Re: [PATCH] Add pidfs filesystem

From: Dmitry V. Levin
Date: Wed Feb 22 2017 - 10:47:07 EST


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


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