fs: introduce vfs_path_lookup

Stackable file systems, among others, frequently need to lookup paths or
path components starting from an arbitrary point in the namespace
(identified by a dentry and a vfsmount).  Currently, such file systems use
lookup_one_len, which is frowned upon [1] as it does not pass the lookup
intent along; not passing a lookup intent, for example, can trigger BUG_ON's
when stacking on top of NFSv4.

The first patch introduces a new lookup function to allow lookup starting
from an arbitrary point in the namespace.  This approach has been suggested
by Christoph Hellwig [2].

The second patch changes sunrpc to use vfs_path_lookup.

The third patch changes nfsctl.c to use vfs_path_lookup.

The fourth patch marks link_path_walk static.

The fifth, and last patch, unexports path_walk because it is no longer
unnecessary to call it directly, and using the new vfs_path_lookup is
cleaner.

For example, the following snippet of code, looks up "some/path/component"
in a directory pointed to by parent_{dentry,vfsmnt}:

err = vfs_path_lookup(parent_dentry, parent_vfsmnt,
		      "some/path/component", 0, &nd);
if (!err) {
	/* exits */

	...

	/* once done, release the references */
	path_release(&nd);
} else if (err == -ENOENT) {
	/* doesn't exist */
} else {
	/* other error */
}

VFS functions such as lookup_create can be used on the nameidata structure
to pass the create intent to the file system.
Signed-off-by: Josef 'Jeff' Sipek <jsipek@cs.sunysb.edu>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Acked-by: Christoph Hellwig <hch@lst.de>
Cc: Trond Myklebust <trond.myklebust@fys.uio.no>
Cc: Neil Brown <neilb@suse.de>
Cc: Michael Halcrow <mhalcrow@us.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/namei.c

@@ -1172,6 +1172,37 @@ int fastcall path_lookup(const char *name, unsigned int flags,
 	return do_path_lookup(AT_FDCWD, name, flags, nd);
 }
 
+/**
+ * vfs_path_lookup - lookup a file path relative to a dentry-vfsmount pair
+ * @dentry:  pointer to dentry of the base directory
+ * @mnt: pointer to vfs mount of the base directory
+ * @name: pointer to file name
+ * @flags: lookup flags
+ * @nd: pointer to nameidata
+ */
+int vfs_path_lookup(struct dentry *dentry, struct vfsmount *mnt,
+		    const char *name, unsigned int flags,
+		    struct nameidata *nd)
+{
+	int retval;
+
+	/* same as do_path_lookup */
+	nd->last_type = LAST_ROOT;
+	nd->flags = flags;
+	nd->depth = 0;
+
+	nd->mnt = mntget(mnt);
+	nd->dentry = dget(dentry);
+
+	retval = path_walk(name, nd);
+	if (unlikely(!retval && !audit_dummy_context() && nd->dentry &&
+				nd->dentry->d_inode))
+		audit_inode(name, nd->dentry->d_inode);
+
+	return retval;
+
+}
+
 static int __path_lookup_intent_open(int dfd, const char *name,
 		unsigned int lookup_flags, struct nameidata *nd,
 		int open_flags, int create_mode)
@@ -2774,6 +2805,7 @@ EXPORT_SYMBOL(__page_symlink);
 EXPORT_SYMBOL(page_symlink);
 EXPORT_SYMBOL(page_symlink_inode_operations);
 EXPORT_SYMBOL(path_lookup);
+EXPORT_SYMBOL(vfs_path_lookup);
 EXPORT_SYMBOL(path_release);
 EXPORT_SYMBOL(path_walk);
 EXPORT_SYMBOL(permission);

include/linux/namei.h

@@ -69,6 +69,8 @@ extern int FASTCALL(__user_walk_fd(int dfd, const char __user *, unsigned, struc
 #define user_path_walk_link(name,nd) \
 	__user_walk_fd(AT_FDCWD, name, 0, nd)
 extern int FASTCALL(path_lookup(const char *, unsigned, struct nameidata *));
+extern int vfs_path_lookup(struct dentry *, struct vfsmount *,
+			   const char *, unsigned int, struct nameidata *);
 extern int FASTCALL(path_walk(const char *, struct nameidata *));
 extern int FASTCALL(link_path_walk(const char *, struct nameidata *));
 extern void path_release(struct nameidata *);