[PATCH v2] dl-load: add memory barrier before updating the next

[Maninder Singh]

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

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

diff --git a/elf/dl-load.c b/elf/dl-load.c
index a5318f9..52df931 100644
--- a/elf/dl-load.c
+++ b/elf/dl-load.c
@@ -418,7 +418,7 @@ 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;
+  atomic_store_release (&(lastp->next), newname);
 }
 
 /* Standard search directories.  */
diff --git a/elf/dl-misc.c b/elf/dl-misc.c
index 1e9a6ee..8f48751 100644
--- a/elf/dl-misc.c
+++ b/elf/dl-misc.c
@@ -295,7 +295,7 @@ _dl_name_match_p (const char *name, const struct link_map *map)
     if (strcmp (name, runp->name) == 0)
       return 1;
     else
-      runp = runp->next;
+      runp = atomic_load_acquire(&(runp->next));
 
   return 0;
 }
-- 
1.7.1