Re: [PATCH v2 1/7] malloc: Add check for top size corruption.

[Istvan Kurucsai <pistukem@gmail.com>]

> Andreas already pointed out style issues.
>
> I'm somewhat surprised that we have accurate accounting in av->system_mem.
>
> Furthermore, for non-main arenas, I think the check should be against the
> size of a single heap, or maybe the minimum of av->system_mem and that size.

I thought about this and believe that we can ensure something more
strict: that the end of the top chunk is the same as the end of the
arena (contiguous main_arena case) or the heap (mmapped arena case),
see below. Tests passed but I'm a bit uncertain if these invariants
are always held.


Ensure that the end of the top chunk is the same as
 the end of the arena/heap.

    * malloc/malloc.c (_int_malloc): Check top size.
---
 malloc/malloc.c | 29 +++++++++++++++++++++++++++++
 1 file changed, 29 insertions(+)

diff --git a/malloc/malloc.c b/malloc/malloc.c
index f5aafd2..fd0f001 100644
--- a/malloc/malloc.c
+++ b/malloc/malloc.c
@@ -2251,6 +2251,33 @@ do_check_malloc_state (mstate av)
 }
 #endif

+static bool
+valid_top_chunk (mstate av, mchunkptr top)
+{
+  size_t size = chunksize(top);
+
+  assert (av);
+  assert (av->top != initial_top (av));
+
+  if (av == &main_arena)
+    {
+      if ((contiguous (&main_arena)
+          && __glibc_unlikely ((uintptr_t) top + size
+                               != (uintptr_t) mp_.sbrk_base + av->system_mem))
+          || (!contiguous (&main_arena)
+              && __glibc_unlikely (size > av->system_mem)))
+        return false;
+    }
+  else
+    {
+      heap_info *heap = heap_for_ptr (top);
+      uintptr_t heap_end = (uintptr_t) heap + heap->size;
+      if (__glibc_unlikely ((uintptr_t) top + size != heap_end))
+        return false;
+    }
+
+  return true;
+}

 /* ----------------- Support for debugging hooks -------------------- */
 #include "hooks.c"
@@ -4088,6 +4115,8 @@ _int_malloc (mstate av, size_t bytes)

       if ((unsigned long) (size) >= (unsigned long) (nb + MINSIZE))
         {
+          if (__glibc_unlikely (!valid_top_chunk (av, victim)))
+            malloc_printerr ("malloc(): corrupted top chunk");
           remainder_size = size - nb;
           remainder = chunk_at_offset (victim, nb);
           av->top = remainder;
-- 
2.7.4