diff mbox series

stdlib: Fix data race in __run_exit_handlers

Message ID 20210417172258.2788076-1-vitalybuka@google.com
State Superseded
Headers show
Series stdlib: Fix data race in __run_exit_handlers | expand

Commit Message

Vitaly Buka April 17, 2021, 5:22 p.m. UTC
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).

The test needs multiple iterations to consistently fail without the fix.
---
 stdlib/Makefile                |   4 +-
 stdlib/exit.c                  |  28 +++++---
 stdlib/test-cxa_atexit-race2.c | 114 +++++++++++++++++++++++++++++++++
 3 files changed, 135 insertions(+), 11 deletions(-)
 create mode 100644 stdlib/test-cxa_atexit-race2.c

Comments

Paul Pluzhnikov April 17, 2021, 6:01 p.m. UTC | #1
On Sat, Apr 17, 2021 at 10:23 AM Vitaly Buka via Libc-alpha
<libc-alpha@sourceware.org> wrote:

> diff --git a/stdlib/test-cxa_atexit-race2.c b/stdlib/test-cxa_atexit-race2.c

> +   This test spawns multiple threads, and check the same desctructor is

Typo. Suggest:

This test spawns multiple threads, and checks that the same destructor is

> +   not called more then once.  */

s/then/than/

> +static void *cb_arg = NULL;
> +static void
> +add_handlers (void)
> +{
> +  int n = 10;
> +  for (int i = 0; i < n; ++i)
> +    __cxa_atexit (&atexit_cb, ++cb_arg, 0);

add_handlers() is called from many threads. This code appears to race on cb_arg.
Vitaly Buka April 20, 2021, 10:51 p.m. UTC | #2
I guess I forgot to reply to these comments.

On Sat, 17 Apr 2021 at 11:02, Paul Pluzhnikov <ppluzhnikov@google.com>
wrote:

> On Sat, Apr 17, 2021 at 10:23 AM Vitaly Buka via Libc-alpha
> <libc-alpha@sourceware.org> wrote:
>
> > diff --git a/stdlib/test-cxa_atexit-race2.c
> b/stdlib/test-cxa_atexit-race2.c
>
> > +   This test spawns multiple threads, and check the same desctructor is
>
> Typo. Suggest:
>
> This test spawns multiple threads, and checks that the same destructor is
>

Done in the last patch.


>
> > +   not called more then once.  */
>
> s/then/than/
>

Done in the last patch.


>
> > +static void *cb_arg = NULL;
> > +static void
> > +add_handlers (void)
> > +{
> > +  int n = 10;
> > +  for (int i = 0; i < n; ++i)
> > +    __cxa_atexit (&atexit_cb, ++cb_arg, 0);
>
> add_handlers() is called from many threads. This code appears to race on
> cb_arg.
>

We don't have a data race as add_handlers is called from a single
background thread.
Previous patches had more threads but they didn't help to reproduce the
issues.


>
>
> --
> Paul Pluzhnikov
>
Paul Pluzhnikov April 20, 2021, 11:40 p.m. UTC | #3
On Tue, Apr 20, 2021 at 3:51 PM Vitaly Buka <vitalybuka@google.com> wrote:

>> > +static void *cb_arg = NULL;
>> > +static void
>> > +add_handlers (void)
>> > +{
>> > +  int n = 10;
>> > +  for (int i = 0; i < n; ++i)
>> > +    __cxa_atexit (&atexit_cb, ++cb_arg, 0);
>>
>> add_handlers() is called from many threads. This code appears to race on cb_arg.
>
>
> We don't have a data race as add_handlers is called from a single background thread.
> Previous patches had more threads but they didn't help to reproduce the issues.

Thanks.

I think cb_arg can be moved into add_handlers() and doesn't have to be
static anymore (if it were a local, there wouldn't be a question of a
race in the first place).

I've looked at the code and the first data race description at the
start of this thread.
I agree that this is the right fix for it.

I have not yet understood the second interaction (between
__run_exit_handlers and __new_exitfn), but I am not sure I really need
to: the patch seems correct.

One other change I would make is to move the unlock before
PTR_DEMANGLE (since it doesn't use any of the data guarded by this
lock).





--
Paul Pluzhnikov
diff mbox series

Patch

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..0aed0e7eae
--- /dev/null
+++ b/stdlib/test-cxa_atexit-race2.c
@@ -0,0 +1,114 @@ 
+/* 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 spawns multiple threads, and check the same desctructor is
+   not called more then 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 (&registered, 1);
+  static void *prev;
+  if (arg == prev)
+    {
+      printf ("%p\n", arg);
+      abort ();
+    }
+  prev = arg;
+
+  while (atomic_load (&todo) > 0 && atomic_load (&registered) < 100)
+    ;
+}
+
+int __cxa_atexit (void (*func) (void *), void *arg, void *d);
+
+static void *cb_arg = NULL;
+static void
+add_handlers (void)
+{
+  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);
+}
+
+static void *
+thread_func (void *arg)
+{
+  while (atomic_load (&todo) > 0)
+    if (atomic_load (&registered) < 10000)
+      add_handlers ();
+  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 (&registered))
+    ;
+  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>