block/copy-before-write: relax permission requirements when no parents

We are going to publish copy-before-write filter. So, user should be
able to create it with blockdev-add first, specifying both filtered and
target children. And then do blockdev-reopen, to actually insert the
filter where needed.

Currently, filter unshares write permission unconditionally on source
node. It's good, but it will not allow to do blockdev-add. So, let's
relax restrictions when filter doesn't have any parent.

Test output is modified, as now permission conflict happens only when
job creates a blk parent for filter node.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Message-Id: <20210824083856.17408-11-vsementsov@virtuozzo.com>
Signed-off-by: Hanna Reitz <hreitz@redhat.com>

1 files changed, 5 insertions, 3 deletions

diff --git a/block/copy-before-write.c b/block/copy-before-write.c
index a7996d5..2a51cc6 100644
--- a/block/copy-before-write.c
+++ b/block/copy-before-write.c
@@ -142,10 +142,12 @@ static void cbw_child_perm(BlockDriverState *bs, BdrvChild *c,
         bdrv_default_perms(bs, c, role, reopen_queue,
                            perm, shared, nperm, nshared);
 
-        if (perm & BLK_PERM_WRITE) {
-            *nperm = *nperm | BLK_PERM_CONSISTENT_READ;
+        if (!QLIST_EMPTY(&bs->parents)) {
+            if (perm & BLK_PERM_WRITE) {
+                *nperm = *nperm | BLK_PERM_CONSISTENT_READ;
+            }
+            *nshared &= ~(BLK_PERM_WRITE | BLK_PERM_RESIZE);
         }
-        *nshared &= ~(BLK_PERM_WRITE | BLK_PERM_RESIZE);
     }
 }