[PATCH] 2.2.18: d_move() with self-root dentries (Dentry Corruption!)

From: Chip Salzenberg (chip@valinux.com)
Date: Tue Nov 21 2000 - 13:18:36 EST

This may be 2.2.18 material after all.... I wrote last night:
> Making nfsd's d_splice() compensate for d_move's limitations is not
> only a kludge, but also it harder to keep nfsd correct.
> someday, nfsd may not be the only creator of this kind of dentry.

Sure enough, there is just such a bug *already* in nfsd. Nfsd's
cleanup after d_move is incomplete: It handles one of the dentries
being parentless, but not the other one. This bug *will* cause dentry
corruption.[1] It may well be what's been causing the hangs that my
recent patches seem to have fixed.

Therefore, in the mainline kernel, we need either the below patch to
d_move (along with a trivial simplifcation of nfsd's use of it), or an
expansion of the kludge in nfsd. You can guess which one I favor....

[1] The bug can only show up when reconstructing pruned dentries, and
    only under a specific pattern of client requests, so it's not
    surprising that it is rarely observed in the wild.

Index: fs/dcache.c
--- fs/dcache.c.prev
+++ fs/dcache.c Mon Nov 20 22:31:09 2000
@@ -749,16 +749,28 @@ void d_move(struct dentry * dentry, stru
         INIT_LIST_HEAD(&target->d_hash);
 
+ /* Switch the names */
+ switch_names(dentry, target);
+ do_switch(dentry->d_name.len, target->d_name.len);
+ do_switch(dentry->d_name.hash, target->d_name.hash);
+
+ /* Switch parentage, allowing for self-parents */
+
         list_del(&dentry->d_child);
         list_del(&target->d_child);
 
- /* Switch the parents and the names.. */
- switch_names(dentry, target);
         do_switch(dentry->d_parent, target->d_parent);
- do_switch(dentry->d_name.len, target->d_name.len);
- do_switch(dentry->d_name.hash, target->d_name.hash);
 
- /* And add them back to the (new) parent lists */
- list_add(&target->d_child, &target->d_parent->d_subdirs);
- list_add(&dentry->d_child, &dentry->d_parent->d_subdirs);
+ if (dentry->d_parent != target)
+ list_add(&dentry->d_child, &dentry->d_parent->d_subdirs);
+ else {
+ INIT_LIST_HEAD(&dentry->d_child);
+ dentry->d_parent = dentry;
+ }
+ if (target->d_parent != dentry)
+ list_add(&target->d_child, &target->d_parent->d_subdirs);
+ else {
+ INIT_LIST_HEAD(&target->d_child);
+ target->d_parent = target;
+ }
 }
  

-- 
Chip Salzenberg            - a.k.a. -            <chip@valinux.com>
   "Give me immortality, or give me death!"  // Firesign Theatre
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