fs: namei: Allow follow_down() to uncover auto mounts

This function is only used by NFSD to cross mount points.
If a mount point is of type auto mount, follow_down() will
not uncover it. Add LOOKUP_AUTOMOUNT to the lookup flags
to have ->d_automount() called when NFSD walks down the
mount tree.
Signed-off-by: Richard Weinberger <richard@nod.at>
Reviewed-by: Ian Kent <raven@themaw.net>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Acked-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>

fs/namei.c

@@ -1458,11 +1458,11 @@ EXPORT_SYMBOL(follow_down_one);
  * point, the filesystem owning that dentry may be queried as to whether the
  * caller is permitted to proceed or not.
  */
-int follow_down(struct path *path)
+int follow_down(struct path *path, unsigned int flags)
 {
 	struct vfsmount *mnt = path->mnt;
 	bool jumped;
-	int ret = traverse_mounts(path, &jumped, NULL, 0);
+	int ret = traverse_mounts(path, &jumped, NULL, flags);
 
 	if (path->mnt != mnt)
 		mntput(mnt);
@@ -2864,7 +2864,7 @@ int path_pts(struct path *path)
 
 	path->dentry = child;
 	dput(parent);
-	follow_down(path);
+	follow_down(path, 0);
 	return 0;
 }
 #endif

fs/nfsd/vfs.c

@@ -126,9 +126,13 @@ nfsd_cross_mnt(struct svc_rqst *rqstp, struct dentry **dpp,
 	struct dentry *dentry = *dpp;
 	struct path path = {.mnt = mntget(exp->ex_path.mnt),
 			    .dentry = dget(dentry)};
+	unsigned int follow_flags = 0;
 	int err = 0;
 
-	err = follow_down(&path);
+	if (exp->ex_flags & NFSEXP_CROSSMOUNT)
+		follow_flags = LOOKUP_AUTOMOUNT;
+
+	err = follow_down(&path, follow_flags);
 	if (err < 0)
 		goto out;
 	if (path.mnt == exp->ex_path.mnt && path.dentry == dentry &&

include/linux/namei.h

@@ -77,7 +77,7 @@ struct dentry *lookup_one_positive_unlocked(struct user_namespace *mnt_userns,
 					    struct dentry *base, int len);
 
 extern int follow_down_one(struct path *);
-extern int follow_down(struct path *);
+extern int follow_down(struct path *path, unsigned int flags);
 extern int follow_up(struct path *);
 
 extern struct dentry *lock_rename(struct dentry *, struct dentry *);