[PATCH] stdlib: Fix data race in __run_exit_handlers
Vitaly Buka
vitalybuka@google.com
Mon Apr 26 19:27:29 GMT 2021
Fixes https://sourceware.org/bugzilla/show_bug.cgi?id=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.
---
stdlib/Makefile | 4 +-
stdlib/exit.c | 28 ++++++---
stdlib/test-cxa_atexit-race2.c | 110 +++++++++++++++++++++++++++++++++
3 files changed, 131 insertions(+), 11 deletions(-)
create mode 100644 stdlib/test-cxa_atexit-race2.c
diff --git a/stdlib/Makefile b/stdlib/Makefile
index b3b30ab73e..f5755a1654 100644
--- a/stdlib/Makefile
+++ b/stdlib/Makefile
@@ -81,7 +81,8 @@ tests := tst-strtol tst-strtod testmb testrand testsort testdiv \
tst-width-stdint tst-strfrom tst-strfrom-locale \
tst-getrandom tst-atexit tst-at_quick_exit \
tst-cxa_atexit tst-on_exit test-atexit-race \
- test-at_quick_exit-race test-cxa_atexit-race \
+ test-at_quick_exit-race test-cxa_atexit-race \
+ test-cxa_atexit-race2 \
test-on_exit-race test-dlclose-exit-race \
tst-makecontext-align test-bz22786 tst-strtod-nan-sign \
tst-swapcontext1 tst-setcontext4 tst-setcontext5 \
@@ -100,6 +101,7 @@ endif
LDLIBS-test-atexit-race = $(shared-thread-library)
LDLIBS-test-at_quick_exit-race = $(shared-thread-library)
LDLIBS-test-cxa_atexit-race = $(shared-thread-library)
+LDLIBS-test-cxa_atexit-race2 = $(shared-thread-library)
LDLIBS-test-on_exit-race = $(shared-thread-library)
LDLIBS-tst-canon-bz26341 = $(shared-thread-library)
diff --git a/stdlib/exit.c b/stdlib/exit.c
index bed82733ad..f095b38ab3 100644
--- a/stdlib/exit.c
+++ b/stdlib/exit.c
@@ -45,6 +45,8 @@ __run_exit_handlers (int status, struct exit_function_list **listp,
if (run_dtors)
__call_tls_dtors ();
+ __libc_lock_lock (__exit_funcs_lock);
+
/* We do it this way to handle recursive calls to exit () made by
the functions registered with `atexit' and `on_exit'. We call
everyone on the list and use the status value in the last
@@ -53,8 +55,6 @@ __run_exit_handlers (int status, struct exit_function_list **listp,
{
struct exit_function_list *cur;
- __libc_lock_lock (__exit_funcs_lock);
-
restart:
cur = *listp;
@@ -63,7 +63,6 @@ __run_exit_handlers (int status, struct exit_function_list **listp,
/* Exit processing complete. We will not allow any more
atexit/on_exit registrations. */
__exit_funcs_done = true;
- __libc_lock_unlock (__exit_funcs_lock);
break;
}
@@ -72,44 +71,52 @@ __run_exit_handlers (int status, struct exit_function_list **listp,
struct exit_function *const f = &cur->fns[--cur->idx];
const uint64_t new_exitfn_called = __new_exitfn_called;
- /* Unlock the list while we call a foreign function. */
- __libc_lock_unlock (__exit_funcs_lock);
switch (f->flavor)
{
void (*atfct) (void);
void (*onfct) (int status, void *arg);
void (*cxafct) (void *arg, int status);
+ void *arg;
case ef_free:
case ef_us:
break;
case ef_on:
onfct = f->func.on.fn;
+ arg = f->func.on.arg;
#ifdef PTR_DEMANGLE
PTR_DEMANGLE (onfct);
#endif
- onfct (status, f->func.on.arg);
+ /* Unlock the list while we call a foreign function. */
+ __libc_lock_unlock (__exit_funcs_lock);
+ onfct (status, arg);
+ __libc_lock_lock (__exit_funcs_lock);
break;
case ef_at:
atfct = f->func.at;
#ifdef PTR_DEMANGLE
PTR_DEMANGLE (atfct);
#endif
+ /* Unlock the list while we call a foreign function. */
+ __libc_lock_unlock (__exit_funcs_lock);
atfct ();
+ __libc_lock_lock (__exit_funcs_lock);
break;
case ef_cxa:
/* To avoid dlclose/exit race calling cxafct twice (BZ 22180),
we must mark this function as ef_free. */
f->flavor = ef_free;
cxafct = f->func.cxa.fn;
+ arg = f->func.cxa.arg;
#ifdef PTR_DEMANGLE
PTR_DEMANGLE (cxafct);
#endif
- cxafct (f->func.cxa.arg, status);
+ /* Unlock the list while we call a foreign function. */
+ __libc_lock_unlock (__exit_funcs_lock);
+ cxafct (arg, status);
+ __libc_lock_lock (__exit_funcs_lock);
break;
}
- /* Re-lock again before looking at global state. */
- __libc_lock_lock (__exit_funcs_lock);
if (__glibc_unlikely (new_exitfn_called != __new_exitfn_called))
/* The last exit function, or another thread, has registered
@@ -123,9 +130,10 @@ __run_exit_handlers (int status, struct exit_function_list **listp,
allocate element. */
free (cur);
- __libc_lock_unlock (__exit_funcs_lock);
}
+ __libc_lock_unlock (__exit_funcs_lock);
+
if (run_list_atexit)
RUN_HOOK (__libc_atexit, ());
diff --git a/stdlib/test-cxa_atexit-race2.c b/stdlib/test-cxa_atexit-race2.c
new file mode 100644
index 0000000000..d8c3d418e7
--- /dev/null
+++ b/stdlib/test-cxa_atexit-race2.c
@@ -0,0 +1,110 @@
+/* Support file for atexit/exit, etc. race tests.
+ Copyright (C) 2017-2021 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/>. */
+
+/* This file must be run from within a directory called "stdlib". */
+
+/* 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/xthread.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 (®istered, 1);
+ static void *prev;
+ if (arg == prev)
+ {
+ printf ("%p\n", arg);
+ abort ();
+ }
+ prev = arg;
+
+ while (atomic_load (&todo) > 0 && atomic_load (®istered) < 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 (®istered) < 10000)
+ {
+ int n = 10;
+ for (int i = 0; i < n; ++i)
+ __cxa_atexit (&atexit_cb, ++cb_arg, 0);
+ atomic_fetch_add (®istered, n);
+ atomic_fetch_sub (&todo, n);
+ }
+ return 0;
+}
+
+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 (®istered))
+ ;
+ exit (0);
+}
+
+static int
+do_test (void)
+{
+ for (int i = 0; i < 20; ++i)
+ {
+ for (int i = 0; i < 10; ++i)
+ if (fork () == 0)
+ test_and_exit ();
+
+ int status;
+ while (wait (&status) > 0)
+ {
+ if (!WIFEXITED (status))
+ {
+ printf ("Failed interation %d\n", i);
+ abort ();
+ }
+ }
+ }
+
+ exit (0);
+}
+
+#define TEST_FUNCTION do_test
+#include <support/test-driver.c>
--
2.31.1.498.g6c1eba8ee3d-goog
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