Fix deadlock when pthread_atfork handler calls pthread_atfork or dlclose

In multi-threaded programs, registering via pthread_atfork,
de-registering implicitly via dlclose, or running pthread_atfork
handlers during fork was protected by an internal lock.  This meant
that a pthread_atfork handler attempting to register another handler or
dlclose a dynamically loaded library would lead to a deadlock.

This commit fixes the deadlock in the following way:

During the execution of handlers at fork time, the atfork lock is
released prior to the execution of each handler and taken again upon its
return.  Any handler registrations or de-registrations that occurred
during the execution of the handler are accounted for before proceeding
with further handler execution.

If a handler that hasn't been executed yet gets de-registered by another
handler during fork, it will not be executed.   If a handler gets
registered by another handler during fork, it will not be executed
during that particular fork.

The possibility that handlers may now be registered or deregistered
during handler execution means that identifying the next handler to be
run after a given handler may register/de-register others requires some
bookkeeping.  The fork_handler struct has an additional field, 'id',
which is assigned sequentially during registration.  Thus, handlers are
executed in ascending order of 'id' during 'prepare', and descending
order of 'id' during parent/child handler execution after the fork.

Two tests are included:

* tst-atfork3: Adhemerval Zanella <adhemerval.zanella@linaro.org>
  This test exercises calling dlclose from prepare, parent, and child
  handlers.

* tst-atfork4: This test exercises calling pthread_atfork and dlclose
  from the prepare handler.

[BZ #24595, BZ #27054]

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

include/register-atfork.h

@@ -26,6 +26,7 @@ struct fork_handler
   void (*parent_handler) (void);
   void (*child_handler) (void);
   void *dso_handle;
+  uint64_t id;
 };
 
 /* Function to call to unregister fork handlers.  */
@@ -39,19 +40,18 @@ enum __run_fork_handler_type
   atfork_run_parent
 };
 
-/* Run the atfork handlers and lock/unlock the internal lock depending
-   of the WHO argument:
+/* Run the atfork prepare handlers in the reverse order of registration and
+   return the ID of the last registered handler.  If DO_LOCKING is true, the
+   internal lock is held locked upon return.  */
+extern uint64_t __run_prefork_handlers (_Bool do_locking) attribute_hidden;
 
-   - atfork_run_prepare: run all the PREPARE_HANDLER in reverse order of
-			 insertion and locks the internal lock.
-   - atfork_run_child: run all the CHILD_HANDLER and unlocks the internal
-		       lock.
-   - atfork_run_parent: run all the PARENT_HANDLER and unlocks the internal
-			lock.
-
-   Perform locking only if DO_LOCKING.  */
-extern void __run_fork_handlers (enum __run_fork_handler_type who,
-				 _Bool do_locking) attribute_hidden;
+/* Given a handler type (parent or child), run all the atfork handlers in
+   the order of registration up to and including the handler with id equal
+   to LASTRUN.  If DO_LOCKING is true, the internal lock is unlocked prior
+   to return.  */
+extern void __run_postfork_handlers (enum __run_fork_handler_type who,
+                                     _Bool do_locking,
+                                     uint64_t lastrun) attribute_hidden;
 
 /* C library side function to register new fork handlers.  */
 extern int __register_atfork (void (*__prepare) (void),

posix/fork.c

@@ -46,8 +46,9 @@ __libc_fork (void)
      best effort to make is async-signal-safe at least for single-thread
      case.  */
   bool multiple_threads = __libc_single_threaded == 0;
+  uint64_t lastrun;
 
-  __run_fork_handlers (atfork_run_prepare, multiple_threads);
+  lastrun = __run_prefork_handlers (multiple_threads);
 
   struct nss_database_data nss_database_data;
 
@@ -105,7 +106,7 @@ __libc_fork (void)
       reclaim_stacks ();
 
       /* Run the handlers registered for the child.  */
-      __run_fork_handlers (atfork_run_child, multiple_threads);
+      __run_postfork_handlers (atfork_run_child, multiple_threads, lastrun);
     }
   else
     {
@@ -123,7 +124,7 @@ __libc_fork (void)
 	}
 
       /* Run the handlers registered for the parent.  */
-      __run_fork_handlers (atfork_run_parent, multiple_threads);
+      __run_postfork_handlers (atfork_run_parent, multiple_threads, lastrun);
 
       if (pid < 0)
 	__set_errno (save_errno);

posix/register-atfork.c

@@ -18,6 +18,8 @@
 #include <libc-lock.h>
 #include <stdbool.h>
 #include <register-atfork.h>
+#include <intprops.h>
+#include <stdio.h>
 
 #define DYNARRAY_ELEMENT           struct fork_handler
 #define DYNARRAY_STRUCT            fork_handler_list
@@ -26,7 +28,7 @@
 #include <malloc/dynarray-skeleton.c>
 
 static struct fork_handler_list fork_handlers;
-static bool fork_handler_init = false;
+static uint64_t fork_handler_counter;
 
 static int atfork_lock = LLL_LOCK_INITIALIZER;
 
@@ -36,11 +38,8 @@ __register_atfork (void (*prepare) (void), void (*parent) (void),
 {
   lll_lock (atfork_lock, LLL_PRIVATE);
 
-  if (!fork_handler_init)
-    {
-      fork_handler_list_init (&fork_handlers);
-      fork_handler_init = true;
-    }
+  if (fork_handler_counter == 0)
+    fork_handler_list_init (&fork_handlers);
 
   struct fork_handler *newp = fork_handler_list_emplace (&fork_handlers);
   if (newp != NULL)
@@ -49,6 +48,13 @@ __register_atfork (void (*prepare) (void), void (*parent) (void),
       newp->parent_handler = parent;
       newp->child_handler = child;
       newp->dso_handle = dso_handle;
+
+      /* IDs assigned to handlers start at 1 and increment with handler
+         registration.  Un-registering a handlers discards the corresponding
+         ID.  It is not reused in future registrations.  */
+      if (INT_ADD_OVERFLOW (fork_handler_counter, 1))
+        __libc_fatal ("fork handler counter overflow");
+      newp->id = ++fork_handler_counter;
     }
 
   /* Release the lock.  */
@@ -103,37 +109,111 @@ __unregister_atfork (void *dso_handle)
   lll_unlock (atfork_lock, LLL_PRIVATE);
 }
 
-void
-__run_fork_handlers (enum __run_fork_handler_type who, _Bool do_locking)
+uint64_t
+__run_prefork_handlers (_Bool do_locking)
 {
-  struct fork_handler *runp;
+  uint64_t lastrun;
 
-  if (who == atfork_run_prepare)
+  if (do_locking)
+    lll_lock (atfork_lock, LLL_PRIVATE);
+
+  /* We run prepare handlers from last to first.  After fork, only
+     handlers up to the last handler found here (pre-fork) will be run.
+     Handlers registered during __run_prefork_handlers or
+     __run_postfork_handlers will be positioned after this last handler, and
+     since their prepare handlers won't be run now, their parent/child
+     handlers should also be ignored.  */
+  lastrun = fork_handler_counter;
+
+  size_t sl = fork_handler_list_size (&fork_handlers);
+  for (size_t i = sl; i > 0;)
     {
-      if (do_locking)
-	lll_lock (atfork_lock, LLL_PRIVATE);
-      size_t sl = fork_handler_list_size (&fork_handlers);
-      for (size_t i = sl; i > 0; i--)
-	{
-	  runp = fork_handler_list_at (&fork_handlers, i - 1);
-	  if (runp->prepare_handler != NULL)
-	    runp->prepare_handler ();
-	}
+      struct fork_handler *runp
+        = fork_handler_list_at (&fork_handlers, i - 1);
+
+      uint64_t id = runp->id;
+
+      if (runp->prepare_handler != NULL)
+        {
+          if (do_locking)
+            lll_unlock (atfork_lock, LLL_PRIVATE);
+
+          runp->prepare_handler ();
+
+          if (do_locking)
+            lll_lock (atfork_lock, LLL_PRIVATE);
+        }
+
+      /* We unlocked, ran the handler, and locked again.  In the
+         meanwhile, one or more deregistrations could have occurred leading
+         to the current (just run) handler being moved up the list or even
+         removed from the list itself.  Since handler IDs are guaranteed to
+         to be in increasing order, the next handler has to have:  */
+
+      /* A. An earlier position than the current one has.  */
+      i--;
+
+      /* B. A lower ID than the current one does.  The code below skips
+         any newly added handlers with higher IDs.  */
+      while (i > 0
+             && fork_handler_list_at (&fork_handlers, i - 1)->id >= id)
+        i--;
     }
-  else
+
+  return lastrun;
+}
+
+void
+__run_postfork_handlers (enum __run_fork_handler_type who, _Bool do_locking,
+                         uint64_t lastrun)
+{
+  size_t sl = fork_handler_list_size (&fork_handlers);
+  for (size_t i = 0; i < sl;)
     {
-      size_t sl = fork_handler_list_size (&fork_handlers);
-      for (size_t i = 0; i < sl; i++)
-	{
-	  runp = fork_handler_list_at (&fork_handlers, i);
-	  if (who == atfork_run_child && runp->child_handler)
-	    runp->child_handler ();
-	  else if (who == atfork_run_parent && runp->parent_handler)
-	    runp->parent_handler ();
-	}
+      struct fork_handler *runp = fork_handler_list_at (&fork_handlers, i);
+      uint64_t id = runp->id;
+
+      /* prepare handlers were not run for handlers with ID > LASTRUN.
+         Thus, parent/child handlers will also not be run.  */
+      if (id > lastrun)
+        break;
+
       if (do_locking)
-	lll_unlock (atfork_lock, LLL_PRIVATE);
+        lll_unlock (atfork_lock, LLL_PRIVATE);
+
+      if (who == atfork_run_child && runp->child_handler)
+        runp->child_handler ();
+      else if (who == atfork_run_parent && runp->parent_handler)
+        runp->parent_handler ();
+
+      if (do_locking)
+        lll_lock (atfork_lock, LLL_PRIVATE);
+
+      /* We unlocked, ran the handler, and locked again.  In the meanwhile,
+         one or more [de]registrations could have occurred.  Due to this,
+         the list size must be updated.  */
+      sl = fork_handler_list_size (&fork_handlers);
+
+      /* The just-run handler could also have moved up the list. */
+
+      if (sl > i && fork_handler_list_at (&fork_handlers, i)->id == id)
+        /* The position of the recently run handler hasn't changed.  The
+           next handler to be run is an easy increment away.  */
+        i++;
+      else
+        {
+          /* The next handler to be run is the first handler in the list
+             to have an ID higher than the current one.  */
+          for (i = 0; i < sl; i++)
+            {
+              if (fork_handler_list_at (&fork_handlers, i)->id > id)
+                break;
+            }
+        }
     }
+
+  if (do_locking)
+    lll_unlock (atfork_lock, LLL_PRIVATE);
 }
 
 

sysdeps/pthread/Makefile

@@ -154,16 +154,36 @@ tests += tst-cancelx2 tst-cancelx3 tst-cancelx6 tst-cancelx8 tst-cancelx9 \
 	 tst-cleanupx0 tst-cleanupx1 tst-cleanupx2 tst-cleanupx3
 
 ifeq ($(build-shared),yes)
-tests += tst-atfork2 tst-pt-tls4 tst-_res1 tst-fini1 tst-create1
+tests += \
+  tst-atfork2 \
+  tst-pt-tls4 \
+  tst-_res1 \
+  tst-fini1 \
+  tst-create1 \
+  tst-atfork3 \
+  tst-atfork4 \
+# tests
+
 tests-nolibpthread += tst-fini1
 endif
 
-modules-names += tst-atfork2mod tst-tls4moda tst-tls4modb \
-		 tst-_res1mod1 tst-_res1mod2 tst-fini1mod \
-		 tst-create1mod
+modules-names += \
+  tst-atfork2mod \
+  tst-tls4moda \
+  tst-tls4modb \
+  tst-_res1mod1 \
+  tst-_res1mod2 \
+  tst-fini1mod \
+  tst-create1mod \
+  tst-atfork3mod \
+  tst-atfork4mod \
+# module-names
+
 test-modules = $(addprefix $(objpfx),$(addsuffix .so,$(modules-names)))
 
 tst-atfork2mod.so-no-z-defs = yes
+tst-atfork3mod.so-no-z-defs = yes
+tst-atfork4mod.so-no-z-defs = yes
 tst-create1mod.so-no-z-defs = yes
 
 ifeq ($(build-shared),yes)
@@ -226,8 +246,18 @@ tst-atfork2-ENV = MALLOC_TRACE=$(objpfx)tst-atfork2.mtrace \
 		  LD_PRELOAD=$(common-objpfx)/malloc/libc_malloc_debug.so
 $(objpfx)tst-atfork2mod.so: $(shared-thread-library)
 
+$(objpfx)tst-atfork3: $(shared-thread-library)
+LDFLAGS-tst-atfork3 = -rdynamic
+$(objpfx)tst-atfork3mod.so: $(shared-thread-library)
+
+$(objpfx)tst-atfork4: $(shared-thread-library)
+LDFLAGS-tst-atfork4 = -rdynamic
+$(objpfx)tst-atfork4mod.so: $(shared-thread-library)
+
 ifeq ($(build-shared),yes)
 $(objpfx)tst-atfork2.out: $(objpfx)tst-atfork2mod.so
+$(objpfx)tst-atfork3.out: $(objpfx)tst-atfork3mod.so
+$(objpfx)tst-atfork4.out: $(objpfx)tst-atfork4mod.so
 endif
 
 ifeq ($(build-shared),yes)

sysdeps/pthread/tst-atfork3.c

@@ -0,0 +1,118 @@
+/* Check if pthread_atfork handler can call dlclose (BZ#24595).
+   Copyright (C) 2022 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
+   <http://www.gnu.org/licenses/>.  */
+
+#include <stdio.h>
+#include <pthread.h>
+#include <unistd.h>
+#include <stdlib.h>
+#include <stdbool.h>
+
+#include <support/check.h>
+#include <support/xthread.h>
+#include <support/capture_subprocess.h>
+#include <support/xdlfcn.h>
+
+/* Check if pthread_atfork handlers do not deadlock when calling a function
+   that might alter the internal fork handle list, such as dlclose.
+
+   The test registers a callback set with pthread_atfork(), dlopen() a shared
+   library (nptl/tst-atfork3mod.c), calls an exported symbol from the library
+   (which in turn also registers atfork handlers), and calls fork to trigger
+   the callbacks.  */
+
+static void *handler;
+static bool run_dlclose_prepare;
+static bool run_dlclose_parent;
+static bool run_dlclose_child;
+
+static void
+prepare (void)
+{
+  if (run_dlclose_prepare)
+    xdlclose (handler);
+}
+
+static void
+parent (void)
+{
+  if (run_dlclose_parent)
+    xdlclose (handler);
+}
+
+static void
+child (void)
+{
+  if (run_dlclose_child)
+    xdlclose (handler);
+}
+
+static void
+proc_func (void *closure)
+{
+}
+
+static void
+do_test_generic (bool dlclose_prepare, bool dlclose_parent, bool dlclose_child)
+{
+  run_dlclose_prepare = dlclose_prepare;
+  run_dlclose_parent = dlclose_parent;
+  run_dlclose_child = dlclose_child;
+
+  handler = xdlopen ("tst-atfork3mod.so", RTLD_NOW);
+
+  int (*atfork3mod_func)(void);
+  atfork3mod_func = xdlsym (handler, "atfork3mod_func");
+
+  atfork3mod_func ();
+
+  struct support_capture_subprocess proc
+    = support_capture_subprocess (proc_func, NULL);
+  support_capture_subprocess_check (&proc, "tst-atfork3", 0, sc_allow_none);
+
+  handler = atfork3mod_func = NULL;
+
+  support_capture_subprocess_free (&proc);
+}
+
+static void *
+thread_func (void *closure)
+{
+  return NULL;
+}
+
+static int
+do_test (void)
+{
+  {
+    /* Make the process acts as multithread.  */
+    pthread_attr_t attr;
+    xpthread_attr_init (&attr);
+    xpthread_attr_setdetachstate (&attr, PTHREAD_CREATE_DETACHED);
+    xpthread_create (&attr, thread_func, NULL);
+  }
+
+  TEST_COMPARE (pthread_atfork (prepare, parent, child), 0);
+
+  do_test_generic (true  /* prepare */, false /* parent */, false /* child */);
+  do_test_generic (false /* prepare */, true  /* parent */, false /* child */);
+  do_test_generic (false /* prepare */, false /* parent */, true  /* child */);
+
+  return 0;
+}
+
+#include <support/test-driver.c>

sysdeps/pthread/tst-atfork3mod.c

@@ -0,0 +1,44 @@
+/* Copyright (C) 2022 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
+   <http://www.gnu.org/licenses/>.  */
+
+#include <unistd.h>
+#include <stdlib.h>
+#include <pthread.h>
+
+#include <support/check.h>
+
+static void
+mod_prepare (void)
+{
+}
+
+static void
+mod_parent (void)
+{
+}
+
+static void
+mod_child (void)
+{
+}
+
+int atfork3mod_func (void)
+{
+  TEST_COMPARE (pthread_atfork (mod_prepare, mod_parent, mod_child), 0);
+
+  return 0;
+}

sysdeps/pthread/tst-atfork4.c

@@ -0,0 +1,128 @@
+/* pthread_atfork supports handlers that call pthread_atfork or dlclose.
+   Copyright (C) 2022 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/>.  */
+
+#include <support/xdlfcn.h>
+#include <stdio.h>
+#include <support/xthread.h>
+#include <sys/types.h>
+#include <sys/wait.h>
+#include <support/xunistd.h>
+#include <support/check.h>
+#include <stdlib.h>
+
+static void *
+thread_func (void *x)
+{
+  return NULL;
+}
+
+static unsigned int second_atfork_handler_runcount = 0;
+
+static void
+second_atfork_handler (void)
+{
+  second_atfork_handler_runcount++;
+}
+
+static void *h = NULL;
+
+static unsigned int atfork_handler_runcount = 0;
+
+static void
+prepare (void)
+{
+  /* These atfork handlers are registered while atfork handlers are being
+     executed and thus will not be executed during the corresponding
+     fork.  */
+  TEST_VERIFY_EXIT (pthread_atfork (second_atfork_handler,
+                                    second_atfork_handler,
+                                    second_atfork_handler) == 0);
+
+  /* This will de-register the atfork handlers registered by the dlopen'd
+     library and so they will not be executed.  */
+  if (h != NULL)
+    {
+      xdlclose (h);
+      h = NULL;
+    }
+
+  atfork_handler_runcount++;
+}
+
+static void
+after (void)
+{
+  atfork_handler_runcount++;
+}
+
+static int
+do_test (void)
+{
+  /* Make sure __libc_single_threaded is 0.  */
+  pthread_attr_t attr;
+  xpthread_attr_init (&attr);
+  xpthread_attr_setdetachstate (&attr, PTHREAD_CREATE_DETACHED);
+  xpthread_create (&attr, thread_func, NULL);
+
+  void (*reg_atfork_handlers) (void);
+
+  h = xdlopen ("tst-atfork4mod.so", RTLD_LAZY);
+
+  reg_atfork_handlers = xdlsym (h, "reg_atfork_handlers");
+
+  reg_atfork_handlers ();
+
+  /* We register our atfork handlers *after* loading the module so that our
+     prepare handler is called first at fork, where we then dlclose the
+     module before its prepare handler has a chance to be called.  */
+  TEST_VERIFY_EXIT (pthread_atfork (prepare, after, after) == 0);
+
+  pid_t pid = xfork ();
+
+  /* Both the parent and the child processes should observe this.  */
+  TEST_VERIFY_EXIT (atfork_handler_runcount == 2);
+  TEST_VERIFY_EXIT (second_atfork_handler_runcount == 0);
+
+  if (pid > 0)
+    {
+      int childstat;
+
+      xwaitpid (-1, &childstat, 0);
+      TEST_VERIFY_EXIT (WIFEXITED (childstat)
+                        && WEXITSTATUS (childstat) == 0);
+
+      /* This time, the second set of atfork handlers should also be called
+         since the handlers are already in place before fork is called.  */
+
+      pid = xfork ();
+
+      TEST_VERIFY_EXIT (atfork_handler_runcount == 4);
+      TEST_VERIFY_EXIT (second_atfork_handler_runcount == 2);
+
+      if (pid > 0)
+        {
+          xwaitpid (-1, &childstat, 0);
+          TEST_VERIFY_EXIT (WIFEXITED (childstat)
+                            && WEXITSTATUS (childstat) == 0);
+        }
+    }
+
+  return 0;
+}
+
+#include <support/test-driver.c>

sysdeps/pthread/tst-atfork4mod.c

@@ -0,0 +1,48 @@
+/* pthread_atfork supports handlers that call pthread_atfork or dlclose.
+   Copyright (C) 2022 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/>.  */
+
+#include <pthread.h>
+#include <stdlib.h>
+
+/* This dynamically loaded library simply registers its atfork handlers when
+   asked to.  The atfork handlers should never be executed because the
+   library is unloaded before fork is called by the test program.  */
+
+static void
+prepare (void)
+{
+  abort ();
+}
+
+static void
+parent (void)
+{
+  abort ();
+}
+
+static void
+child (void)
+{
+  abort ();
+}
+
+void
+reg_atfork_handlers (void)
+{
+  pthread_atfork (prepare, parent, child);
+}