Fix BZ# 22180.

POSIX requires that dlclose() and exit() be thread safe, therefore
you can have one thread in the middle of dlclose() and another thread
executing exit() without causing any undefined behaviour on the part
of the implementation.

The existing implementation had a flaw that exit() exit handler processing
did not consider a concurrent dlclose() and would not mark already run
exit handlers using the ef_free flavour. The consequence of this is that
a concurrent exit() with dlclose() will run all the exit handlers that
dlclose() had not yet run, but then will block on the loader lock. The
concurrent dlclose() will continue to run all the exit handlers again
(twice) in violation of the Itanium C++ ABI requirements for __cxa_atexit().

This commit fixes this by having exit() mark all handlers with ef_free to
ensure that concurrent dlclose() won't re-run registered exit handlers that
have already run.

1 files changed, 80 insertions, 0 deletions

diff --git a/stdlib/test-dlclose-exit-race.c b/stdlib/test-dlclose-exit-race.c
new file mode 100644
index 0000000000..9ec294e168
--- /dev/null
+++ b/stdlib/test-dlclose-exit-race.c
@@ -0,0 +1,80 @@
+/* Test for exit/dlclose race (Bug 22180).
+   Copyright (C) 2017 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
+   <http://www.gnu.org/licenses/>.  */
+
+/* This file must be run from within a directory called "stdlib".  */
+
+/* This test verifies that when dlopen in one thread races against exit
+   in another thread, we don't call registered destructor twice.
+
+   Expected result:
+     second
+     first
+     ... clean termination
+*/
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <semaphore.h>
+#include <support/check.h>
+#include <support/xdlfcn.h>
+#include <support/xthread.h>
+
+/* Semaphore to ensure we have a happens-before between the first function
+   starting and exit being called.  */
+sem_t order1;
+
+/* Semaphore to ensure we have a happens-before between the second function
+   starting and the first function returning.  */
+sem_t order2;
+
+void *
+exit_thread (void *arg)
+{
+  /* Wait for the dlclose to start...  */
+  sem_wait (&order1);
+  /* Then try to run the exit sequence which should call all
+     __cxa_atexit registered functions and in parallel with
+     the executing dlclose().  */
+  exit (0);
+}
+
+
+void
+last (void)
+{
+  /* Let dlclose thread proceed.  */
+  sem_post (&order2);
+}
+
+int
+main (void)
+{
+  void *dso;
+  pthread_t thread;
+
+  atexit (last);
+
+  dso = xdlopen ("$ORIGIN/test-dlclose-exit-race-helper.so",
+		 RTLD_NOW|RTLD_GLOBAL);
+  thread = xpthread_create (NULL, exit_thread, NULL);
+
+  xdlclose (dso);
+  xpthread_join (thread);
+
+  FAIL_EXIT1 ("Did not terminate via exit(0) in exit_thread() as expected.");
+}