linux-kernel - Re: [RFC][PATCHSET] non-recursive link_path

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Message-ID: <20150422201238.GW889@ZenIV.linux.org.uk>
Date:	Wed, 22 Apr 2015 21:12:38 +0100
From:	Al Viro <viro@...IV.linux.org.uk>
To:	NeilBrown <neilb@...e.de>
Cc:	Christoph Hellwig <hch@...radead.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: [RFC][PATCHSET] non-recursive link_path_walk() and reducing
 stack footprint

On Wed, Apr 22, 2015 at 07:07:59PM +0100, Al Viro wrote:
> And one more: may_follow_link() is now potentially oopsable.  Look: suppose
> we've reached the link in RCU mode, just as it got unlinked.  link->dentry
> has become negative and may_follow_link() steps into
>         /* Allowed if owner and follower match. */
>         inode = link->dentry->d_inode;
>         if (uid_eq(current_cred()->fsuid, inode->i_uid))
>                 return 0;
> Oops...  Incidentally, I suspect that your __read_seqcount_retry() in
> follow_link() might be lacking a barrier; why isn't full read_seqcount_retry()
> needed?
> 
> FWIW, I would rather fetch ->d_inode *and* checked ->seq proir to calling
> get_link(), and passed inode to it as an explicit argument.  And passed it
> to may_follow_link() as well...

Hrm...  You know, something really weird is going on here.  Where are
you setting nd->seq?  I don't see anything in follow_link() doing that.
And nd->seq _definitely_ needs setting if you want to stay in RCU mode -
at that point it matches the dentry of symlink, not that of nd->path
(== parent directory).  Neil, could you tell me which kernel you'd been
testing (ideally - commit ID is a public git tree), what config and what
tests had those been?
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