stdlib: Fix data race in __run_exit_handlers

Message ID 20210426192036.1743585-1-vitalybuka@google.com
State Superseded
Headers show
Series
  • stdlib: Fix data race in __run_exit_handlers
Related show

Commit Message

Florian Weimer via Libc-alpha April 26, 2021, 7:20 p.m.
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 | 109 +++++++++++++++++++++++++++++++++
 3 files changed, 130 insertions(+), 11 deletions(-)
 create mode 100644 stdlib/test-cxa_atexit-race2.c

-- 
2.31.1.498.g6c1eba8ee3d-goog

Comments

Florian Weimer via Libc-alpha April 26, 2021, 7:23 p.m. | #1
The last patch removes static cb_arg and improves description of race case
2.

On Mon, 26 Apr 2021 at 12:20, Vitaly Buka <vitalybuka@google.com> wrote:

> 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 | 109 +++++++++++++++++++++++++++++++++

>  3 files changed, 130 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..9ab110ebaf

> --- /dev/null

> +++ b/stdlib/test-cxa_atexit-race2.c

> @@ -0,0 +1,109 @@

> +/* 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 (&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 *

> +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 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>

> --

> 2.31.1.498.g6c1eba8ee3d-goog

>

>

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..9ab110ebaf
--- /dev/null
+++ b/stdlib/test-cxa_atexit-race2.c
@@ -0,0 +1,109 @@ 
+/* 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 (&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 *
+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 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>