On 06/03/2016 11:21 AM, Kevin Wolf wrote: > This patch changes the units that qcow2_get_cluster_offset() uses > internally, without touching the interface just yet. This will be done > in another patch. > > Signed-off-by: Kevin Wolf <kw...@redhat.com> > --- > block/qcow2-cluster.c | 43 ++++++++++++++++++++++--------------------- > 1 file changed, 22 insertions(+), 21 deletions(-) > > diff --git a/block/qcow2-cluster.c b/block/qcow2-cluster.c > index d901d89..b2405b1 100644 > --- a/block/qcow2-cluster.c > +++ b/block/qcow2-cluster.c > @@ -482,29 +482,27 @@ int qcow2_get_cluster_offset(BlockDriverState *bs, > uint64_t offset, > unsigned int l2_index; > uint64_t l1_index, l2_offset, *l2_table; > int l1_bits, c; > - unsigned int index_in_cluster, nb_clusters; > - uint64_t nb_available, nb_needed; > + unsigned int offset_in_cluster, nb_clusters; > + uint64_t bytes_available, bytes_needed; > int ret; > + unsigned int bytes; > > - index_in_cluster = (offset >> 9) & (s->cluster_sectors - 1); > - nb_needed = *num + index_in_cluster; > + bytes = *num * BDRV_SECTOR_SIZE;
bytes can overflow if num > BDRV_REQUEST_MAX_SECTORS. Is that ever a
problem? Would an assert() help, or else clamping it (since it is really
just a hint of how far we are allowed to look for similar clusters, but
we can always quit looking early):
bytes = MIN(*num * BDRV_SECTOR_SIZE, UINT32_MAX);
Then again, the interface is about to change, so this may just be a
transient problem.
>
> - l1_bits = s->l2_bits + s->cluster_bits;
> -
> - /* compute how many bytes there are between the offset and
> - * the end of the l1 entry
> - */
> + offset_in_cluster = offset_into_cluster(s, offset);
> + bytes_needed = bytes + offset_in_cluster;
Hmm, my idea for clamped bytes above may need tweaking (that is,
UINT32_MAX is inappropriate as the clamp value, if you are going to
round it up here). Or, since bytes_needed is 64-bits, make either bytes
or offset_in_cluster also be 64 bits, or start with '0ULL +', to avoid
overflow.
>
> - nb_available = (1ULL << l1_bits) - (offset & ((1ULL << l1_bits) - 1));
> -
> - /* compute the number of available sectors */
> + l1_bits = s->l2_bits + s->cluster_bits;
>
> - nb_available = (nb_available >> 9) + index_in_cluster;
> + /* compute how many bytes there are between the start of the cluster
> + * containing offset and the end of the l1 entry */
> + bytes_available = (1ULL << l1_bits) - (offset & ((1ULL << l1_bits) - 1))
> + + offset_in_cluster;
>
> - if (nb_needed > nb_available) {
> - nb_needed = nb_available;
> + if (bytes_needed > bytes_available) {
> + bytes_needed = bytes_available;
> }
> - assert(nb_needed <= INT_MAX);
> + assert(bytes_needed <= INT_MAX);
Is this assertion too late given the spots I pointed out above as
possible overflow points?
>
> *cluster_offset = 0;
>
> @@ -542,7 +540,7 @@ int qcow2_get_cluster_offset(BlockDriverState *bs,
> uint64_t offset,
> *cluster_offset = be64_to_cpu(l2_table[l2_index]);
>
> /* nb_needed <= INT_MAX, thus nb_clusters <= INT_MAX, too */
> - nb_clusters = size_to_clusters(s, nb_needed << 9);
> + nb_clusters = size_to_clusters(s, bytes_needed);
>
> ret = qcow2_get_cluster_type(*cluster_offset);
> switch (ret) {
> @@ -589,13 +587,16 @@ int qcow2_get_cluster_offset(BlockDriverState *bs,
> uint64_t offset,
>
> qcow2_cache_put(bs, s->l2_table_cache, (void**) &l2_table);
>
> - nb_available = (c * s->cluster_sectors);
> + bytes_available = (c * s->cluster_size);
>
> out:
> - if (nb_available > nb_needed)
> - nb_available = nb_needed;
> + if (bytes_available > bytes_needed) {
> + bytes_available = bytes_needed;
> + }
>
> - *num = nb_available - index_in_cluster;
> + bytes = bytes_available - offset_in_cluster;
> + assert((bytes & (BDRV_SECTOR_SIZE - 1)) == 0);
> + *num = bytes >> BDRV_SECTOR_BITS;
>
> return ret;
Looks like it is headed in the right direction, though.
--
Eric Blake eblake redhat com +1-919-301-3266
Libvirt virtualization library http://libvirt.org
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