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>

5 files changed, 56 insertions, 9 deletions

diff --git a/elf/dl-close.c b/elf/dl-close.c
index 93ff5c9..cfe0f1c 100644
--- a/elf/dl-close.c
+++ b/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))
diff --git a/elf/dl-open.c b/elf/dl-open.c
index 5295e93..6ea5dd2 100644
--- a/elf/dl-open.c
+++ b/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.  */
diff --git a/elf/dl-support.c b/elf/dl-support.c
index 02e2ed7..d99c1f1 100644
--- a/elf/dl-support.c
+++ b/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
diff --git a/elf/dl-tls.c b/elf/dl-tls.c
index d554ae4..9260d2d 100644
--- a/elf/dl-tls.c
+++ b/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))
diff --git a/elf/rtld.c b/elf/rtld.c
index 8d2bba3..9642eb9 100644
--- a/elf/rtld.c
+++ b/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 =