[PATCH] malloc: Correct size computation in realloc for dumped fake mmapped chunks

[fweimer at redhat dot com (Florian Weimer)]

For regular mmapped chunks there are two size fields (hence a reduction
by 2 * SIZE_SZ bytes), but for fake chunks, we only have one size field,
so we need to subtract SIZE_SZ bytes.

This was initially reported as Emacs bug 23726.

2016-06-08  Florian Weimer  <fweimer@redhat.com>

	Emacs bug 23726.
	* malloc/malloc.c (dumped_main_arena_start): Update comment.
	(__libc_realloc): Correct size computation for dumped fake mmapped
	chunks.

diff --git a/malloc/malloc.c b/malloc/malloc.c
index ead9a21..6f77d37 100644
--- a/malloc/malloc.c
+++ b/malloc/malloc.c
@@ -1748,7 +1748,9 @@ static struct malloc_state main_arena =
 
 /* These variables are used for undumping support.  Chunked are marked
    as using mmap, but we leave them alone if they fall into this
-   range.  */
+   range.  NB: The chunk size for these chunks only includes the
+   initial size field (of SIZE_SZ bytes), there is no trailing size
+   field (unlike with regular mmapped chunks).  */
 static mchunkptr dumped_main_arena_start; /* Inclusive.  */
 static mchunkptr dumped_main_arena_end;   /* Exclusive.  */
 
@@ -3029,9 +3031,11 @@ __libc_realloc (void *oldmem, size_t bytes)
 	  if (newmem == 0)
 	    return NULL;
 	  /* Copy as many bytes as are available from the old chunk
-	     and fit into the new size.  */
-	  if (bytes > oldsize - 2 * SIZE_SZ)
-	    bytes = oldsize - 2 * SIZE_SZ;
+	     and fit into the new size.  NB: The overhead for faked
+	     mmapped chunks is only SIZE_SZ, not 2 * SIZE_SZ as for
+	     regular mmapped chunks.  */
+	  if (bytes > oldsize - SIZE_SZ)
+	    bytes = oldsize - SIZE_SZ;
 	  memcpy (newmem, oldmem, bytes);
 	  return newmem;
 	}