[PATCH] vhost-user-blk-server: do not advertise VIRTIO_BLK_F_SIZE_MAX with size_max=0

[PATCH] vhost-user-blk-server: do not advertise VIRTIO_BLK_F_SIZE_MAX with size_max=0

[Max Makarov]

The QSD vhost-user-blk backend advertises VIRTIO_BLK_F_SIZE_MAX but
sets size_max=0 in config, creating a contradictory state. The feature
bit tells the guest that size_max is valid, but the value is zero.

Windows viostor trusts the feature bit, reads size_max=0, and uses it
in scatter-gather calculations producing zero-length segments that hang
I/O. The disk appears empty (no GPT, no partitions), causing
INACCESSIBLE_BOOT_DEVICE BSOD. Linux tolerates this because
blk_validate_limits() silently corrects max_segment_size=0 to
BLK_MAX_SEGMENT_SIZE (65536).

This affects any VMM using QSD as a vhost-user-blk backend (QEMU,
Cloud Hypervisor, etc.), since both pass through SIZE_MAX from the
backend to the guest via feature intersection.

Remove VIRTIO_BLK_F_SIZE_MAX from the backend feature bits. The
size_max config field remains zero but is now ignored by guests since
the feature is not negotiated.

Signed-off-by: Max Makarov <maxpain@linux.com>
---
 block/export/vhost-user-blk-server.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/block/export/vhost-user-blk-server.c b/block/export/vhost-user-blk-server.c
index e89422b..b6b03b0 100644
--- a/block/export/vhost-user-blk-server.c
+++ b/block/export/vhost-user-blk-server.c
@@ -119,7 +119,7 @@ static uint64_t vu_blk_get_features(VuDev *dev)
     uint64_t features;
     VuServer *server = container_of(dev, VuServer, vu_dev);
     VuBlkExport *vexp = container_of(server, VuBlkExport, vu_server);
-    features = 1ull << VIRTIO_BLK_F_SIZE_MAX |
+    features =
                1ull << VIRTIO_BLK_F_SEG_MAX |
                1ull << VIRTIO_BLK_F_TOPOLOGY |
                1ull << VIRTIO_BLK_F_BLK_SIZE |
-- 
2.50.1 (Apple Git-155)