linux-kernel - Re: linux-next: build failure after merge of the userns tree

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Message-ID: <20131108155547.GP13318@ZenIV.linux.org.uk>
Date:	Fri, 8 Nov 2013 15:55:47 +0000
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Christoph Hellwig <hch@...radead.org>
Cc:	Stephen Rothwell <sfr@...b.auug.org.au>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	linux-next@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: linux-next: build failure after merge of the userns tree

On Thu, Nov 07, 2013 at 11:27:32PM -0800, Christoph Hellwig wrote:
> On Fri, Nov 08, 2013 at 05:58:48PM +1100, Stephen Rothwell wrote:
> > Hi Eric,
> > 
> > After merging the userns tree, today's linux-next build (powerpc
> > ppc64_defconfig) failed like this:
> > 
> > fs/namei.c: In function 'covered':
> > fs/namei.c:3528:2: error: too many arguments to function '__lookup_mnt'
> >   is_covered = d_mountpoint(dentry) && __lookup_mnt(mnt, dentry, 1);
> >   ^
> > 
> > Caused by my incomplete merge resolution between commits 474279dc0f77
> > ("split __lookup_mnt() in two functions") from the vfs tree and
> > a3b4491433f2 ("vfs: Don't allow overwriting mounts in the current mount
> > namespace") from the userns tree.
> 
> Btw, I don't think the userns tree has any business touching lookup
> and mount semantics in namei.c without an explicit VFS signoff.
> 
> Please drop the tree for now.

I'll probably put some form of that stuff through the vfs.git - the idea
is sane, but I would really like to see Eric's answer to the question
I've asked about the checks he adds in the first commit in this series;
AFAICS, to make them non-racy one needs to change locking rules for mount(2).
As it is, we have namespace_sem held exclusive _and_ ->i_mutex of mountpoint
to be held for all places where we turn something into a mountpoint.  His code
appears to assume that we are actually using ->i_mutex on _parent_ instead;
either that, or these checks are deliberately racy.

I'm not saying that change of lock_mount(9) behaviour is out of question -
we could change these locking rules, but such change isn't there in that
series and it's not even discussed there.
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