[PATCH v4 07/12] block: reject zone appends that are not a multiple of the sector size

[Niklas Cassel <cassel@kernel.org>]

Zone write pointers are tracked and reported in units of
BDRV_SECTOR_SIZE, so an append whose data size is not a multiple of it
would leave a write pointer that cannot be represented, neither in a
BlockZoneDescriptor nor in the virtio and NVMe zone reports derived from
one.

Nothing states that invariant. file-posix, the only driver that carries
out an append itself, does enforce it, but only as a side effect of
checking each iovec against BlockLimits.write_granularity, which is never
smaller than the sector size. That conflates two constraints: the sector
granularity holds for every driver, whereas write_granularity describes a
coarser requirement of the medium below a driver, and only a driver that
has such a medium should express it.

Check the invariant once in bdrv_co_zone_append(), so that it no longer
rests on a driver that happens to have a granularity of its own to
report.

This is a lower bound, not the alignment that a guest has to observe. The
block layer cannot know that one, because it also depends on the logical
block size the device is configured with, which is a property of the
frontend. The alignment that applies to a guest is the larger of the two,
and it is the frontend that reports it, that validates requests against
it, and that has to refuse a device whose write pointers do not satisfy
it.

Reviewed-by: Damien Le Moal <dlemoal@kernel.org>
Signed-off-by: Niklas Cassel <cassel@kernel.org>
---
 block/io.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/block/io.c b/block/io.c
index cef23ee5bc..64e2f2b046 100644
--- a/block/io.c
+++ b/block/io.c
@@ -3355,6 +3355,16 @@ int coroutine_fn bdrv_co_zone_append(BlockDriverState *bs, int64_t *offset,
         return ret;
     }
 
+    /*
+     * Zone write pointers are kept and reported in units of BDRV_SECTOR_SIZE,
+     * so an append that would leave a write pointer at a finer granularity
+     * cannot be represented. Drivers may impose a coarser granularity of their
+     * own, see BlockLimits.write_granularity.
+     */
+    if (!QEMU_IS_ALIGNED(qiov->size, BDRV_SECTOR_SIZE)) {
+        return -EINVAL;
+    }
+
     bdrv_inc_in_flight(bs);
     if (!drv || !drv->bdrv_co_zone_append || bs->bl.zoned == BLK_Z_NONE) {
         co.ret = -ENOTSUP;
-- 
2.55.0