[CRIU] [PATCH 2/2] restore: remount /proc after clone(CLONE_NEWPID)

[Tycho Andersen]

Hi Andrew,

On Wed, Aug 06, 2014 at 11:54:06PM +0400, Andrew Vagin wrote:
> On Wed, Aug 06, 2014 at 01:52:17PM -0500, Tycho Andersen wrote:
> > Hi Andrew,
> > 
> > On Wed, Aug 06, 2014 at 10:44:12PM +0400, Andrew Vagin wrote:
> > > On Wed, Aug 06, 2014 at 01:34:07PM -0500, Tycho Andersen wrote:
> > > > Hi Andrew,
> > > > 
> > > > On Wed, Aug 06, 2014 at 10:25:10PM +0400, Andrew Vagin wrote:
> > > > > On Wed, Aug 06, 2014 at 01:06:29PM -0500, Tycho Andersen wrote:
> > > > > > We need to remount /proc after the clone because things like getpid() return
> > > > > > the pid in the new namespace, but /proc still has the old namespace's info in
> > > > > > it. This causes problems when e.g. there are some things in criu's private
> > > > > > mount namespace but not in (the original) init's namespace.
> > > > > 
> > > > > Could you show an example of problems?
> > > > 
> > > > Yes, if you:
> > > > 
> > > > 1. unshare(CLONE_NEWNS)
> > > > 2. mount() some directory to pass as --root
> > > > 3. exec(criu)
> > > 
> > > Could you show how criu fails in this case?
> > 
> > Yes,
> > 
> > > > 
> > > > criu fails with "(mount.c:1958): New root and old root are the same".
> 
> And this patch fixes this error? I don't understand how it can be.

Yes. When /proc is remounted, the process' "private" (i.e. whatever
happened after unshare()) mount namespace is visible in
/proc/1/mountinfo. Since it is visible, criu parses it and resolves it
correctly and all is happy.

> Tycho, could you look at https://github.com/avagin/criu/commit/6d1cc9acb7e3f442cafbb4b6ff2596ad32b7ac34
> This patch reworks code, which reports the error "(mount.c:1958): New
> root and old root are the same".

This would fix my issue as well (I think). Won't it leak the mount if
criu doesn't create a new mount namespace, though? (I do think this
nicer than forcing the user to mount their own --root.)

> > 
> > what other information would be useful?
> 
> How do you mount the directory to pass as --root?

mount(rootfs->path, rootfs->mount, NULL, MS_BIND, NULL)

where rootfs here is some internal lxc structure. It is also the
reason I ask the above question: rootfs->mount happens to be the same
for (most) lxcs, it is just mounted in a different mount namespace for
each of them.

Tycho