elf: Fix data race in _dl_name_match_p [BZ #21349]

dlopen updates libname_list by writing to lastp->next, but concurrent
reads in _dl_name_match_p were not synchronized when it was called
without holding GL(dl_load_lock), which can happen during lazy symbol
resolution.

This patch fixes the race between _dl_name_match_p reading lastp->next
and add_name_to_object writing to it. This could cause segfault on
targets with weak memory order when lastp->next->name is read, which
was observed on an arm system. Fixes bug 21349.

(Code is from Maninder Singh, comments and description is from Szabolcs
Nagy.)

Co-authored-by: Vaneet Narang <v.narang@samsung.com>
Co-authored-by: Szabolcs Nagy <szabolcs.nagy@arm.com>
Reviewed-by: Adhemerval Zanella  <adhemerval.zanella@linaro.org>

elf/dl-load.c

@@ -438,7 +438,23 @@ add_name_to_object (struct link_map *l, const char *name)
   newname->name = memcpy (newname + 1, name, name_len);
   newname->next = NULL;
   newname->dont_free = 0;
-  lastp->next = newname;
+  /* CONCURRENCY NOTES:
+
+     Make sure the initialization of newname happens before its address is
+     read from the lastp->next store below.
+
+     GL(dl_load_lock) is held here (and by other writers, e.g. dlclose), so
+     readers of libname_list->next (e.g. _dl_check_caller or the reads above)
+     can use that for synchronization, however the read in _dl_name_match_p
+     may be executed without holding the lock during _dl_runtime_resolve
+     (i.e. lazy symbol resolution when a function of library l is called).
+
+     The release MO store below synchronizes with the acquire MO load in
+     _dl_name_match_p.  Other writes need to synchronize with that load too,
+     however those happen either early when the process is single threaded
+     (dl_main) or when the library is unloaded (dlclose) and the user has to
+     synchronize library calls with unloading.  */
+  atomic_store_release (&lastp->next, newname);
 }
 
 /* Standard search directories.  */

elf/dl-misc.c

@@ -347,7 +347,9 @@ _dl_name_match_p (const char *name, const struct link_map *map)
     if (strcmp (name, runp->name) == 0)
       return 1;
     else
-      runp = runp->next;
+      /* Synchronize with the release MO store in add_name_to_object.
+	 See CONCURRENCY NOTES in add_name_to_object in dl-load.c.  */
+      runp = atomic_load_acquire (&runp->next);
 
   return 0;
 }