link_path_walk: nd->depth massage, part 1

nd->stack[0] is unused until the handling of trailing symlinks and
we want to get rid of that.  Having fucked that transformation up
several times, I went for bloody pedantic series of provably equivalent
transformations.  Sorry.

Step 1: keep nd->depth higher by one in link_path_walk() - increment upon
entry, decrement on exits, adjust the arithmetics inside and surround the
calls of functions that care about nd->depth value (nd_alloc_stack(),
get_link(), put_link()) with decrement/increment pairs.

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

diff --git a/fs/namei.c b/fs/namei.c
index 1ae34cd..e408f4d 100644 (file)
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -1763,6 +1763,7 @@ static int link_path_walk(const char *name, struct nameidata *nd)
        if (!*name)
                return 0;
 
+       nd->depth++;
        /* At this point we know we have a real path component. */
        for(;;) {
                u64 hash_len;
@@ -1823,15 +1824,18 @@ Walked:
                if (err) {
                        const char *s;
 
+                       nd->depth--;
                        err = nd_alloc_stack(nd);
+                       nd->depth++;
                        if (unlikely(err)) {
                                path_to_nameidata(&nd->link, nd);
                                break;
                        }
 
                        nd->depth++;
-
+                       nd->depth--;
                        s = get_link(nd);
+                       nd->depth++;
 
                        if (unlikely(IS_ERR(s))) {
                                err = PTR_ERR(s);
@@ -1841,7 +1845,9 @@ Walked:
                        err = 0;
                        if (unlikely(!s)) {
                                /* jumped */
+                               nd->depth--;
                                put_link(nd);
+                               nd->depth++;
                                nd->depth--;
                        } else {
                                if (*s == '/') {
@@ -1855,7 +1861,7 @@ Walked:
                                                ;
                                }
                                nd->inode = nd->path.dentry->d_inode;
-                               nd->stack[nd->depth].name = name;
+                               nd->stack[nd->depth - 1].name = name;
                                if (!*s)
                                        goto OK;
                                name = s;
@@ -1869,19 +1875,25 @@ Walked:
        }
        terminate_walk(nd);
 Err:
-       while (unlikely(nd->depth)) {
+       while (unlikely(nd->depth > 1)) {
+               nd->depth--;
                put_link(nd);
+               nd->depth++;
                nd->depth--;
        }
+       nd->depth--;
        return err;
 OK:
-       if (unlikely(nd->depth)) {
-               name = nd->stack[nd->depth].name;
+       if (unlikely(nd->depth > 1)) {
+               name = nd->stack[nd->depth - 1].name;
                err = walk_component(nd, LOOKUP_FOLLOW);
+               nd->depth--;
                put_link(nd);
+               nd->depth++;
                nd->depth--;
                goto Walked;
        }
+       nd->depth--;
        return 0;
 }