[Qemu-devel] [PULL 3/4] block/copy-on-read: Fix permissions for inactive node

[Kevin Wolf <kwolf@redhat.com>]

The copy-on-read drive must not request the WRITE_UNCHANGED permission
for its child if the node is inactive, otherwise starting a migration
destination with -incoming will fail because the child cannot provide
write access yet:

  qemu-system-x86_64: -blockdev copy-on-read,file=img,node-name=cor: Block node is read-only

Earlier QEMU versions additionally ran into an abort() on the migration
source side: bdrv_inactivate_recurse() failed to update permissions.
This is silently ignored today because it was only supposed to loosen
restrictions. This is the symptom that was originally reported here:

  https://bugzilla.redhat.com/show_bug.cgi?id=1733022

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
---
 block/copy-on-read.c | 16 +++++++---------
 1 file changed, 7 insertions(+), 9 deletions(-)

diff --git a/block/copy-on-read.c b/block/copy-on-read.c
index 22f24fd0db..6631f30205 100644
--- a/block/copy-on-read.c
+++ b/block/copy-on-read.c
@@ -56,16 +56,14 @@ static void cor_child_perm(BlockDriverState *bs, BdrvChild *c,
                            uint64_t perm, uint64_t shared,
                            uint64_t *nperm, uint64_t *nshared)
 {
-    if (c == NULL) {
-        *nperm = (perm & PERM_PASSTHROUGH) | BLK_PERM_WRITE_UNCHANGED;
-        *nshared = (shared & PERM_PASSTHROUGH) | PERM_UNCHANGED;
-        return;
-    }
+    *nperm = perm & PERM_PASSTHROUGH;
+    *nshared = (shared & PERM_PASSTHROUGH) | PERM_UNCHANGED;
 
-    *nperm = (perm & PERM_PASSTHROUGH) |
-             (c->perm & PERM_UNCHANGED);
-    *nshared = (shared & PERM_PASSTHROUGH) |
-               (c->shared_perm & PERM_UNCHANGED);
+    /* We must not request write permissions for an inactive node, the child
+     * cannot provide it. */
+    if (!(bs->open_flags & BDRV_O_INACTIVE)) {
+        *nperm |= BLK_PERM_WRITE_UNCHANGED;
+    }
 }
 
 
-- 
2.20.1