Return required size when encode_fh size too small

Quoting <linux/exportfs.h>:

> encode_fh() should return the fileid_type on success and on error
> returns 255 (if the space needed to encode fh is greater than
> @max_len*4 bytes). On error @max_len contains the minimum size (in 4
> byte unit) needed to encode the file handle.

ZFS was not setting max_len in the case where the handle was too
small.  As a result of this, the `t_name_to_handle_at.c' example in
name_to_handle_at(2) did not work on ZFS.

zfsctl_fid() will itself set max_len if called with a fid that is too
small, so if we give zfs_fid() that behavior as well, the fix is quite
easy: if the handle is too small, just use a zero-size fid instead of
the handle.

Tested by running t_name_to_handle_at on a normal file, a directory, a
.zfs directory, and a snapshot.

Thanks-to: Puck Meerburg <[email protected]>
Reviewed-by: Brian Behlendorf <[email protected]>
Reviewed-by: Tony Nguyen <[email protected]>
Signed-off-by: Alyssa Ross <[email protected]>
Closes #11995 

2 files changed, 15 insertions, 4 deletions

diff --git a/module/os/linux/zfs/zfs_vnops_os.c b/module/os/linux/zfs/zfs_vnops_os.c
index e95d051ed..6f3faab04 100644
--- a/module/os/linux/zfs/zfs_vnops_os.c
+++ b/module/os/linux/zfs/zfs_vnops_os.c
@@ -3950,6 +3950,13 @@ zfs_fid(struct inode *ip, fid_t *fidp)
 	int		size, i, error;
 
 	ZFS_ENTER(zfsvfs);
+
+	if (fidp->fid_len < SHORT_FID_LEN) {
+		fidp->fid_len = SHORT_FID_LEN;
+		ZFS_EXIT(zfsvfs);
+		return (SET_ERROR(ENOSPC));
+	}
+
 	ZFS_VERIFY_ZP(zp);
 
 	if ((error = sa_lookup(zp->z_sa_hdl, SA_ZPL_GEN(zfsvfs),
diff --git a/module/os/linux/zfs/zpl_export.c b/module/os/linux/zfs/zpl_export.c
index eaf048c38..5be63532d 100644
--- a/module/os/linux/zfs/zpl_export.c
+++ b/module/os/linux/zfs/zpl_export.c
@@ -41,15 +41,19 @@ zpl_encode_fh(struct dentry *dentry, __u32 *fh, int *max_len, int connectable)
 	struct inode *ip = dentry->d_inode;
 #endif /* HAVE_ENCODE_FH_WITH_INODE */
 	fstrans_cookie_t cookie;
-	fid_t *fid = (fid_t *)fh;
+	ushort_t empty_fid = 0;
+	fid_t *fid;
 	int len_bytes, rc;
 
 	len_bytes = *max_len * sizeof (__u32);
 
-	if (len_bytes < offsetof(fid_t, fid_data))
-		return (255);
+	if (len_bytes < offsetof(fid_t, fid_data)) {
+		fid = (fid_t *)&empty_fid;
+	} else {
+		fid = (fid_t *)fh;
+		fid->fid_len = len_bytes - offsetof(fid_t, fid_data);
+	}
 
-	fid->fid_len = len_bytes - offsetof(fid_t, fid_data);
 	cookie = spl_fstrans_mark();
 
 	if (zfsctl_is_node(ip))