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

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Date:   Sat, 11 Mar 2017 22:51:02 +0100
From:   Alexey Gladkov <gladkov.alexey@...il.com>
To:     Djalal Harouni <tixxdz@...il.com>
Cc:     Andy Lutomirski <luto@...capital.net>,
        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>,
        Oleg Nesterov <oleg@...hat.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 09, 2017 at 12:26:49PM +0100, Djalal Harouni wrote:
> I'm bit lost in the two discussion, however the main concern I was
> discussing with Andy was if you have per superblock proc mounts then
> each mount will end up with its own device ID st_dev, right now they
> share the same ID if they are in the same pid namespace, but if we
> change that then we may break the following:
> http://man7.org/linux/man-pages/man7/namespaces.7.html

In fact, nothing has changed. I added a parameter that affects
the mountpoint, not the entire pid namespace.

The procfs will still be global. The device ID will be the same as before.

> Both new NS_GET_PARENT and NS_GET_USERNS ioctl() that return an fd,
> suggests to follow up with fstat() to identify the namespaces..
> "By applying fstat(2) to the returned file descriptor, one obtains a
> stat structure whose st_dev (resident device) and st_ino (inode
> number) fields together identify the owning/parent namespace."
> 
> Other /proc/self/ns/* comparison and stat() logic...
> 
> Andy suggested that we may have the same st_dev for mounts in the same
> pid namespace... I'm not sure which side effect this may bring!

Basically we have here a issue because other proc options (hidepid for
example) affect the entire pid namespace, but, I guess, have to affect
the mountpoint.

# grep ^proc /proc/mounts 
proc /proc proc rw,relatime 0 0

# mount -t proc proc /tmp/proc
# mount -o remount,hidepid=2 -t proc proc /tmp/proc

# grep ^proc /proc/mounts 
proc /proc proc rw,relatime,hidepid=2 0 0
proc /tmp/proc proc rw,relatime,hidepid=2 0 0

-- 
Rgrds, legion