[PATCH v2 5/7] malloc: Verify the integrity of mmapped chunks in calloc.

[Florian Weimer]

On 11/07/2017 04:27 PM, Istvan Kurucsai wrote:
>      * malloc/malloc.c (__libc_calloc): Check mmapped chunks.
> ---
>   malloc/malloc.c | 9 +++++++++
>   1 file changed, 9 insertions(+)
> 
> diff --git a/malloc/malloc.c b/malloc/malloc.c
> index 8e48952..5eb661e 100644
> --- a/malloc/malloc.c
> +++ b/malloc/malloc.c
> @@ -3447,6 +3447,15 @@ __libc_calloc (size_t n, size_t elem_size)
>     /* Two optional cases in which clearing not necessary */
>     if (chunk_is_mmapped (p))
>       {
> +      size_t pagesize = GLRO (dl_pagesize);
> +      INTERNAL_SIZE_T offset = prev_size (p);
> +      INTERNAL_SIZE_T size = chunksize (p);
> +      uintptr_t block = (uintptr_t) p - offset;
> +      size_t total_size = offset + size;
> +      if (__glibc_unlikely ((block | total_size) & (pagesize - 1)) != 0
> +          || __glibc_unlikely (!powerof2 ((uintptr_t) mem & (pagesize - 1))))
> +        malloc_printerr ("calloc(): invalid mmapped chunk");
> +
>         if (__builtin_expect (perturb_byte, 0))
>           return memset (mem, 0, sz);

Sorry, I don't understand the powerof2 check.  Could you elaborate?

Thanks,
Florian