Re: [Qemu-devel] [PATCH 6/8] qcow2: add autoclear bit for dirty bitmaps

[Stefan Hajnoczi]

On Mon, Jun 08, 2015 at 06:21:24PM +0300, Vladimir Sementsov-Ogievskiy wrote:
> @@ -928,8 +936,9 @@ static int qcow2_open(BlockDriverState *bs, QDict 
> *options, int flags,
>      }
>  
>      /* Clear unknown autoclear feature bits */
> -    if (!bs->read_only && !(flags & BDRV_O_INCOMING) && 
> s->autoclear_features) {
> -        s->autoclear_features = 0;
> +    if (!bs->read_only && !(flags & BDRV_O_INCOMING) &&
> +        (s->autoclear_features & ~QCOW2_AUTOCLEAR_MASK)) {
> +        s->autoclear_features |= QCOW2_AUTOCLEAR_MASK;

This should be bitwise-and instead of bitwise-or:

s->autoclear_features &= QCOW2_AUTOCLEAR_MASK

Otherwise we set features that happen to be in QCOW2_AUTOCLEAR_MASK but
were not enabled by the user.  Right now that's not fatal but if other
features are added to QCOW2_AUTOCLEAR_MASK it could introduce a bug,
depending on the feature semantics.

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