[PATCH v2 16/21] fuse: allow backing_id=0 in open to inherit inode's backing file

With FUSE_PASSTHROUGH_INO, the backing file is associated with the inode
at lookup time. Allow the server to pass in backing_id=0 in the open
response. On the kernel side, this uses the backing file already set up
on the inode.

Signed-off-by: Joanne Koong <[email protected]>
---
 fs/fuse/passthrough.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/fs/fuse/passthrough.c b/fs/fuse/passthrough.c
index d62a1c751157..ae0137caa06d 100644
--- a/fs/fuse/passthrough.c
+++ b/fs/fuse/passthrough.c
@@ -193,13 +193,17 @@ int fuse_passthrough_readdir(struct file *file, struct dir_context *ctx)
  */
 struct fuse_backing *fuse_passthrough_open(struct file *file, int backing_id)
 {
+	struct fuse_inode *fi = get_fuse_inode(file_inode(file));
 	struct fuse_file *ff = file->private_data;
 	struct fuse_conn *fc = ff->fm->fc;
 	struct fuse_backing *fb = NULL;
 	struct file *backing_file;
 	int err;
 
-	fb = fuse_backing_id_get(fc, backing_id);
+	if (!backing_id && fc->passthrough_ino)
+		fb = fuse_backing_get(fuse_inode_backing(fi));
+	else
+		fb = fuse_backing_id_get(fc, backing_id);
 	if (IS_ERR_OR_NULL(fb)) {
 		err = fb ? PTR_ERR(fb) : -ENOENT;
 		fb = NULL;
-- 
2.52.0