Re: [Qemu-devel] [Qemu-block] [PATCH for-2.11] qcow2: Fix overly broad madvise()

[Darren Kenny]

FWIW,

Reviewed-by: Darren Kenny <address@hidden>

Thanks,

Darren.

On Tue, Nov 14, 2017 at 07:41:27PM +0100, Max Reitz wrote:
> @mem_size and @offset are both size_t, thus subtracting them from one
> another will just return a big size_t if mem_size < offset -- even more
> obvious here because the result is stored in another size_t.
>
> Checking that result to be positive is therefore not sufficient to
> excluse the case that offset > mem_size.  Thus, we currently sometimes
> issue an madvise() over a very large address range.
>
> This is triggered by iotest 163, but with -m64, this does not result in
> tangible problems.  But with -m32, this test produces three segfaults,
> all of which are fixed by this patch.
>
> Signed-off-by: Max Reitz <address@hidden>
> ---
> block/qcow2-cache.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/block/qcow2-cache.c b/block/qcow2-cache.c
> index 75746a7f43..5222a7b94d 100644
> --- a/block/qcow2-cache.c
> +++ b/block/qcow2-cache.c
> @@ -73,7 +73,7 @@ static void qcow2_cache_table_release(BlockDriverState *bs, 
> Qcow2Cache *c,
>     size_t mem_size = (size_t) s->cluster_size * num_tables;
>     size_t offset = QEMU_ALIGN_UP((uintptr_t) t, align) - (uintptr_t) t;
>     size_t length = QEMU_ALIGN_DOWN(mem_size - offset, align);
> -    if (length > 0) {
> +    if (mem_size > offset && length > 0) {
>         madvise((uint8_t *) t + offset, length, MADV_DONTNEED);
>     }
> #endif
> --
> 2.13.6