From f6ba993e0cda0ca5554fd47b00e6a87be5fdf05e Mon Sep 17 00:00:00 2001 From: Adhemerval Zanella Date: Thu, 25 Jul 2024 15:41:44 -0300 Subject: stdlib: Allow concurrent exit (BZ 31997) Even if C/POSIX standard states that exit is not formally thread-unsafe, calling it more than once is UB. The glibc already supports it for the single-thread, and both elf/nodelete2.c and tst-rseq-disable.c call exit from a DSO destructor (which is called by _dl_fini, registered at program startup with __cxa_atexit). However, there are still race issues when it is called more than once concurrently by multiple threads. A recent Rust PR triggered this issue [1], which resulted in an Austin Group ask for clarification [2]. Besides it, there is a discussion to make concurrent calling not UB [3], wtih a defined semantic where any remaining callers block until the first call to exit has finished (reentrant calls, leaving through longjmp, and exceptions are still undefined). For glibc, at least reentrant calls are required to be supported to avoid changing the current behaviour. This requires locking using a recursive lock, where any exit called by atexit() handlers resumes at the point of the current handler (thus avoiding calling the current handle multiple times). Checked on x86_64-linux-gnu and aarch64-linux-gnu. [1] https://github.com/rust-lang/rust/issues/126600 [2] https://austingroupbugs.net/view.php?id=1845 [3] https://www.openwall.com/lists/libc-coord/2024/07/24/4 Reviewed-by: Carlos O'Donell --- stdlib/exit.c | 8 ++++++++ 1 file changed, 8 insertions(+) (limited to 'stdlib/exit.c') diff --git a/stdlib/exit.c b/stdlib/exit.c index 5166c78044..bbaf138806 100644 --- a/stdlib/exit.c +++ b/stdlib/exit.c @@ -132,9 +132,17 @@ __run_exit_handlers (int status, struct exit_function_list **listp, } +/* The lock handles concurrent exit(), even though the C/POSIX standard states + that calling exit() more than once is UB. The recursive lock allows + atexit() handlers or destructors to call exit() itself. In this case, the + handler list execution will resume at the point of the current handler. */ +__libc_lock_define_initialized_recursive (static, __exit_lock) + void exit (int status) { + /* The exit should never return, so there is no need to unlock it. */ + __libc_lock_lock_recursive (__exit_lock); __run_exit_handlers (status, &__exit_funcs, true, true); } libc_hidden_def (exit) -- cgit v1.2.3