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Re: [Qemu-devel] [PATCH] qcow2: fix range check


From: Kevin Wolf
Subject: Re: [Qemu-devel] [PATCH] qcow2: fix range check
Date: Mon, 12 Sep 2011 10:43:15 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:6.0) Gecko/20110816 Thunderbird/6.0

Am 10.09.2011 10:23, schrieb Frediano Ziglio:
> QCowL2Meta::offset is not cluster aligned but only sector aligned
> however nb_clusters count cluster from cluster start.
> This fix range check. Note that old code have no corruption issues
> related to this check cause it only cause intersection to occur
> when shouldn't.

Are you sure? See below. (I think it doesn't corrupt the image, but for
a different reason)

> 
> Signed-off-by: Frediano Ziglio <address@hidden>
> ---
>  block/qcow2-cluster.c |   14 +++++++-------
>  1 files changed, 7 insertions(+), 7 deletions(-)
> 
> diff --git a/block/qcow2-cluster.c b/block/qcow2-cluster.c
> index 428b5ad..2f76311 100644
> --- a/block/qcow2-cluster.c
> +++ b/block/qcow2-cluster.c
> @@ -776,17 +776,17 @@ again:
>       */
>      QLIST_FOREACH(old_alloc, &s->cluster_allocs, next_in_flight) {
>  
> -        uint64_t end_offset = offset + nb_clusters * s->cluster_size;
> -        uint64_t old_offset = old_alloc->offset;
> -        uint64_t old_end_offset = old_alloc->offset +
> -            old_alloc->nb_clusters * s->cluster_size;
> +        uint64_t start = offset >> s->cluster_bits;
> +        uint64_t end = start + nb_clusters;
> +        uint64_t old_start = old_alloc->offset >> s->cluster_bits;
> +        uint64_t old_end = old_start + old_alloc->nb_clusters;
>  
> -        if (end_offset < old_offset || offset > old_end_offset) {
> +        if (end < old_start || start > old_end) {
>              /* No intersection */

Consider request A from 0x0 + 0x1000 bytes and request B from 0x2000 +
0x1000 bytes. Both touch the same cluster and therefore should be
serialised, but 0x2000 > 0x1000, so we decided here that there is no
intersection and we don't have to care.

Note that this doesn't corrupt the image, qcow2 can handle parallel
requests allocating the same cluster. In qcow2_alloc_cluster_link_l2()
we get an additional COW operation, so performance will be hurt, but
correctness is maintained.

>          } else {
> -            if (offset < old_offset) {
> +            if (start < old_start) {
>                  /* Stop at the start of a running allocation */
> -                nb_clusters = (old_offset - offset) >> s->cluster_bits;
> +                nb_clusters = old_start - start;
>              } else {
>                  nb_clusters = 0;
>              }

Anyway, the patch looks good. Applied to the block branch.

Kevin



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