[Qemu-devel] [PATCH v6 03/10] qcow2: Assert that cluster operations are aligned

[Eric Blake <eblake@redhat.com>]

qcow2_discard_clusters() is set up to silently ignore sub-cluster
head or tail on unaligned requests.  However, it is easy to audit
the various callers: qcow2_snapshot_create() has always passed
aligned data since the call was introduced in 1ebf561;
qcow2_co_pdiscard() has passed aligned clusters since commit
ecdbead taught the block layer the preferred discard alignment (the
block layer can still pass sub-cluster values, but those are
handled directly in qcow2_co_pdiscard()); and qcow2_make_empty()
was fixed to pass aligned clusters in commit a3e1505.  Replace
rounding with assertions to hold us to the tighter contract,
eliminating the now-impossible case of an early exit for a
sub-cluster request.

qcow2_zero_clusters() has always been called with cluster-aligned
arguments from its lone caller qcow2_co_pwrite_zeroes() (like
qcow2_co_pdiscard(), the caller takes care of sub-cluster requests
from the block layer; and qcow2_zero_clusters() would have
misbehaved on unaligned requests), but it deserves the same
assertion for symmetry.

Signed-off-by: Eric Blake <eblake@redhat.com>

---
v6: avoid assertion on non-cluster-aligned image, use s->cluster_sectors
to avoid a shift, drop R-b
v5: no change
v4: new patch
---
 block/qcow2-cluster.c | 15 ++++++++-------
 1 file changed, 8 insertions(+), 7 deletions(-)

diff --git a/block/qcow2-cluster.c b/block/qcow2-cluster.c
index 78c11d4..046fbd8 100644
--- a/block/qcow2-cluster.c
+++ b/block/qcow2-cluster.c
@@ -1519,13 +1519,10 @@ int qcow2_discard_clusters(BlockDriverState *bs, uint64_t offset,

     end_offset = offset + (nb_sectors << BDRV_SECTOR_BITS);

-    /* Round start up and end down */
-    offset = align_offset(offset, s->cluster_size);
-    end_offset = start_of_cluster(s, end_offset);
-
-    if (offset > end_offset) {
-        return 0;
-    }
+    /* Caller must pass aligned values, except at image end */
+    assert(QEMU_IS_ALIGNED(offset, s->cluster_size));
+    assert(QEMU_IS_ALIGNED(end_offset, s->cluster_size) ||
+           end_offset == bs->total_sectors << BDRV_SECTOR_BITS);

     nb_clusters = size_to_clusters(s, end_offset - offset);

@@ -1600,6 +1597,10 @@ int qcow2_zero_clusters(BlockDriverState *bs, uint64_t offset, int nb_sectors,
     uint64_t nb_clusters;
     int ret;

+    /* Caller must pass aligned values */
+    assert(QEMU_IS_ALIGNED(offset, s->cluster_size));
+    assert(QEMU_IS_ALIGNED(nb_sectors, s->cluster_sectors));
+
     /* The zero flag is only supported by version 3 and newer */
     if (s->qcow_version < 3) {
         return -ENOTSUP;
-- 
2.9.3