fuse: add FUSE_NO_OPEN_SUPPORT flag to INIT

Here some additional changes to set a capability flag so that clients can
detect when it's appropriate to return -ENOSYS from open.

This amends the following commit introduced in 3.14:

  7678ac50615d  fuse: support clients that don't implement 'open'

However we can only add the flag to 3.15 and later since there was no
protocol version update in 3.14.

Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>
Cc: <stable@vger.kernel.org> # v3.15+

fs/fuse/inode.c

@@ -935,7 +935,7 @@ static void fuse_send_init(struct fuse_conn *fc, struct fuse_req *req)
 		FUSE_SPLICE_WRITE | FUSE_SPLICE_MOVE | FUSE_SPLICE_READ |
 		FUSE_FLOCK_LOCKS | FUSE_IOCTL_DIR | FUSE_AUTO_INVAL_DATA |
 		FUSE_DO_READDIRPLUS | FUSE_READDIRPLUS_AUTO | FUSE_ASYNC_DIO |
-		FUSE_WRITEBACK_CACHE;
+		FUSE_WRITEBACK_CACHE | FUSE_NO_OPEN_SUPPORT;
 	req->in.h.opcode = FUSE_INIT;
 	req->in.numargs = 1;
 	req->in.args[0].size = sizeof(*arg);

include/uapi/linux/fuse.h

@@ -101,6 +101,7 @@
  *  - add FATTR_CTIME
  *  - add ctime and ctimensec to fuse_setattr_in
  *  - add FUSE_RENAME2 request
+ *  - add FUSE_NO_OPEN_SUPPORT flag
  */
 
 #ifndef _LINUX_FUSE_H
@@ -229,6 +230,7 @@ struct fuse_file_lock {
  * FUSE_READDIRPLUS_AUTO: adaptive readdirplus
  * FUSE_ASYNC_DIO: asynchronous direct I/O submission
  * FUSE_WRITEBACK_CACHE: use writeback cache for buffered writes
+ * FUSE_NO_OPEN_SUPPORT: kernel supports zero-message opens
  */
 #define FUSE_ASYNC_READ		(1 << 0)
 #define FUSE_POSIX_LOCKS	(1 << 1)
@@ -247,6 +249,7 @@ struct fuse_file_lock {
 #define FUSE_READDIRPLUS_AUTO	(1 << 14)
 #define FUSE_ASYNC_DIO		(1 << 15)
 #define FUSE_WRITEBACK_CACHE	(1 << 16)
+#define FUSE_NO_OPEN_SUPPORT	(1 << 17)
 
 /**
  * CUSE INIT request/reply flags