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Date: Wed, 22 Apr 2015 19:07:59 +0100
From: Al Viro <viro@ZenIV.linux.org.uk>
To: NeilBrown <neilb@suse.de>
Cc: Christoph Hellwig <hch@infradead.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org
Subject: Re: [RFC][PATCHSET] non-recursive link_path_walk() and reducing
 stack footprint
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On Tue, Apr 21, 2015 at 10:20:07PM +0100, Al Viro wrote:

> I agree that unlazy_walk() attempted when walking a symlink ought to fail
> with -ECHILD; we can't legitimize the symlink itself, so once we are out
> of RCU mode, there's nothing to hold the inode of symlink (and its body)
> from getting freed.  Solution is wrong though; for example, when
> nested symlink occurs in the middle of a trailing one, we should *not*
> remove the flag upon leaving the nested symlink.
> 
> Another unpleasant thing is that ->follow_link() saying "can't do that in
> RCU mode" ends up with restart from scratch - that actually risks to be
> worse than the mainline; there we would attempt unlazy_walk() and normally
> it would've succeed.
> 
> AFAICS, the real rule is "can't unlazy if nd->last.name points into a symlink
> body and we might still need to access it"...

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...