[RFC][PATCH 4/7] malloc: Ensure lower bound on chunk size in __libc_realloc.

[Istvan Kurucsai <pistukem at gmail dot com>]

Under some circumstances, a chunk size of SIZE_SZ could lead to an underflow
when calculating the length argument of memcpy.

   * malloc/malloc.c (__libc_realloc): Check chunk size.
---
 malloc/malloc.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/malloc/malloc.c b/malloc/malloc.c
index 873aa29..424c69d 100644
--- a/malloc/malloc.c
+++ b/malloc/malloc.c
@@ -2989,8 +2989,9 @@ __libc_realloc (void *oldmem, size_t bytes)
      accident or by "design" from some intruder.  We need to bypass
      this check for dumped fake mmap chunks from the old main arena
      because the new malloc may provide additional alignment.  */
-  if ((__builtin_expect ((uintptr_t) oldp > (uintptr_t) -oldsize, 0)
-       || __builtin_expect (misaligned_chunk (oldp), 0))
+  if ((__glibc_unlikely ((uintptr_t) oldp > (uintptr_t) -oldsize)
+       || __glibc_unlikely (misaligned_chunk (oldp))
+       || __glibc_unlikely (oldsize <= 2 * SIZE_SZ))
       && !DUMPED_MAIN_ARENA_CHUNK (oldp))
     {
       malloc_printerr (check_action, "realloc(): invalid pointer", oldmem,
-- 
2.7.4