub fn exit(code: i32) -> ! {
    cfg_select! {
        target_os = "hermit" => {
            unsafe { hermit_abi::exit(code) }
        }
        target_os = "linux" => {
            unsafe {
                unique_thread_exit();
                libc::exit(code)
            }
        }
        target_os = "motor" => {
            moto_rt::process::exit(code)
        }
        all(target_vendor = "fortanix", target_env = "sgx") => {
            crate::sys::pal::abi::exit_with_code(code as _)
        }
        target_os = "solid_asp3" => {
            rtabort!("exit({}) called", code)
        }
        target_os = "teeos" => {
            let _ = code;
            panic!("TA should not call `exit`")
        }
        target_os = "uefi" => {
            use r_efi::base::Status;

            use crate::os::uefi::env;

            if let (Some(boot_services), Some(handle)) =
                (env::boot_services(), env::try_image_handle())
            {
                let boot_services = boot_services.cast::<r_efi::efi::BootServices>();
                let _ = unsafe {
                    ((*boot_services.as_ptr()).exit)(
                        handle.as_ptr(),
                        Status::from_usize(code as usize),
                        0,
                        crate::ptr::null_mut(),
                    )
                };
            }
            crate::intrinsics::abort()
        }
        any(
            target_family = "unix",
            target_os = "wasi",
        ) => {
            unsafe { libc::exit(code as crate::ffi::c_int) }
        }
        target_os = "vexos" => {
            let _ = code;

            unsafe {
                vex_sdk::vexSystemExitRequest();

                loop {
                    vex_sdk::vexTasksRun();
                }
            }
        }
        target_os = "windows" => {
            unsafe { crate::sys::pal::c::ExitProcess(code as u32) }
        }
        target_os = "xous" => {
            crate::os::xous::ffi::exit(code as u32)
        }
        _ => {
            let _ = code;
            crate::intrinsics::abort()
        }
    }
}