On Tue, 16 Jun 2026 at 17:20, Jimmy Zuber [off-list ref] wrote:

+/*
+ * A server can stall syncfs()/sync(), so only honor FUSE_HAS_SYNCFS for
+ * mounts owned by the initial user namespace, i.e. set up with host
+ * privilege (like virtiofs and fuseblk).
+ */
+static bool fuse_syncfs_enable(struct fuse_conn *fc, u64 flags)
+{
+       return (flags & FUSE_HAS_SYNCFS) && fc->user_ns == &init_user_ns;
+}

Sounds really easy to trick: start the server in the initial user ns,
then clone the mounter with a new user/mount namespace.   The
init_user_ns test will pass happily, since the server is running in
the initial namespace.

Thanks,
Miklos