[PATCH v2 08/21] fuse: handle zero ops_mask in FUSE_DEV_IOC_BACKING_OPEN

[Joanne Koong <joannelkoong@gmail.com>]

Servers that pre-date the ops_mask field will pass a zero ops_mask in
FUSE_DEV_IOC_BACKING_OPEN. Default this to FUSE_PASSTHROUGH_RW_OPS to
maintain backwards compatibility.

For FUSE_PASSTHROUGH_INO servers, an ops_mask must be set.
FUSE_PASSTHROUGH_INO is a new feature with no backwards compatibliity
requirements.

Signed-off-by: Joanne Koong <joannelkoong@gmail.com>
---
 fs/fuse/backing.c | 20 +++++++++++++++++---
 1 file changed, 17 insertions(+), 3 deletions(-)

diff --git a/fs/fuse/backing.c b/fs/fuse/backing.c
index 2234ab47406a..b499860e1185 100644
--- a/fs/fuse/backing.c
+++ b/fs/fuse/backing.c
@@ -119,9 +119,23 @@ int fuse_backing_open(struct fuse_conn *fc, struct fuse_backing_map *map)
 	if (map->flags || map->ops_mask & ~FUSE_BACKING_MAP_VALID_OPS)
 		goto out;
 
-	/* For now passthrough inode operations requires FUSE_PASSTHROUGH_INO */
-	if (!fc->passthrough_ino && map->ops_mask & FUSE_PASSTHROUGH_INODE_OPS)
-		goto out;
+	if (!fc->passthrough_ino) {
+		/*
+		 * For now passthrough inode operations requires
+		 * FUSE_PASSTHROUGH_INO
+		 */
+		if (map->ops_mask & FUSE_PASSTHROUGH_INODE_OPS)
+			goto out;
+		/*
+		 * To maintain backwards compatibility with servers that
+		 * pre-date the ops_mask field, a zero ops_mask defaults
+		 * to passing through both reads and writes
+		 */
+		if (!map->ops_mask)
+			map->ops_mask |= FUSE_PASSTHROUGH_RW_OPS;
+	} else if (!map->ops_mask) {
+		return -EINVAL;
+	}
 
 	file = fget_raw(map->fd);
 	res = -EBADF;
-- 
2.52.0