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SubjectRe: Linux 2.6.38-rc5
On Wed, Feb 16, 2011 at 04:06:43PM +0000, Al Viro wrote:

> Sigh... I see what's going on. We'd got inode from dentry that is getting
> crapped under us. We will *not* survive dropping RCU - it's bad enough for
> full restart in normal mode. So right after we'd seen that (already wrong)
> inode has ->follow_link(), we decide to drop RCU. Originally this BUG_ON
> hadn't been reached in that case - we had already failed with -ECHILD before
> we got to it. Now we don't...
>
> _However_, I don't like passing inode to do_follow_link(). I'd rather set
> nd->inode to inode first and use it there. Let me think a bit and see if
> it's feasible...

No, that won't do. The damn thing uses previous value of nd->inode if
it walks into relative symlink...

Let's shift that call of nameidata_dentry_drop_rcu_maybe() into both
callers of do_follow_link() instead. Marginally less obvious that we won't
reach the guts of do_follow_link() in RCU mode, just as obvious that overall
structure is ugly as hell and avoids making it even uglier by passing inode
down there. How about this:

diff --git a/fs/namei.c b/fs/namei.c
index 9e701e2..d7003cf 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -800,10 +800,6 @@ static inline int do_follow_link(struct path *path, struct nameidata *nd)
void *cookie;
int err = -ELOOP;

- /* We drop rcu-walk here */
- if (nameidata_dentry_drop_rcu_maybe(nd, path->dentry))
- return -ECHILD;
-
if (current->link_count >= MAX_NESTED_LINKS)
goto loop;
if (current->total_link_count >= 40)
@@ -1413,6 +1409,9 @@ exec_again:
goto out_dput;

if (inode->i_op->follow_link) {
+ /* We drop rcu-walk here */
+ if (nameidata_dentry_drop_rcu_maybe(nd, next.dentry))
+ return -ECHILD;
BUG_ON(inode != next.dentry->d_inode);
err = do_follow_link(&next, nd);
if (err)
@@ -1458,6 +1457,8 @@ last_component:
break;
if (inode && unlikely(inode->i_op->follow_link) &&
(lookup_flags & LOOKUP_FOLLOW)) {
+ if (nameidata_dentry_drop_rcu_maybe(nd, next.dentry))
+ return -ECHILD;
BUG_ON(inode != next.dentry->d_inode);
err = do_follow_link(&next, nd);
if (err)

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