block: Default .bdrv_child_perm() for format drivers

Almost all format drivers have the same characteristics as far as
permissions are concerned: They have one or more children for storing
their own data and, more importantly, metadata (can be written to and
grow even without external write requests, must be protected against
other writers and present consistent data) and optionally a backing file
(this is just data, so like for a filter, it only depends on what the
parent nodes need).

This provides a default implementation that can be shared by most of
our format drivers.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Acked-by: Fam Zheng <famz@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>

1 files changed, 44 insertions, 0 deletions

diff --git a/block.c b/block.c
index 064e9d77fa..d67819f2a2 100644
--- a/block.c
+++ b/block.c
@@ -1560,6 +1560,50 @@ void bdrv_filter_default_perms(BlockDriverState *bs, BdrvChild *c,
                (c->shared_perm & DEFAULT_PERM_UNCHANGED);
 }
 
+void bdrv_format_default_perms(BlockDriverState *bs, BdrvChild *c,
+                               const BdrvChildRole *role,
+                               uint64_t perm, uint64_t shared,
+                               uint64_t *nperm, uint64_t *nshared)
+{
+    bool backing = (role == &child_backing);
+    assert(role == &child_backing || role == &child_file);
+
+    if (!backing) {
+        /* Apart from the modifications below, the same permissions are
+         * forwarded and left alone as for filters */
+        bdrv_filter_default_perms(bs, c, role, perm, shared, &perm, &shared);
+
+        /* Format drivers may touch metadata even if the guest doesn't write */
+        if (!bdrv_is_read_only(bs)) {
+            perm |= BLK_PERM_WRITE | BLK_PERM_RESIZE;
+        }
+
+        /* bs->file always needs to be consistent because of the metadata. We
+         * can never allow other users to resize or write to it. */
+        perm |= BLK_PERM_CONSISTENT_READ;
+        shared &= ~(BLK_PERM_WRITE | BLK_PERM_RESIZE);
+    } else {
+        /* We want consistent read from backing files if the parent needs it.
+         * No other operations are performed on backing files. */
+        perm &= BLK_PERM_CONSISTENT_READ;
+
+        /* If the parent can deal with changing data, we're okay with a
+         * writable and resizable backing file. */
+        /* TODO Require !(perm & BLK_PERM_CONSISTENT_READ), too? */
+        if (shared & BLK_PERM_WRITE) {
+            shared = BLK_PERM_WRITE | BLK_PERM_RESIZE;
+        } else {
+            shared = 0;
+        }
+
+        shared |= BLK_PERM_CONSISTENT_READ | BLK_PERM_GRAPH_MOD |
+                  BLK_PERM_WRITE_UNCHANGED;
+    }
+
+    *nperm = perm;
+    *nshared = shared;
+}
+
 static void bdrv_replace_child(BdrvChild *child, BlockDriverState *new_bs,
                                bool check_new_perm)
 {