linux-kernel - Re: [PATCH] vfs: fix race in rcu lookup of pruned dentry

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Date:	Mon, 18 Jul 2011 14:42:51 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Hugh Dickins <hughd@...gle.com>
Cc:	Al Viro <viro@...iv.linux.org.uk>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Nick Piggin <npiggin@...nel.dk>, linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH] vfs: fix race in rcu lookup of pruned dentry

On Mon, Jul 18, 2011 at 2:19 PM, Hugh Dickins <hughd@...gle.com> wrote:
>
> __d_lookup_rcu() is being careful about *inode, yes.
>
> But I'd forgotten it was even setting it: doesn't that setting get
> overridden later by the more careless *inode = path->d_entry->d_inode
> at the head of __follow_mount_rcu()'s loop?
>
> Perhaps that line just needs to be moved to the tail of the loop?

Ahh. Bingo. Yes, I think you found it.

I don't think it should touch that *inode value in
__follow_mount_rcu() unless we actually followed a mount, exactly
because it will overwrite the thing that we were so careful about in
__d_lookup_rcu().

So how about this patch that replaces the earlier mount-point sequence
number one. The only difference is (as you mention) to just do the
*inode update at the end of the loop, so that we don't overwrite the
valid inode data with a non-checked one when we don't do anything.

Untested. But this should make my propised change to fs/dcache.c be
irrelevant, because whether we clear d_inode or not, the existing
sequence number checks will catch it. Agreed?

                      Linus

View attachment "mount-sequence.diff" of type "text/x-patch" (1442 bytes)