diff mbox series

stdlib: Fix data race in __run_exit_handlers

Message ID 20200920120957.852821-1-vitalybuka@google.com
State Superseded
Headers show
Series stdlib: Fix data race in __run_exit_handlers | expand

Commit Message

Vitaly Buka Sept. 20, 2020, 12:09 p.m. UTC
Read f->func.cxa under the lock.

There is a clear data race:
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

It's incredibly rare so the test without patch fails only about 10%.
---
 stdlib/Makefile                |  4 +-
 stdlib/exit.c                  | 20 +++++---
 stdlib/test-cxa_atexit-race2.c | 94 ++++++++++++++++++++++++++++++++++
 3 files changed, 111 insertions(+), 7 deletions(-)
 create mode 100644 stdlib/test-cxa_atexit-race2.c

Comments

Paul Pluzhnikov Sept. 20, 2020, 8:41 p.m. UTC | #1
On Sun, Sep 20, 2020 at 5:10 AM Vitaly Buka via Libc-alpha
<libc-alpha@sourceware.org> wrote:

> +static void *
> +threadfunc (void *unused)
> +{
> +  for (; done < 1e6;)
> +    {
> +      if (added < done + 100)
> +        {
> +          __cxa_atexit (&atexitcb, (void *)(++added), __dso_handle);

Isn't there a data race on "added" here (in addition to a data race on "done")?
What prevents two threads from observing "added == 100" at the same
time and adding two calls with value of 101, which would later trigger
abort() in exitcb()?

> +  /* With default 8MiB Linux stack size, creating 1024 threads can cause
> +     VM exhausiton on 32-bit machines.  Reduce stack size of each thread to
> +     128KiB for a maximum required VM size of 128MiB.  */

This comment is far removed from the computation of kStacksize (and
the name violates the naming conventions used here).

I suggest:

  size_t stack_size = 128 << 10; /* 128KiB  */
  if (stack_size < PTHREAD_STACK_MIN) stack_size = PTHREAD_STACK_MIN;

Also, I suspect that 32KiB would be more than enough for stack size here.

> +  for (i = 0; i < kNumThreads; ++i)

Since kNumThreads isn't used anywhere else, I suggest making it a local:

  const int num_threads = 50;
Vitaly Buka Sept. 20, 2020, 9:26 p.m. UTC | #2
On Sun, 20 Sep 2020 at 13:42, Paul Pluzhnikov <ppluzhnikov@google.com>
wrote:

> On Sun, Sep 20, 2020 at 5:10 AM Vitaly Buka via Libc-alpha
> <libc-alpha@sourceware.org> wrote:
>
> > +static void *
> > +threadfunc (void *unused)
> > +{
> > +  for (; done < 1e6;)
> > +    {
> > +      if (added < done + 100)
> > +        {
> > +          __cxa_atexit (&atexitcb, (void *)(++added), __dso_handle);
>
> Isn't there a data race on "added" here (in addition to a data race on
> "done")?
> What prevents two threads from observing "added == 100" at the same
> time and adding two calls with value of 101, which would later trigger
> abort() in exitcb()?
>

They are atomic. Isn't (++added) guarantee to return different values in
all threads?


>
> > +  /* With default 8MiB Linux stack size, creating 1024 threads can cause
> > +     VM exhausiton on 32-bit machines.  Reduce stack size of each
> thread to
> > +     128KiB for a maximum required VM size of 128MiB.  */
>
> This comment is far removed from the computation of kStacksize (and
> the name violates the naming conventions used here).
>
> I suggest:
>
>   size_t stack_size = 128 << 10; /* 128KiB  */
>   if (stack_size < PTHREAD_STACK_MIN) stack_size = PTHREAD_STACK_MIN;
>
> Also, I suspect that 32KiB would be more than enough for stack size here.
>
> > +  for (i = 0; i < kNumThreads; ++i)
>
> Since kNumThreads isn't used anywhere else, I suggest making it a local:
>
>   const int num_threads = 50;
>
> --
> Paul Pluzhnikov
>
Vitaly Buka Sept. 20, 2020, 11:36 p.m. UTC | #3
Oh, this atomics are not what I thought. So the test reproduced bugs in the
test.
I tried different approaches and I failed to reproduce the data race.

On Sun, 20 Sep 2020 at 14:26, Vitaly Buka <vitalybuka@google.com> wrote:

>
>
> On Sun, 20 Sep 2020 at 13:42, Paul Pluzhnikov <ppluzhnikov@google.com>
> wrote:
>
>> On Sun, Sep 20, 2020 at 5:10 AM Vitaly Buka via Libc-alpha
>> <libc-alpha@sourceware.org> wrote:
>>
>> > +static void *
>> > +threadfunc (void *unused)
>> > +{
>> > +  for (; done < 1e6;)
>> > +    {
>> > +      if (added < done + 100)
>> > +        {
>> > +          __cxa_atexit (&atexitcb, (void *)(++added), __dso_handle);
>>
>> Isn't there a data race on "added" here (in addition to a data race on
>> "done")?
>> What prevents two threads from observing "added == 100" at the same
>> time and adding two calls with value of 101, which would later trigger
>> abort() in exitcb()?
>>
>
> They are atomic. Isn't (++added) guarantee to return different values in
> all threads?
>
>
>>
>> > +  /* With default 8MiB Linux stack size, creating 1024 threads can
>> cause
>> > +     VM exhausiton on 32-bit machines.  Reduce stack size of each
>> thread to
>> > +     128KiB for a maximum required VM size of 128MiB.  */
>>
>> This comment is far removed from the computation of kStacksize (and
>> the name violates the naming conventions used here).
>>
>> I suggest:
>>
>>   size_t stack_size = 128 << 10; /* 128KiB  */
>>   if (stack_size < PTHREAD_STACK_MIN) stack_size = PTHREAD_STACK_MIN;
>>
>> Also, I suspect that 32KiB would be more than enough for stack size here.
>>
>> > +  for (i = 0; i < kNumThreads; ++i)
>>
>> Since kNumThreads isn't used anywhere else, I suggest making it a local:
>>
>>   const int num_threads = 50;
>>
>> --
>> Paul Pluzhnikov
>>
>
diff mbox series

Patch

diff --git a/stdlib/Makefile b/stdlib/Makefile
index 4615f6dfe7..f622fa9e0b 100644
--- a/stdlib/Makefile
+++ b/stdlib/Makefile
@@ -82,7 +82,8 @@  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 \
@@ -101,6 +102,7 @@  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-test-dlclose-exit-race = $(shared-thread-library) $(libdl)
diff --git a/stdlib/exit.c b/stdlib/exit.c
index 7bca1cdc14..93cdec82f0 100644
--- a/stdlib/exit.c
+++ b/stdlib/exit.c
@@ -72,44 +72,52 @@  __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;
+	      /* Unlock the list while we call a foreign function.  */
+	      __libc_lock_unlock (__exit_funcs_lock);
 #ifdef PTR_DEMANGLE
 	      PTR_DEMANGLE (onfct);
 #endif
-	      onfct (status, f->func.on.arg);
+	      onfct (status, arg);
+	      __libc_lock_lock (__exit_funcs_lock);
 	      break;
 	    case ef_at:
 	      atfct = f->func.at;
+	      /* Unlock the list while we call a foreign function.  */
+	      __libc_lock_unlock (__exit_funcs_lock);
 #ifdef PTR_DEMANGLE
 	      PTR_DEMANGLE (atfct);
 #endif
 	      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;
+	      /* Unlock the list while we call a foreign function.  */
+	      __libc_lock_unlock (__exit_funcs_lock);
 #ifdef PTR_DEMANGLE
 	      PTR_DEMANGLE (cxafct);
 #endif
-	      cxafct (f->func.cxa.arg, status);
+	      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
diff --git a/stdlib/test-cxa_atexit-race2.c b/stdlib/test-cxa_atexit-race2.c
new file mode 100644
index 0000000000..08acafe19f
--- /dev/null
+++ b/stdlib/test-cxa_atexit-race2.c
@@ -0,0 +1,94 @@ 
+/* Support file for atexit/exit, etc. race tests.
+   Copyright (C) 2017-2020 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 accessing destructor function list (Bug 14333).
+
+   This test spawns large number of threads, and check the same
+   desctructor is not called more then once.  */
+
+#include <limits.h>
+#include <stdatomic.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <support/xthread.h>
+
+extern void *__dso_handle;
+
+const size_t kNumThreads = 50;
+const size_t kStacksize =
+#ifdef PTHREAD_STACK_MIN
+    0x20000 < PTHREAD_STACK_MIN ? PTHREAD_STACK_MIN :
+#endif
+                                0x20000;
+
+static atomic_intptr_t added;
+static atomic_intptr_t done;
+static void *prev;
+
+static void
+atexitcb (void *arg)
+{
+  if (arg == prev)
+    abort ();
+  prev = arg;
+  ++done;
+}
+
+int __cxa_atexit (void (*func) (void *), void *arg, void *d);
+
+static void *
+threadfunc (void *unused)
+{
+  for (; done < 1e6;)
+    {
+      if (added < done + 100)
+        {
+          __cxa_atexit (&atexitcb, (void *)(++added), __dso_handle);
+        }
+    }
+  return 0;
+}
+
+static int
+do_test (void)
+{
+  size_t i;
+  pthread_attr_t attr;
+
+  xpthread_attr_init (&attr);
+  xpthread_attr_setdetachstate (&attr, 1);
+
+  /* With default 8MiB Linux stack size, creating 1024 threads can cause
+     VM exhausiton on 32-bit machines.  Reduce stack size of each thread to
+     128KiB for a maximum required VM size of 128MiB.  */
+  xpthread_attr_setstacksize (&attr, kStacksize);
+
+  for (i = 0; i < kNumThreads; ++i)
+    {
+      xpthread_create (&attr, threadfunc, NULL);
+    }
+  xpthread_attr_destroy (&attr);
+
+  exit (0);
+}
+
+#define TEST_FUNCTION do_test
+#include <support/test-driver.c>