[Qemu-devel] [PATCH v12 15/16] qcow2: Force "no other writer" lock on bs->file

[Fam Zheng <famz@redhat.com>]

Writing to the same qcow2 file from two QEMU processes at the same time
will never work correctly, so disable it even when the caller specifies
BDRV_O_RDWR.

Other formats are less vulnerable because they don't have internal
snapshots thus qemu-img is less often misused to create live snapshot.

Signed-off-by: Fam Zheng <famz@redhat.com>
---
 block/qcow2.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/block/qcow2.c b/block/qcow2.c
index 96fb8a8..879361a 100644
--- a/block/qcow2.c
+++ b/block/qcow2.c
@@ -1177,6 +1177,17 @@ static int qcow2_open(BlockDriverState *bs, QDict *options, int flags,
         }
     }
 
+    if ((flags & BDRV_O_SHARE_RW) && (flags & BDRV_O_RDWR)) {
+        /* Shared write is never a good idea for qcow2, override it.
+         * XXX: Use permission propagation and masking mechanism in op blockers
+         * API once it's there. */
+        ret = bdrv_reopen(bs->file->bs, flags & ~BDRV_O_SHARE_RW, &local_err);
+        if (ret) {
+            error_propagate(errp, local_err);
+            goto fail;
+        }
+    }
+
 #ifdef DEBUG_ALLOC
     {
         BdrvCheckResult result = {0};
-- 
2.9.3