Re: fs/namei.c: Misuse of sequence counts?

From: Al Viro
Date: Sun Oct 12 2014 - 00:29:49 EST

On Sat, Oct 11, 2014 at 10:55:10PM -0500, Eric Biggers wrote:
> On Sun, Oct 12, 2014 at 12:46:35AM +0100, Al Viro wrote:
> >
> > Nope. What we do is
> > * pick parent inode and seqcount (in whatever order)
> > * THEN check that child is still unchanged.
> > The second part guarantees that parent dentry had been the parent of
> > child all along, since the moment we'd first fetched _child's_ seqcount.
> > And since a pinned positive dentry can't have its ->d_inode changed,
> > we know that the value of parent's inode we'd fetched remained valid
> > at least until we'd checked the child's seqcount and found it unchanged.
> > Which means that we had it valid at some point after we'd fetched parent's
> > seqcount.
> Ah, very tricky. And I take it that the other two fetches of d_inode in
> follow_dotdot_rcu() can likewise be unordered with respect to
> read_seqcount_begin(), because the underlying dentries are pinned as either
> mnt_mountpoint or mnt_root --- which in RCU mode, is only guaranteed because of
> the call to synchronize_rcu() in namespace_unlock() prior to dropping
> references?

The last one is actually covered by read_seqretry(&mount_lock, nd->m_seq) -
if it still matches, we know that whatever we got from __lookup_mnt() must
have been valid through fetching ->d_inode and ->d_seq of its mnt_root.
Which means that those two are consistent regardless of that synchronize_rcu().

The one before it would probably be better off with similar check on mount_lock
as well. That code *is* correct for the reason you've mentioned, but I wonder
if explicit check of mount_lock would be better - right now it's more subtle
than I'd like it to be. I don't think the cost would be noticable - it's
smp_rmb() + fetch + comparison when we cross a mountpoint while following ..
in lazy pathwalk, but that needs profiling - handwaving is not good enough...
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