stdlib: Fix data race in __run_exit_handlers [BZ #27749]

Keep __exit_funcs_lock almost all the time and unlock it only to execute
callbacks. This fixed two issues.

1. f->func.cxa was modified outside the lock with rare data race like:
	thread 0: __run_exit_handlers unlock __exit_funcs_lock
	thread 1: __internal_atexit locks __exit_funcs_lock
	thread 0: f->flavor = ef_free;
	thread 1: sees ef_free and use it as new
	thread 1: new->func.cxa.fn = (void (*) (void *, int)) func;
	thread 1: new->func.cxa.arg = arg;
	thread 1: new->flavor = ef_cxa;
	thread 0: cxafct = f->func.cxa.fn;  // it's wrong fn!
	thread 0: cxafct (f->func.cxa.arg, status);  // it's wrong arg!
	thread 0: goto restart;
	thread 0: call the same exit_function again as it's ef_cxa

2. Don't unlock in main while loop after *listp = cur->next. If *listp
   is NULL and __exit_funcs_done is false another thread may fail in
   __new_exitfn on assert (l != NULL):
	 thread 0: *listp = cur->next;  // It can be the last: *listp = NULL.
	 thread 0: __libc_lock_unlock
	 thread 1: __libc_lock_lock in __on_exit
	 thread 1: __new_exitfn
	 thread 1: if (__exit_funcs_done)  // false: thread 0 isn't there yet.
	 thread 1: l = *listp
	 thread 1: moves one and crashes on assert (l != NULL);

The test needs multiple iterations to consistently fail without the fix.

Fixes https://sourceware.org/bugzilla/show_bug.cgi?id=27749

Checked on x86_64-linux-gnu.

Reviewed-by: Adhemerval Zanella  <adhemerval.zanella@linaro.org>

1 files changed, 110 insertions, 0 deletions

diff --git a/stdlib/test-cxa_atexit-race2.c b/stdlib/test-cxa_atexit-race2.c
new file mode 100644
index 0000000000..be6ba9d4be
--- /dev/null
+++ b/stdlib/test-cxa_atexit-race2.c
@@ -0,0 +1,110 @@
+/* Support file for atexit/exit, etc. race tests (BZ #27749).
+   Copyright (C) 2021 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/>.  */
+
+/* The atexit/exit, at_quick_exit/quick_exit, __cxa_atexit/exit, etc.
+   exhibited data race while calling destructors.
+
+   This test registers destructors from the background thread, and checks that
+   the same destructor is not called more than once.  */
+
+#include <stdatomic.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <support/check.h>
+#include <support/xthread.h>
+#include <support/xunistd.h>
+#include <sys/wait.h>
+#include <unistd.h>
+
+static atomic_int registered;
+static atomic_int todo = 100000;
+
+static void
+atexit_cb (void *arg)
+{
+  atomic_fetch_sub (&registered, 1);
+  static void *prev;
+  if (arg == prev)
+    FAIL_EXIT1 ("%s: %p\n", __func__, arg);
+  prev = arg;
+
+  while (atomic_load (&todo) > 0 && atomic_load (&registered) < 100)
+    ;
+}
+
+int __cxa_atexit (void (*func) (void *), void *arg, void *d);
+
+static void *
+thread_func (void *arg)
+{
+  void *cb_arg = NULL;
+  while (atomic_load (&todo) > 0)
+    {
+      if (atomic_load (&registered) < 10000)
+        {
+          int n = 10;
+          for (int i = 0; i < n; ++i)
+            __cxa_atexit (&atexit_cb, ++cb_arg, 0);
+          atomic_fetch_add (&registered, n);
+          atomic_fetch_sub (&todo, n);
+        }
+    }
+
+  return NULL;
+}
+
+_Noreturn static void
+test_and_exit (void)
+{
+  pthread_attr_t attr;
+
+  xpthread_attr_init (&attr);
+  xpthread_attr_setdetachstate (&attr, 1);
+
+  xpthread_create (&attr, thread_func, NULL);
+  xpthread_attr_destroy (&attr);
+
+  while (atomic_load (&registered) == 0)
+    ;
+  exit (0);
+}
+
+static int
+do_test (void)
+{
+  for (int i = 0; i < 20; ++i)
+    {
+      for (int i = 0; i < 10; ++i)
+        if (xfork () == 0)
+          test_and_exit ();
+
+      for (int i = 0; i < 10; ++i)
+        {
+          int status;
+          xwaitpid (0, &status, 0);
+          if (!WIFEXITED (status))
+            FAIL_EXIT1 ("Failed iterations %d", i);
+          TEST_COMPARE (WEXITSTATUS (status), 0);
+        }
+    }
+
+  return 0;
+}
+
+#define TEST_FUNCTION do_test
+#include <support/test-driver.c>