linux-kernel - Re: [PATCH 3/4] autofs - make mountpoint checks namespace aware

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Message-ID: <1474507987.12887.5.camel@themaw.net>
Date:   Thu, 22 Sep 2016 09:33:07 +0800
From:   Ian Kent <raven@...maw.net>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>,
        Mateusz Guzik <mguzik@...hat.com>
Cc:     NeilBrown <neilb@...e.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        autofs mailing list <autofs@...r.kernel.org>,
        Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Al Viro <viro@...IV.linux.org.uk>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Omar Sandoval <osandov@...ndov.com>
Subject: Re: [PATCH 3/4] autofs - make mountpoint checks namespace aware

On Wed, 2016-09-21 at 07:00 +0800, Ian Kent wrote:
> On Wed, 2016-09-21 at 06:44 +0800, Ian Kent wrote:
> > On Tue, 2016-09-20 at 11:50 -0500, Eric W. Biederman wrote:
> > > Ian Kent <raven@...maw.net> writes:
> > > 
> > > > On Sat, 2016-09-17 at 22:10 +0200, Mateusz Guzik wrote:
> > > > > On Wed, Sep 14, 2016 at 02:14:45PM +0800, Ian Kent wrote:
> > > > > > If an automount mount is clone(2)ed into a file system that is
> > > > > > propagation private, when it later expires in the originating
> > > > > > namespace subsequent calls to autofs ->d_automount() for that
> > > > > > dentry in the original namespace will return ELOOP until the
> > > > > > mount is manually umounted in the cloned namespace.
> > > > > > 
> > > > > > In the same way, if an autofs mount is triggered by automount(8)
> > > > > > running within a container the dentry will be seen as mounted in
> > > > > > the root init namespace and calls to ->d_automount() in that
> > > > > > namespace
> > > > > > will return ELOOP until the mount is umounted within the container.
> > > > > > 
> > > > > > Also, have_submounts() can return an incorect result when a mount
> > > > > > exists in a namespace other than the one being checked.
> > > > > > 
> > > > > > @@ -460,7 +460,7 @@ static int autofs4_d_manage(struct dentry
> > > > > > *dentry,
> > > > > > bool
> > > > > > rcu_walk)
> > > > > >  
> > > > > >  		if (ino->flags & AUTOFS_INF_WANT_EXPIRE)
> > > > > >  			return 0;
> > > > > > -		if (d_mountpoint(dentry))
> > > > > > +		if (is_local_mountpoint(dentry))
> > > > > >  			return 0;
> > > > > >  		inode = d_inode_rcu(dentry);
> > > > > >  		if (inode && S_ISLNK(inode->i_mode))
> > > > > 
> > > > > This change is within RCU lookup.
> > > > > 
> > > > > is_local_mountpoint may end up calling __is_local_mountpoint, which
> > > > > will
> > > > > optionally take the namespace_sem lock, resulting in a splat:
> > > > 
> > > > Yes, that's a serious problem I missed.
> > > > 
> > > > snip ...
> > > > 
> > > > > I don't know this code. Perhaps it will be perfectly fine performance
> > > > > wise
> > > > > to
> > > > > just drop out of RCU lookup in this case.
> > > > 
> > > > It's a bit worse than that.
> > > > 
> > > > I think being able to continue the rcu-walk for an already mounted
> > > > dentry
> > > > that
> > > > is not being expired is an important part of the performance improvement
> > > > given
> > > > by the series that added this.
> > > > 
> > > > Can you confirm that Neil?
> > > > 
> > > > But for the case here the existing test can allow rcu-walk to continue
> > > > for
> > > > a
> > > > dentry that would attempt to trigger an automount so it's also a bug in
> > > > the
> > > > existing code.
> > > 
> > > I don't think the existing code is buggy.  As I read __follow_mount_rcu
> > > if DCACHE_NEED_AUTOMOUNT is set on the dentry after return from d_manage
> > > the code will break out of the rcu walk.
> > 
> > That's right.
> > 
> > I'm working on follow up patches now.
> > 
> > > 
> > > > Any thoughts on how we can handle this Neil, I'm having a bit of trouble
> > > > working
> > > > out how to resolve this one.
> > > 
> > > I believe in this case d_mountpoint is enough for the rcu walk case.  If
> > > the mountpoint turns out not to be local __follow_mount_rcu will kick
> > > out of the rcu walk as it will return false.  Because:
> > > 	return !mounted && !(path->dentry->d_flags & DCACHE_NEED_AUTOMOUNT);
> > > 
> > > I am not quite certain about the non-rcu case.  That can't be
> > > is_local_mountpoint as that is inside a spinlock and is_local_mountpoint
> > > can sleep.  Perhaps d_mountpoint is simply correct for autofs4_d_manage.
> > 
> > That's right, I think I have other mistakes there too.
> > 
> > I'm checking those too.
> 
> It may be that is_local_mountpoint() isn't the right thing to use for this.
> 
> When I originally set out to do this I used a lookup_mnt() type check which
> should be sufficient in this case so I might need to go back to that.

Eric, Mateusz, I appreciate your spending time on this and particularly pointing
out my embarrassingly stupid is_local_mountpoint() usage mistake.

Please accept my apology for the inconvenience.

If all goes well (in testing) I'll have follow up patches to correct this fairly
soon.

Ian