linux-kernel - Re: [RFC] Add option to mount only a pids subset

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Message-ID: <20170323225710.GI4554@comp-core-i7-2640m-0182e6>
Date:   Thu, 23 Mar 2017 23:57:10 +0100
From:   Alexey Gladkov <gladkov.alexey@...il.com>
To:     Oleg Nesterov <oleg@...hat.com>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux API <linux-api@...r.kernel.org>,
        "Kirill A. Shutemov" <kirill@...temov.name>,
        Vasiliy Kulikov <segoon@...nwall.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Pavel Emelyanov <xemul@...allels.com>,
        James Bottomley <James.Bottomley@...senPartnership.com>,
        "Dmitry V. Levin" <ldv@...linux.org>
Subject: Re: [RFC] Add option to mount only a pids subset

On Thu, Mar 23, 2017 at 05:05:07PM +0100, Oleg Nesterov wrote:
> Again, I can't really review this, I know nothing about vfs, but since
> nobody else replied...

Thanks anyway :)

> On 03/20, Alexey Gladkov wrote:
> >
> > @@ -97,7 +169,23 @@ static struct dentry *proc_mount(struct file_system_type *fs_type,
> >  		ns = task_active_pid_ns(current);
> >  	}
> >
> > -	return mount_ns(fs_type, flags, data, ns, ns->user_ns, proc_fill_super);
> > +	root = mount_ns(fs_type, flags, data, ns, ns->user_ns, proc_fill_super);
> > +
> > +	if (!IS_ERR(root)) {
> > +		if (!proc_fill_options(data, &opts))
> > +			return ERR_PTR(-EINVAL);
> 
> So we have to call proc_fill_options() twice, not good... Yes, I understand
> why, but perhaps we factor it out somehow, we can pack options + pid_ns into
> sb->s_fs_info. Nevermind, this is minor.

It happens only once, when we don't have the s_root yet.

> > +		if (opts.pid_only) {
> > +			int ret;
> > +
> > +			if (!ns->pidfs && (ret = fill_pidfs_root(root->d_sb)))
> > +				return ERR_PTR(ret);
> > +
> > +			root = ns->pidfs;
> 
> Afaics this lacks dget(ns->pidfs) which should pair with dput(mnt.mnt_root)
> in cleanup_mnt(). IIUC otherwise ns->pidfs can go away after umount, OTOH,
> if we return ns->pidfs then dget(sb->s_root) in mount_ns() is not balanced.
> But this all is fixeable.
> 
> So with this change "mount -opidonly" creates another IS_ROOT() dentry which
> is not equal to sb->s_root. I simply do not know if this is technically
> correct or not... but, say, the "Only bind mounts can have disconnected paths"
> comment in path_connected() makes me worry ;)
> 
> And this obviously means that /path-to-pidonly-mnt/ won't share dentries with
> the normal /proc mount. Not really good imo even if not really wrong... Lets
> look at proc_flush_task(). The exiting task will flush its $pid dentries in
> /proc/ but not in /path-to-pidonly-mnt/ iiuc. Again, not really a bug, but
> still...

I know that I'm cheater, but I did not start first :)

-- 
Rgrds, legion