fs: add nd_jump_link

Add a helper that abstracts out the jump to an already parsed struct path
from ->follow_link operation from procfs.  Not only does this clean up
the code by moving the two sides of this game into a single helper, but
it also prepares for making struct nameidata private to namei.c

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/namei.c

@@ -586,6 +586,21 @@ static inline void path_to_nameidata(const struct path *path,
 	nd->path.dentry = path->dentry;
 }
 
+/*
+ * Helper to directly jump to a known parsed path from ->follow_link,
+ * caller must have taken a reference to path beforehand.
+ */
+void nd_jump_link(struct nameidata *nd, struct path *path)
+{
+	path_put(&nd->path);
+
+	nd->path = *path;
+	nd->inode = nd->path.dentry->d_inode;
+	nd->flags |= LOOKUP_JUMPED;
+
+	BUG_ON(nd->inode->i_op->follow_link);
+}
+
 static inline void put_link(struct nameidata *nd, struct path *link, void *cookie)
 {
 	struct inode *inode = link->dentry->d_inode;
@@ -630,17 +645,9 @@ follow_link(struct path *link, struct nameidata *nd, void **p)
 	s = nd_get_link(nd);
 	if (s) {
 		error = __vfs_follow_link(nd, s);
-	} else if (nd->last_type == LAST_BIND) {
+		if (unlikely(error))
-		nd->flags |= LOOKUP_JUMPED;
+			put_link(nd, link, *p);
-		nd->inode = nd->path.dentry->d_inode;
-		if (nd->inode->i_op->follow_link) {
-			/* stepped on a _really_ weird one */
-			path_put(&nd->path);
-			error = -ELOOP;
-		}
 	}
-	if (unlikely(error))
-		put_link(nd, link, *p);
 
 	return error;
 

fs/proc/base.c

@@ -1438,8 +1438,7 @@ static void *proc_pid_follow_link(struct dentry *dentry, struct nameidata *nd)
 	if (error)
 		goto out;
 
-	path_put(&nd->path);
+	nd_jump_link(nd, &path);
-	nd->path = path;
 	return NULL;
 out:
 	return ERR_PTR(error);

include/linux/namei.h

@@ -80,6 +80,8 @@ extern int follow_up(struct path *);
 extern struct dentry *lock_rename(struct dentry *, struct dentry *);
 extern void unlock_rename(struct dentry *, struct dentry *);
 
+extern void nd_jump_link(struct nameidata *nd, struct path *path);
+
 static inline void nd_set_link(struct nameidata *nd, char *path)
 {
 	nd->saved_names[nd->depth] = path;