ovl: relax permission checking on underlying layers

Make permission checking more consistent:

 - special files don't need any access check on underling fs

 - exec permission check doesn't need to be performed on underlying fs

Reported-by: "J. Bruce Fields" <bfields@fieldses.org>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

fs/overlayfs/inode.c

@@ -286,13 +286,22 @@ int ovl_permission(struct inode *inode, int mask)
 	if (err)
 		return err;
 
-	old_cred = ovl_override_creds(inode->i_sb);
-	if (!upperinode &&
-	    !special_file(realinode->i_mode) && mask & MAY_WRITE) {
+	/* No need to do any access on underlying for special files */
+	if (special_file(realinode->i_mode))
+		return 0;
+
+	/* No need to access underlying for execute */
+	mask &= ~MAY_EXEC;
+	if ((mask & (MAY_READ | MAY_WRITE)) == 0)
+		return 0;
+
+	/* Lower files get copied up, so turn write access into read */
+	if (!upperinode && mask & MAY_WRITE) {
 		mask &= ~(MAY_WRITE | MAY_APPEND);
-		/* Make sure mounter can read file for copy up later */
 		mask |= MAY_READ;
 	}
+
+	old_cred = ovl_override_creds(inode->i_sb);
 	err = inode_permission(realinode, mask);
 	revert_creds(old_cred);