FUSE: Respect userspace FS "do-not-cache" of path components

The FUSE protocol demands that kernel implementations cache user filesystem
path components (lookup/cnp data) for a maximum period of time in the range
of [0, ULONG_MAX] seconds.  In practice, typical requests are for 0, 1, or
10 seconds; or "a long time" to represent indefinite caching.

Historically, FreeBSD FUSE has ignored this client directive entirely.  This
works fine for local-only filesystems, but causes consistency issues with
multi-writer network filesystems.

For now, respect 0 second cache TTLs and do not cache such metadata.
Non-zero metadata caching TTLs in the range [0.000000001, ULONG_MAX] seconds
are still cached indefinitely, because it is unclear how a userspace
filesystem could do anything sensible with those semantics even if
implemented.

Pass fuse_entry_out to fuse_vnode_get when available and only cache lookup
if the user filesystem did not set a zero second TTL.

PR:		230258 (inspired by; does not fix)

1 files changed, 2 insertions, 1 deletions

diff --git a/sys/fs/fuse/fuse_vfsops.c b/sys/fs/fuse/fuse_vfsops.c
index 5685b04ee0d8..ae17768ad2e6 100644
--- a/sys/fs/fuse/fuse_vfsops.c
+++ b/sys/fs/fuse/fuse_vfsops.c
@@ -444,7 +444,8 @@ fuse_vfsop_root(struct mount *mp, int lkflags, struct vnode **vpp)
 		if (err == 0)
 			*vpp = data->vroot;
 	} else {
-		err = fuse_vnode_get(mp, FUSE_ROOT_ID, NULL, vpp, NULL, VDIR);
+		err = fuse_vnode_get(mp, NULL, FUSE_ROOT_ID, NULL, vpp, NULL,
+		    VDIR);
 		if (err == 0) {
 			FUSE_LOCK();
 			MPASS(data->vroot == NULL || data->vroot == *vpp);