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

From: Eric Biggers
Date: Sat Oct 11 2014 - 23:55:23 EST

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
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