linux-kernel - Re: [PATCH] Add pidfs filesystem

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Date:   Wed, 22 Feb 2017 18:37:19 +0300
From:   "Dmitry V. Levin" <ldv@...linux.org>
To:     Oleg Nesterov <oleg@...hat.com>
Cc:     Alexey Gladkov <gladkov.alexey@...il.com>,
        Linux Kernel Mailing List <linux-kernel@...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>
Subject: Re: [PATCH] Add pidfs filesystem

On Tue, Feb 21, 2017 at 03:57:47PM +0100, Oleg Nesterov wrote:
> On 02/18, Alexey Gladkov wrote:
> >
> > This patch allows to mount only the part of /proc related to pids
> > without rest objects. Since this is an addon to /proc, flags applied to
> > /proc have an effect on this pidfs filesystem.
> 
> I leave this to you and Eric, but imo it would be nice to avoid another
> filesystem.
> 
> > Why not implement it as another flag to /proc ?
> >
> > The /proc flags is stored in the pid_namespace and are global for
> > namespace. It means that if you add a flag to hide all except the pids,
> > then it will act on all mounted instances of /proc.
> 
> But perhaps we can use mnt_flags? For example, lets abuse MNT_NODEV, see
> the simple patch below. Not sure it is correct/complete, just to illustrate
> the idea.
> 
> With this patch you can mount proc with -onodev and it will only show
> pids/self/thread_self:
> 
> 	# mkdir /tmp/D
> 	# mount -t proc -o nodev none /tmp/D
> 	# ls /tmp/D
> 	1   11	13  15	17  19	20  22	24  28	3   31	33  4  56  7  9     thread-self
> 	10  12	14  16	18  2	21  23	27  29	30  32	34  5  6   8  self
> 	# cat /tmp/D/meminfo
> 	cat: /tmp/D/meminfo: No such file or directory
> 	# ls /tmp/D/irq
> 	ls: cannot open directory /tmp/D/irq: No such file or directory
> 
> No?

I like the idea of using mnt_flags to turn procfs into pidfs, thus
avoiding yet another filesystem, but MNT_NODEV has a different meaning,
namely "do not interpret character or block special devices on the file
system".  I've actually found a system nearby that already mounts /proc
with nodev:

# grep 'proc.*nodev' /proc/mounts
proc /var/lib/vz/root/1002/proc proc rw,nosuid,nodev,noexec,relatime,gid=19,hidepid=2 0 0
proc /var/lib/vz/root/1003/proc proc rw,nosuid,nodev,noexec,relatime,gid=19,hidepid=2 0 0
proc /var/lib/vz/root/1004/proc proc rw,nosuid,nodev,noexec,relatime,gid=19,hidepid=2 0 0
proc /var/lib/vz/root/1005/proc proc rw,nosuid,nodev,noexec,relatime,gid=19,hidepid=2 0 0


-- 
ldv

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