replication: clarify permissions

Even if hidden_disk, secondary_disk are backing files, they all need
write permissions in replication scenario. Otherwise we will encouter
below exceptions on secondary side during adding nbd server:

{'execute': 'nbd-server-add', 'arguments': {'device': 'colo-disk', 'writable': true } }
{"error": {"class": "GenericError", "desc": "Conflicts with use by hidden-qcow2-driver as 'backing', which does not allow 'write' on sec-qcow2-driver-for-nbd"}}

CC: Zhang Hailiang <zhang.zhanghailiang@huawei.com>
CC: Zhang Chen <zhangchen.fnst@cn.fujitsu.com>
CC: Wen Congyang <wencongyang2@huawei.com>
Signed-off-by: NChanglong Xie <xiecl.fnst@cn.fujitsu.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>

block/replication.c

@@ -155,6 +155,18 @@ static void replication_close(BlockDriverState *bs)
     replication_remove(s->rs);
 }
 
+static void replication_child_perm(BlockDriverState *bs, BdrvChild *c,
+                                   const BdrvChildRole *role,
+                                   uint64_t perm, uint64_t shared,
+                                   uint64_t *nperm, uint64_t *nshared)
+{
+    *nperm = *nshared = BLK_PERM_CONSISTENT_READ \
+                        | BLK_PERM_WRITE \
+                        | BLK_PERM_WRITE_UNCHANGED;
+
+    return;
+}
+
 static int64_t replication_getlength(BlockDriverState *bs)
 {
     return bdrv_getlength(bs->file->bs);
@@ -660,7 +672,7 @@ BlockDriver bdrv_replication = {
 
     .bdrv_open                  = replication_open,
     .bdrv_close                 = replication_close,
-    .bdrv_child_perm            = bdrv_filter_default_perms,
+    .bdrv_child_perm            = replication_child_perm,
 
     .bdrv_getlength             = replication_getlength,
     .bdrv_co_readv              = replication_co_readv,