vfs: re-introduce MAY_CHDIR

Currently MAY_ACCESS means that filesystems must check the permissions
right then and not rely on cached results or the results of future
operations on the object.  This can be because of a call to sys_access() or
because of a call to chdir() which needs to check search without relying on
any future operations inside that dir.  I plan to use MAY_ACCESS for other
purposes in the security system, so I split the MAY_ACCESS and the
MAY_CHDIR cases.

Signed-off-by: Eric Paris <eparis@redhat.com>
Acked-by:  Stephen D. Smalley <sds@tycho.nsa.gov>
Signed-off-by: James Morris <jmorris@namei.org>

fs/nfs/dir.c

@@ -1953,7 +1953,7 @@ int nfs_permission(struct inode *inode, int mask)
 	if ((mask & (MAY_READ | MAY_WRITE | MAY_EXEC)) == 0)
 		goto out;
 	/* Is this sys_access() ? */
-	if (mask & MAY_ACCESS)
+	if (mask & (MAY_ACCESS | MAY_CHDIR))
 		goto force_lookup;
 
 	switch (inode->i_mode & S_IFMT) {