block/mirror: Fix mirror_top's permissions

mirror_top currently shares all permissions, and takes only the WRITE
permission (if some parent has taken that permission, too).

That is wrong, though; mirror_top is a filter, so it should take
permissions like any other filter does.  For example, if the parent
needs CONSISTENT_READ, we need to take that, too, and if it cannot share
the WRITE permission, we cannot share it either.

The exception is when mirror_top is used for active commit, where we
cannot take CONSISTENT_READ (because it is deliberately unshared above
the base node) and where we must share WRITE (so that it is shared for
all images in the backing chain, so the mirror job can take it for the
target BB).

Signed-off-by: Max Reitz <mreitz@redhat.com>
Message-Id: <20210211172242.146671-2-mreitz@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>

1 files changed, 25 insertions, 7 deletions

diff --git a/block/mirror.c b/block/mirror.c
index 6af02a5..d7e54c0 100644
--- a/block/mirror.c
+++ b/block/mirror.c
@@ -89,6 +89,7 @@ typedef struct MirrorBlockJob {
 typedef struct MirrorBDSOpaque {
     MirrorBlockJob *job;
     bool stop;
+    bool is_commit;
 } MirrorBDSOpaque;
 
 struct MirrorOp {
@@ -1522,13 +1523,27 @@ static void bdrv_mirror_top_child_perm(BlockDriverState *bs, BdrvChild *c,
         return;
     }
 
-    /* Must be able to forward guest writes to the real image */
-    *nperm = 0;
-    if (perm & BLK_PERM_WRITE) {
-        *nperm |= BLK_PERM_WRITE;
-    }
+    bdrv_default_perms(bs, c, role, reopen_queue,
+                       perm, shared, nperm, nshared);
 
-    *nshared = BLK_PERM_ALL;
+    if (s->is_commit) {
+        /*
+         * For commit jobs, we cannot take CONSISTENT_READ, because
+         * that permission is unshared for everything above the base
+         * node (except for filters on the base node).
+         * We also have to force-share the WRITE permission, or
+         * otherwise we would block ourselves at the base node (if
+         * writes are blocked for a node, they are also blocked for
+         * its backing file).
+         * (We could also share RESIZE, because it may be needed for
+         * the target if its size is less than the top node's; but
+         * bdrv_default_perms_for_cow() automatically shares RESIZE
+         * for backing nodes if WRITE is shared, so there is no need
+         * to do it here.)
+         */
+        *nperm &= ~BLK_PERM_CONSISTENT_READ;
+        *nshared |= BLK_PERM_WRITE;
+    }
 }
 
 /* Dummy node that provides consistent read to its users without requiring it
@@ -1591,6 +1606,8 @@ static BlockJob *mirror_start_job(
         return NULL;
     }
 
+    target_is_backing = bdrv_chain_contains(bs, target);
+
     /* In the case of active commit, add dummy driver to provide consistent
      * reads on the top, while disabling it in the intermediate nodes, and make
      * the backing chain writable. */
@@ -1613,6 +1630,8 @@ static BlockJob *mirror_start_job(
     bs_opaque = g_new0(MirrorBDSOpaque, 1);
     mirror_top_bs->opaque = bs_opaque;
 
+    bs_opaque->is_commit = target_is_backing;
+
     /* bdrv_append takes ownership of the mirror_top_bs reference, need to keep
      * it alive until block_job_create() succeeds even if bs has no parent. */
     bdrv_ref(mirror_top_bs);
@@ -1653,7 +1672,6 @@ static BlockJob *mirror_start_job(
     target_perms = BLK_PERM_WRITE;
     target_shared_perms = BLK_PERM_WRITE_UNCHANGED;
 
-    target_is_backing = bdrv_chain_contains(bs, target);
     if (target_is_backing) {
         int64_t bs_size, target_size;
         bs_size = bdrv_getlength(bs);