elf: Avoid deadlock between pthread_create and ctors [BZ #28357]

The fix for bug 19329 caused a regression such that pthread_create can
deadlock when concurrent ctors from dlopen are waiting for it to finish.
Use a new GL(dl_load_tls_lock) in pthread_create that is not taken
around ctors in dlopen.

The new lock is also used in __tls_get_addr instead of GL(dl_load_lock).

The new lock is held in _dl_open_worker and _dl_close_worker around
most of the logic before/after the init/fini routines.  When init/fini
routines are running then TLS is in a consistent, usable state.
In _dl_open_worker the new lock requires catching and reraising dlopen
failures that happen in the critical section.

The new lock is reinitialized in a fork child, to keep the existing
behaviour and it is kept recursive in case malloc interposition or TLS
access from signal handlers can retake it.  It is not obvious if this
is necessary or helps, but avoids changing the preexisting behaviour.

The new lock may be more appropriate for dl_iterate_phdr too than
GL(dl_load_write_lock), since TLS state of an incompletely loaded
module may be accessed.  If the new lock can replace the old one,
that can be a separate change.

Fixes bug 28357.

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

elf/dl-close.c

@@ -549,6 +549,9 @@ _dl_close_worker (struct link_map *map, bool force)
   size_t tls_free_end;
   tls_free_start = tls_free_end = NO_TLS_OFFSET;
 
+  /* Protects global and module specitic TLS state.  */
+  __rtld_lock_lock_recursive (GL(dl_load_tls_lock));
+
   /* We modify the list of loaded objects.  */
   __rtld_lock_lock_recursive (GL(dl_load_write_lock));
 
@@ -784,6 +787,9 @@ _dl_close_worker (struct link_map *map, bool force)
 	GL(dl_tls_static_used) = tls_free_start;
     }
 
+  /* TLS is cleaned up for the unloaded modules.  */
+  __rtld_lock_unlock_recursive (GL(dl_load_tls_lock));
+
 #ifdef SHARED
   /* Auditing checkpoint: we have deleted all objects.  */
   if (__glibc_unlikely (do_audit))

elf/dl-open.c

@@ -66,6 +66,9 @@ struct dl_open_args
      libc_map value in the namespace in case of a dlopen failure.  */
   bool libc_already_loaded;
 
+  /* Set to true if the end of dl_open_worker_begin was reached.  */
+  bool worker_continue;
+
   /* Original parameters to the program and the current environment.  */
   int argc;
   char **argv;
@@ -482,7 +485,7 @@ call_dl_init (void *closure)
 }
 
 static void
-dl_open_worker (void *a)
+dl_open_worker_begin (void *a)
 {
   struct dl_open_args *args = a;
   const char *file = args->file;
@@ -774,6 +777,36 @@ dl_open_worker (void *a)
       _dl_call_libc_early_init (libc_map, false);
     }
 
+  args->worker_continue = true;
+}
+
+static void
+dl_open_worker (void *a)
+{
+  struct dl_open_args *args = a;
+
+  args->worker_continue = false;
+
+  {
+    /* Protects global and module specific TLS state.  */
+    __rtld_lock_lock_recursive (GL(dl_load_tls_lock));
+
+    struct dl_exception ex;
+    int err = _dl_catch_exception (&ex, dl_open_worker_begin, args);
+
+    __rtld_lock_unlock_recursive (GL(dl_load_tls_lock));
+
+    if (__glibc_unlikely (ex.errstring != NULL))
+      /* Reraise the error.  */
+      _dl_signal_exception (err, &ex, NULL);
+  }
+
+  if (!args->worker_continue)
+    return;
+
+  int mode = args->mode;
+  struct link_map *new = args->map;
+
   /* Run the initializer functions of new objects.  Temporarily
      disable the exception handler, so that lazy binding failures are
      fatal.  */

elf/dl-support.c

@@ -228,6 +228,13 @@ __rtld_lock_define_initialized_recursive (, _dl_load_lock)
    list of loaded objects while an object is added to or removed from
    that list.  */
 __rtld_lock_define_initialized_recursive (, _dl_load_write_lock)
+  /* This lock protects global and module specific TLS related data.
+     E.g. it is held in dlopen and dlclose when GL(dl_tls_generation),
+     GL(dl_tls_max_dtv_idx) or GL(dl_tls_dtv_slotinfo_list) are
+     accessed and when TLS related relocations are processed for a
+     module.  It was introduced to keep pthread_create accessing TLS
+     state that is being set up.  */
+__rtld_lock_define_initialized_recursive (, _dl_load_tls_lock)
 
 
 #ifdef HAVE_AUX_VECTOR

elf/dl-tls.c

@@ -532,7 +532,7 @@ _dl_allocate_tls_init (void *result)
   size_t maxgen = 0;
 
   /* Protects global dynamic TLS related state.  */
-  __rtld_lock_lock_recursive (GL(dl_load_lock));
+  __rtld_lock_lock_recursive (GL(dl_load_tls_lock));
 
   /* Check if the current dtv is big enough.   */
   if (dtv[-1].counter < GL(dl_tls_max_dtv_idx))
@@ -606,7 +606,7 @@ _dl_allocate_tls_init (void *result)
       listp = listp->next;
       assert (listp != NULL);
     }
-  __rtld_lock_unlock_recursive (GL(dl_load_lock));
+  __rtld_lock_unlock_recursive (GL(dl_load_tls_lock));
 
   /* The DTV version is up-to-date now.  */
   dtv[0].counter = maxgen;
@@ -745,7 +745,7 @@ _dl_update_slotinfo (unsigned long int req_modid)
 
 	 Here the dtv needs to be updated to new_gen generation count.
 
-	 This code may be called during TLS access when GL(dl_load_lock)
+	 This code may be called during TLS access when GL(dl_load_tls_lock)
 	 is not held.  In that case the user code has to synchronize with
 	 dlopen and dlclose calls of relevant modules.  A module m is
 	 relevant if the generation of m <= new_gen and dlclose of m is
@@ -867,11 +867,11 @@ tls_get_addr_tail (GET_ADDR_ARGS, dtv_t *dtv, struct link_map *the_map)
   if (__glibc_unlikely (the_map->l_tls_offset
 			!= FORCED_DYNAMIC_TLS_OFFSET))
     {
-      __rtld_lock_lock_recursive (GL(dl_load_lock));
+      __rtld_lock_lock_recursive (GL(dl_load_tls_lock));
       if (__glibc_likely (the_map->l_tls_offset == NO_TLS_OFFSET))
 	{
 	  the_map->l_tls_offset = FORCED_DYNAMIC_TLS_OFFSET;
-	  __rtld_lock_unlock_recursive (GL(dl_load_lock));
+	  __rtld_lock_unlock_recursive (GL(dl_load_tls_lock));
 	}
       else if (__glibc_likely (the_map->l_tls_offset
 			       != FORCED_DYNAMIC_TLS_OFFSET))
@@ -883,7 +883,7 @@ tls_get_addr_tail (GET_ADDR_ARGS, dtv_t *dtv, struct link_map *the_map)
 #else
 # error "Either TLS_TCB_AT_TP or TLS_DTV_AT_TP must be defined"
 #endif
-	  __rtld_lock_unlock_recursive (GL(dl_load_lock));
+	  __rtld_lock_unlock_recursive (GL(dl_load_tls_lock));
 
 	  dtv[GET_ADDR_MODULE].pointer.to_free = NULL;
 	  dtv[GET_ADDR_MODULE].pointer.val = p;
@@ -891,7 +891,7 @@ tls_get_addr_tail (GET_ADDR_ARGS, dtv_t *dtv, struct link_map *the_map)
 	  return (char *) p + GET_ADDR_OFFSET;
 	}
       else
-	__rtld_lock_unlock_recursive (GL(dl_load_lock));
+	__rtld_lock_unlock_recursive (GL(dl_load_tls_lock));
     }
   struct dtv_pointer result = allocate_and_init (the_map);
   dtv[GET_ADDR_MODULE].pointer = result;
@@ -962,7 +962,7 @@ _dl_tls_get_addr_soft (struct link_map *l)
     return NULL;
 
   dtv_t *dtv = THREAD_DTV ();
-  /* This may be called without holding the GL(dl_load_lock).  Reading
+  /* This may be called without holding the GL(dl_load_tls_lock).  Reading
      arbitrary gen value is fine since this is best effort code.  */
   size_t gen = atomic_load_relaxed (&GL(dl_tls_generation));
   if (__glibc_unlikely (dtv[0].counter != gen))

elf/rtld.c

@@ -322,6 +322,7 @@ struct rtld_global _rtld_global =
 #ifdef _LIBC_REENTRANT
     ._dl_load_lock = _RTLD_LOCK_RECURSIVE_INITIALIZER,
     ._dl_load_write_lock = _RTLD_LOCK_RECURSIVE_INITIALIZER,
+    ._dl_load_tls_lock = _RTLD_LOCK_RECURSIVE_INITIALIZER,
 #endif
     ._dl_nns = 1,
     ._dl_ns =

posix/fork.c

@@ -99,6 +99,9 @@ __libc_fork (void)
       /* Reset the lock the dynamic loader uses to protect its data.  */
       __rtld_lock_initialize (GL(dl_load_lock));
 
+      /* Reset the lock protecting dynamic TLS related data.  */
+      __rtld_lock_initialize (GL(dl_load_tls_lock));
+
       reclaim_stacks ();
 
       /* Run the handlers registered for the child.  */

sysdeps/generic/ldsodefs.h

@@ -369,6 +369,13 @@ struct rtld_global
      list of loaded objects while an object is added to or removed
      from that list.  */
   __rtld_lock_define_recursive (EXTERN, _dl_load_write_lock)
+  /* This lock protects global and module specific TLS related data.
+     E.g. it is held in dlopen and dlclose when GL(dl_tls_generation),
+     GL(dl_tls_max_dtv_idx) or GL(dl_tls_dtv_slotinfo_list) are
+     accessed and when TLS related relocations are processed for a
+     module.  It was introduced to keep pthread_create accessing TLS
+     state that is being set up.  */
+  __rtld_lock_define_recursive (EXTERN, _dl_load_tls_lock)
 
   /* Incremented whenever something may have been added to dl_loaded.  */
   EXTERN unsigned long long _dl_load_adds;
@@ -1268,7 +1275,7 @@ extern int _dl_scope_free (void *) attribute_hidden;
 
 /* Add module to slot information data.  If DO_ADD is false, only the
    required memory is allocated.  Must be called with GL
-   (dl_load_lock) acquired.  If the function has already been called
+   (dl_load_tls_lock) acquired.  If the function has already been called
    for the link map L with !do_add, then this function will not raise
    an exception, otherwise it is possible that it encounters a memory
    allocation failure.  */

sysdeps/pthread/Makefile

@@ -152,15 +152,17 @@ 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
+tests += tst-atfork2 tst-pt-tls4 tst-_res1 tst-fini1 tst-create1
 tests-nolibpthread += tst-fini1
 endif
 
 modules-names += tst-atfork2mod tst-tls4moda tst-tls4modb \
-		 tst-_res1mod1 tst-_res1mod2 tst-fini1mod
+		 tst-_res1mod1 tst-_res1mod2 tst-fini1mod \
+		 tst-create1mod
 test-modules = $(addprefix $(objpfx),$(addsuffix .so,$(modules-names)))
 
 tst-atfork2mod.so-no-z-defs = yes
+tst-create1mod.so-no-z-defs = yes
 
 ifeq ($(build-shared),yes)
 # Build all the modules even when not actually running test programs.
@@ -279,4 +281,8 @@ LDFLAGS-tst-join7mod.so = -Wl,-soname,tst-join7mod.so
 
 CFLAGS-tst-unwind-thread.c += -funwind-tables
 
+LDFLAGS-tst-create1 = -Wl,-export-dynamic
+$(objpfx)tst-create1: $(shared-thread-library)
+$(objpfx)tst-create1.out: $(objpfx)tst-create1mod.so
+
 endif

sysdeps/pthread/tst-create1.c

@@ -0,0 +1,119 @@
+/* Verify that pthread_create does not deadlock when ctors take locks.
+   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/>.  */
+
+#include <stdio.h>
+#include <support/xdlfcn.h>
+#include <support/xthread.h>
+
+/*
+Check if ctor and pthread_create deadlocks in
+
+thread 1: dlopen -> ctor -> lock(user_lock)
+thread 2: lock(user_lock) -> pthread_create
+
+or in
+
+thread 1: dlclose -> dtor -> lock(user_lock)
+thread 2: lock(user_lock) -> pthread_create
+*/
+
+static pthread_barrier_t bar_ctor;
+static pthread_barrier_t bar_dtor;
+static pthread_mutex_t user_lock = PTHREAD_MUTEX_INITIALIZER;
+
+void
+ctor (void)
+{
+  xpthread_barrier_wait (&bar_ctor);
+  dprintf (1, "thread 1: in ctor: started.\n");
+  xpthread_mutex_lock (&user_lock);
+  dprintf (1, "thread 1: in ctor: locked user_lock.\n");
+  xpthread_mutex_unlock (&user_lock);
+  dprintf (1, "thread 1: in ctor: unlocked user_lock.\n");
+  dprintf (1, "thread 1: in ctor: done.\n");
+}
+
+void
+dtor (void)
+{
+  xpthread_barrier_wait (&bar_dtor);
+  dprintf (1, "thread 1: in dtor: started.\n");
+  xpthread_mutex_lock (&user_lock);
+  dprintf (1, "thread 1: in dtor: locked user_lock.\n");
+  xpthread_mutex_unlock (&user_lock);
+  dprintf (1, "thread 1: in dtor: unlocked user_lock.\n");
+  dprintf (1, "thread 1: in dtor: done.\n");
+}
+
+static void *
+thread3 (void *a)
+{
+  dprintf (1, "thread 3: started.\n");
+  dprintf (1, "thread 3: done.\n");
+  return 0;
+}
+
+static void *
+thread2 (void *a)
+{
+  pthread_t t3;
+  dprintf (1, "thread 2: started.\n");
+
+  xpthread_mutex_lock (&user_lock);
+  dprintf (1, "thread 2: locked user_lock.\n");
+  xpthread_barrier_wait (&bar_ctor);
+  t3 = xpthread_create (0, thread3, 0);
+  xpthread_mutex_unlock (&user_lock);
+  dprintf (1, "thread 2: unlocked user_lock.\n");
+  xpthread_join (t3);
+
+  xpthread_mutex_lock (&user_lock);
+  dprintf (1, "thread 2: locked user_lock.\n");
+  xpthread_barrier_wait (&bar_dtor);
+  t3 = xpthread_create (0, thread3, 0);
+  xpthread_mutex_unlock (&user_lock);
+  dprintf (1, "thread 2: unlocked user_lock.\n");
+  xpthread_join (t3);
+
+  dprintf (1, "thread 2: done.\n");
+  return 0;
+}
+
+static void
+thread1 (void)
+{
+  dprintf (1, "thread 1: started.\n");
+  xpthread_barrier_init (&bar_ctor, NULL, 2);
+  xpthread_barrier_init (&bar_dtor, NULL, 2);
+  pthread_t t2 = xpthread_create (0, thread2, 0);
+  void *p = xdlopen ("tst-create1mod.so", RTLD_NOW | RTLD_GLOBAL);
+  dprintf (1, "thread 1: dlopen done.\n");
+  xdlclose (p);
+  dprintf (1, "thread 1: dlclose done.\n");
+  xpthread_join (t2);
+  dprintf (1, "thread 1: done.\n");
+}
+
+static int
+do_test (void)
+{
+  thread1 ();
+  return 0;
+}
+
+#include <support/test-driver.c>

sysdeps/pthread/tst-create1mod.c

@@ -0,0 +1,41 @@
+/* Verify that pthread_create does not deadlock when ctors take locks.
+   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/>.  */
+
+#include <stdio.h>
+
+/* Require TLS setup for the module.  */
+__thread int tlsvar;
+
+void ctor (void);
+void dtor (void);
+
+static void __attribute__ ((constructor))
+do_init (void)
+{
+  dprintf (1, "constructor started: %d.\n", tlsvar++);
+  ctor ();
+  dprintf (1, "constructor done: %d.\n", tlsvar++);
+}
+
+static void __attribute__ ((destructor))
+do_end (void)
+{
+  dprintf (1, "destructor started: %d.\n", tlsvar++);
+  dtor ();
+  dprintf (1, "destructor done: %d.\n", tlsvar++);
+}