stdlib: Fix data race in __run_exit_handlers [BZ #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.

Fixes https://sourceware.org/bugzilla/show_bug.cgi?id=27749

Checked on x86_64-linux-gnu.

Reviewed-by: Adhemerval Zanella  <adhemerval.zanella@linaro.org>

stdlib/Makefile

@@ -82,6 +82,7 @@ tests		:= tst-strtol tst-strtod testmb testrand testsort testdiv   \
 		   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-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)
 

stdlib/exit.c

@@ -45,25 +45,21 @@ __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
      exit (). */
   while (true)
     {
-      struct exit_function_list *cur;
+      struct exit_function_list *cur = *listp;
-
-      __libc_lock_lock (__exit_funcs_lock);
-
-    restart:
-      cur = *listp;
 
       if (cur == NULL)
 	{
 	  /* Exit processing complete.  We will not allow any more
 	     atexit/on_exit registrations.  */
 	  __exit_funcs_done = true;
-	  __libc_lock_unlock (__exit_funcs_lock);
 	  break;
 	}
 
@@ -72,49 +68,57 @@ __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
 	       more exit functions.  Start the loop over.  */
-	    goto restart;
+            continue;
 	}
 
       *listp = cur->next;
@@ -122,9 +126,9 @@ __run_exit_handlers (int status, struct exit_function_list **listp,
 	/* Don't free the last element in the chain, this is the statically
 	   allocate element.  */
 	free (cur);
+    }
 
   __libc_lock_unlock (__exit_funcs_lock);
-    }
 
   if (run_list_atexit)
     RUN_HOOK (__libc_atexit, ());

stdlib/test-cxa_atexit-race2.c

@@ -0,0 +1,110 @@
+/* Support file for atexit/exit, etc. race tests (BZ #27749).
+   Copyright (C) 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/>.  */
+
+/* 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/check.h>
+#include <support/xthread.h>
+#include <support/xunistd.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)
+    FAIL_EXIT1 ("%s: %p\n", __func__, arg);
+  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 NULL;
+}
+
+_Noreturn 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) == 0)
+    ;
+  exit (0);
+}
+
+static int
+do_test (void)
+{
+  for (int i = 0; i < 20; ++i)
+    {
+      for (int i = 0; i < 10; ++i)
+        if (xfork () == 0)
+          test_and_exit ();
+
+      for (int i = 0; i < 10; ++i)
+        {
+          int status;
+          xwaitpid (0, &status, 0);
+          if (!WIFEXITED (status))
+            FAIL_EXIT1 ("Failed iterations %d", i);
+          TEST_COMPARE (WEXITSTATUS (status), 0);
+        }
+    }
+
+  return 0;
+}
+
+#define TEST_FUNCTION do_test
+#include <support/test-driver.c>