vvfat: Implement .bdrv_child_perm()

vvfat is the last remaining driver that can have children, but doesn't
implement .bdrv_child_perm() yet. The default handlers aren't suitable
here, so let's implement a very simple driver-specific one that protects
the internal child from being used by other users as good as our
permissions permit.
Signed-off-by: NKevin Wolf <kwolf@redhat.com>
Reviewed-by: NMax Reitz <mreitz@redhat.com>
Acked-by: NFam Zheng <famz@redhat.com>

block.c

@@ -823,7 +823,7 @@ static void bdrv_backing_options(int *child_flags, QDict *child_options,
     *child_flags = flags;
 }
 
-static const BdrvChildRole child_backing = {
+const BdrvChildRole child_backing = {
     .inherit_options = bdrv_backing_options,
     .drained_begin   = bdrv_child_cb_drained_begin,
     .drained_end     = bdrv_child_cb_drained_end,

block/vvfat.c

@@ -3052,6 +3052,27 @@ err:
     return ret;
 }
 
+static void vvfat_child_perm(BlockDriverState *bs, BdrvChild *c,
+                             const BdrvChildRole *role,
+                             uint64_t perm, uint64_t shared,
+                             uint64_t *nperm, uint64_t *nshared)
+{
+    BDRVVVFATState *s = bs->opaque;
+
+    assert(c == s->qcow || role == &child_backing);
+
+    if (c == s->qcow) {
+        /* This is a private node, nobody should try to attach to it */
+        *nperm = BLK_PERM_CONSISTENT_READ | BLK_PERM_WRITE;
+        *nshared = BLK_PERM_WRITE_UNCHANGED;
+    } else {
+        /* The backing file is there so 'commit' can use it. vvfat doesn't
+         * access it in any way. */
+        *nperm = 0;
+        *nshared = BLK_PERM_ALL;
+    }
+}
+
 static void vvfat_close(BlockDriverState *bs)
 {
     BDRVVVFATState *s = bs->opaque;
@@ -3077,6 +3098,7 @@ static BlockDriver bdrv_vvfat = {
     .bdrv_file_open         = vvfat_open,
     .bdrv_refresh_limits    = vvfat_refresh_limits,
     .bdrv_close             = vvfat_close,
+    .bdrv_child_perm        = vvfat_child_perm,
 
     .bdrv_co_preadv         = vvfat_co_preadv,
     .bdrv_co_pwritev        = vvfat_co_pwritev,

include/block/block_int.h

@@ -466,6 +466,7 @@ struct BdrvChildRole {
 
 extern const BdrvChildRole child_file;
 extern const BdrvChildRole child_format;
+extern const BdrvChildRole child_backing;
 
 struct BdrvChild {
     BlockDriverState *bs;