Re: [PATCH 01/16] malloc: Fix a realloc crash with heap tagging [BZ 27468]

[DJ Delorie <dj@redhat.com>]

Szabolcs Nagy <szabolcs.nagy@arm.com> writes:
> diff --git a/malloc/malloc.c b/malloc/malloc.c
> index 1f4bbd8edf..10ea6aa441 100644
> --- a/malloc/malloc.c
> +++ b/malloc/malloc.c
> @@ -3446,7 +3446,9 @@ __libc_realloc (void *oldmem, size_t bytes)
>        newp = __libc_malloc (bytes);
>        if (newp != NULL)
>          {
> -          memcpy (newp, oldmem, oldsize - SIZE_SZ);

> +	  size_t sz = CHUNK_AVAILABLE_SIZE (oldp) - CHUNK_HDR_SZ;

I think this is semantically wrong, because the chunk size
(mptr->mchunk_size) does not include the mchunk_prev_size that's
accounted for in CHUNK_HDR_SZ.  I suspect the problem is that
CHUNK_AVAILABLE_SIZE is wrong, in that it adds SIZE_SZ in the non-tagged
case, and shouldn't, or that it's defined (or named) wrong.

chunksize(p) is the difference between this chunk and the corresponding
address in the next chunk.  i.e. it's prev_ptr to prev_ptr, or
user-bytes to user-bytes.

A "chunk pointer" does NOT point to the beginning of the chunk, but to
the prev_ptr in the PREVIOUS chunk.  So CHUNK_HDR_SZ is the offset from
a chunk pointer to the user data, but it is NOT the difference between
the chunk size and the user data size.  Using CHUNK_HDR_SZ in any
user-data-size computations is suspect logic.

That the resulting value happens to be correct is irrelevent here,
although I suspect it will be off by a word when tagging is enabled, and
not memcpy enough data, if the prev_ptr word is still part of the "user
data" when tagging is enabled.