[PATCH v3] dl-load: add memory barrier before updating the next

[Maninder Singh <maninder1 dot s at samsung dot com>]

[BZ #21349]: race condition between dl_open and rtld lazy symbol resolve.

This patch adds C11 memory barrier before updating the liblist next.

Issue Fix: race condition between add_name_to_object  & _dl_name_match_p.
One threads calling dlopen which further calls add_name_to_object &
other thread trying to resolve RTLD_LAZY symbols through
_dl_runtime_resolve which further calls.

_dl_name_match_p checks if libname->next is valid, then it assumes
libname->next->name to be valid. Also add_name_to_object initialized
name first and then sets valid next pointer.

This patch avoids any reorder of instruction when next is set before
name to avoid any race.

Signed-off-by: Vaneet Narang <v.narang@samsung.com>
Signed-off-by: Maninder Singh <maninder1.s@samsung.com>
---
v1 -> v2: use C11 atomics rather than direct memory barriers.
v2 -> v3: use comments for barriers and enter Bugzilla ID.

 elf/dl-load.c |    5 ++++-
 elf/dl-misc.c |    5 ++++-
 2 files changed, 8 insertions(+), 2 deletions(-)

diff --git a/elf/dl-load.c b/elf/dl-load.c
index a5318f9..03c6afb 100644
--- a/elf/dl-load.c
+++ b/elf/dl-load.c
@@ -418,7 +418,10 @@ 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;
+  /* We need release memory order here because we need to synchronize
+     with other thread doing _dl_runtime_resolve which calls _dl_name_match_p
+     to traverse all names added to libname_list*/
+  atomic_store_release (&(lastp->next), newname);
 }
 
 /* Standard search directories.  */
diff --git a/elf/dl-misc.c b/elf/dl-misc.c
index 1e9a6ee..a26d6f6 100644
--- a/elf/dl-misc.c
+++ b/elf/dl-misc.c
@@ -295,7 +295,10 @@ _dl_name_match_p (const char *name, const struct link_map *map)
     if (strcmp (name, runp->name) == 0)
       return 1;
     else
-      runp = runp->next;
+      /* We need to acquire memory order here because we need to synchronize
+         with other thread calling dlopen and adding new name to libname_list
+         through add_name_to_object */
+      runp = atomic_load_acquire(&(runp->next));
 
   return 0;
 }
-- 
1.7.1