namei: new calling conventions for walk_component()

instead of a single flag (!= 0 => we want to follow symlinks) pass
two bits - WALK_GET (want to follow symlinks) and WALK_PUT (put_link()
once we are done looking at the name).  The latter matters only for
success exits - on failure the caller will discard everything anyway.

Suggestions for better variant are welcome; what this thing aims for
is making sure that pending put_link() is done *before* walk_component()
decides to pick a symlink up, rather than between picking it up and
acting upon it.  See the next commit for payoff.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

diff --git a/fs/namei.c b/fs/namei.c
index 19e5c8a..1c9af92 100644 (file)
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -1577,7 +1577,9 @@ static inline int should_follow_link(struct dentry *dentry, int follow)
        return unlikely(d_is_symlink(dentry)) ? follow : 0;
 }
 
-static int walk_component(struct nameidata *nd, int follow)
+enum {WALK_GET = 1, WALK_PUT = 2};
+
+static int walk_component(struct nameidata *nd, int flags)
 {
        struct path path;
        struct inode *inode;
@@ -1587,8 +1589,12 @@ static int walk_component(struct nameidata *nd, int follow)
         * to be able to know about the current root directory and
         * parent relationships.
         */
-       if (unlikely(nd->last_type != LAST_NORM))
-               return handle_dots(nd, nd->last_type);
+       if (unlikely(nd->last_type != LAST_NORM)) {
+               err = handle_dots(nd, nd->last_type);
+               if (flags & WALK_PUT)
+                       put_link(nd);
+               return err;
+       }
        err = lookup_fast(nd, &path, &inode);
        if (unlikely(err)) {
                if (err < 0)
@@ -1604,7 +1610,9 @@ static int walk_component(struct nameidata *nd, int follow)
                        goto out_path_put;
        }
 
-       if (should_follow_link(path.dentry, follow)) {
+       if (flags & WALK_PUT)
+               put_link(nd);
+       if (should_follow_link(path.dentry, flags & WALK_GET)) {
                if (nd->flags & LOOKUP_RCU) {
                        if (unlikely(nd->path.mnt != path.mnt ||
                                     unlazy_walk(nd, path.dentry))) {
@@ -1816,10 +1824,9 @@ OK:
                        if (!name)
                                return 0;
                        /* last component of nested symlink */
-                       err = walk_component(nd, LOOKUP_FOLLOW);
-                       put_link(nd);
+                       err = walk_component(nd, WALK_GET | WALK_PUT);
                } else {
-                       err = walk_component(nd, LOOKUP_FOLLOW);
+                       err = walk_component(nd, WALK_GET);
                }
                if (err < 0)
                        break;
@@ -2017,9 +2024,12 @@ static inline int lookup_last(struct nameidata *nd)
                nd->flags |= LOOKUP_FOLLOW | LOOKUP_DIRECTORY;
 
        nd->flags &= ~LOOKUP_PARENT;
-       err = walk_component(nd, nd->flags & LOOKUP_FOLLOW);
-       if (nd->depth)
-               put_link(nd);
+       err = walk_component(nd,
+                       nd->flags & LOOKUP_FOLLOW
+                               ? nd->depth
+                                       ? WALK_PUT | WALK_GET
+                                       : WALK_GET
+                               : 0);
        if (err < 0)
                terminate_walk(nd);
        return err;