[PATCH v2] virtiofsd: Do not support blocking flock

[Sebastian Hasler <sebastian.hasler@stuvus.uni-stuttgart.de>]

With the current implementation, blocking flock can lead to
deadlock. Thus, it's better to return EOPNOTSUPP if a user attempts
to perform a blocking flock request.

Signed-off-by: Sebastian Hasler <sebastian.hasler@stuvus.uni-stuttgart.de>
---
 tools/virtiofsd/passthrough_ll.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/tools/virtiofsd/passthrough_ll.c b/tools/virtiofsd/passthrough_ll.c
index 64b5b4fbb1..faa62278c5 100644
--- a/tools/virtiofsd/passthrough_ll.c
+++ b/tools/virtiofsd/passthrough_ll.c
@@ -2442,6 +2442,15 @@ static void lo_flock(fuse_req_t req, fuse_ino_t ino, struct fuse_file_info *fi,
     int res;
     (void)ino;
 
+    if (!(op & LOCK_NB)) {
+        /*
+         * Blocking flock can deadlock as there is only one thread
+         * serving the queue.
+         */
+        fuse_reply_err(req, EOPNOTSUPP);
+        return;
+    }
+
     res = flock(lo_fi_fd(req, fi), op);
 
     fuse_reply_err(req, res == -1 ? errno : 0);
-- 
2.33.1