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] rust-lang/rust#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 <carlos@redhat.com>

stdlib/Makefile

@@ -273,6 +273,7 @@ tests := \
   tst-bsearch \
   tst-bz20544 \
   tst-canon-bz26341 \
+  tst-concurrent-exit \
   tst-cxa_atexit \
   tst-environ \
   tst-getrandom \

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)

stdlib/tst-concurrent-exit.c

@@ -0,0 +1,157 @@
+/* Check if exit can be called concurrently by multiple threads.
+   Copyright (C) 2024 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <https://www.gnu.org/licenses/>.  */
+
+#include <array_length.h>
+#include <stdlib.h>
+#include <support/check.h>
+#include <support/xthread.h>
+#include <stdio.h>
+#include <support/xunistd.h>
+#include <string.h>
+
+#define MAX_atexit 32
+
+static pthread_barrier_t barrier;
+
+static void *
+tf (void *closure)
+{
+  xpthread_barrier_wait (&barrier);
+  exit (0);
+
+  return NULL;
+}
+
+static const char expected[] = "00000000000000000000000003021121130211";
+static char crumbs[sizeof (expected)];
+static int next_slot = 0;
+
+static void
+exit_with_flush (int code)
+{
+  fflush (stdout);
+  /* glibc allows recursive exit, the atexit handlers execution will be
+     resumed from the where the previous exit was interrupted.  */
+  exit (code);
+}
+
+/* Take some time, so another thread potentially issue exit.  */
+#define SETUP_NANOSLEEP \
+  if (nanosleep (&(struct timespec) { .tv_sec = 0, .tv_nsec = 1000L },	\
+		 NULL) != 0)						\
+    FAIL_EXIT1 ("nanosleep: %m")
+
+static void
+fn0 (void)
+{
+  crumbs[next_slot++] = '0';
+  SETUP_NANOSLEEP;
+}
+
+static void
+fn1 (void)
+{
+  crumbs[next_slot++] = '1';
+  SETUP_NANOSLEEP;
+}
+
+static void
+fn2 (void)
+{
+  crumbs[next_slot++] = '2';
+  atexit (fn1);
+  SETUP_NANOSLEEP;
+}
+
+static void
+fn3 (void)
+{
+  crumbs[next_slot++] = '3';
+  atexit (fn2);
+  atexit (fn0);
+  SETUP_NANOSLEEP;
+}
+
+static void
+fn_final (void)
+{
+  TEST_COMPARE_STRING (crumbs, expected);
+  exit_with_flush (0);
+}
+
+_Noreturn static void
+child (void)
+{
+  enum { nthreads = 8 };
+
+  xpthread_barrier_init (&barrier, NULL, nthreads + 1);
+
+  pthread_t thr[nthreads];
+  for (int i = 0; i < nthreads; i++)
+    thr[i] = xpthread_create (NULL, tf, NULL);
+
+  xpthread_barrier_wait (&barrier);
+
+  for (int i = 0; i < nthreads; i++)
+    {
+      pthread_join (thr[i], NULL);
+      /* It should not be reached, it means that thread did not exit for
+	 some reason.  */
+      support_record_failure ();
+    }
+
+  exit (2);
+}
+
+static int
+do_test (void)
+{
+  /* Register a large number of handler that will trigger a heap allocation
+     for the handle state.  On exit, each block will be freed after the
+     handle is processed.  */
+  int slots_remaining = MAX_atexit;
+
+  /* Register this first so it can verify expected order of the rest.  */
+  atexit (fn_final); --slots_remaining;
+
+  TEST_VERIFY_EXIT (atexit (fn1) == 0); --slots_remaining;
+  TEST_VERIFY_EXIT (atexit (fn3) == 0); --slots_remaining;
+  TEST_VERIFY_EXIT (atexit (fn1) == 0); --slots_remaining;
+  TEST_VERIFY_EXIT (atexit (fn2) == 0); --slots_remaining;
+  TEST_VERIFY_EXIT (atexit (fn1) == 0); --slots_remaining;
+  TEST_VERIFY_EXIT (atexit (fn3) == 0); --slots_remaining;
+
+  while (slots_remaining > 0)
+    {
+      TEST_VERIFY_EXIT (atexit (fn0) == 0); --slots_remaining;
+    }
+
+  pid_t pid = xfork ();
+  if (pid != 0)
+    {
+      int status;
+      xwaitpid (pid, &status, 0);
+      TEST_VERIFY (WIFEXITED (status));
+    }
+  else
+    child ();
+
+  return 0;
+}
+
+#include <support/test-driver.c>