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

[Darren Kenny <darren.kenny@oracle.com>]

FWIW,

Reviewed-by: Darren Kenny <darren.kenny@oracle.com>

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 <mreitz@redhat.com>
>---
> 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
>
>