Re: [PATCH v2 3/3] virtiofsd: probe unshare(CLONE_FS) and print an error

From: Stefan Hajnoczi <stefanha@redhat.com>
To: Roman Mohr <rmohr@redhat.com>
Cc: "vromanso@redhat.com" <vromanso@redhat.com>,
	Daniel Walsh <dwalsh@redhat.com>,
	"qemu-devel@nongnu.org" <qemu-devel@nongnu.org>,
	"Dr. David Alan Gilbert" <dgilbert@redhat.com>,
	"virtio-fs@redhat.com" <virtio-fs@redhat.com>,
	"misono.tomohiro@fujitsu.com" <misono.tomohiro@fujitsu.com>,
	"mpatel@redhat.com" <mpatel@redhat.com>,
	"vgoyal@redhat.com" <vgoyal@redhat.com>
Subject: Re: [PATCH v2 3/3] virtiofsd: probe unshare(CLONE_FS) and print an error
Date: Tue, 28 Jul 2020 16:32:19 +0100	[thread overview]
Message-ID: <20200728153219.GC21660@stefanha-x1.localdomain> (raw)
In-Reply-To: <CALDPj7syG0KPhtZEma5n403=YFZ2ptcD4MtP=GdrY9n1eUs5Eg@mail.gmail.com>

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On Tue, Jul 28, 2020 at 12:00:20PM +0200, Roman Mohr wrote:
> On Tue, Jul 28, 2020 at 3:07 AM misono.tomohiro@fujitsu.com <
> misono.tomohiro@fujitsu.com> wrote:
> 
> > > Subject: [PATCH v2 3/3] virtiofsd: probe unshare(CLONE_FS) and print an
> > error
> > >
> > > An assertion failure is raised during request processing if
> > > unshare(CLONE_FS) fails. Implement a probe at startup so the problem can
> > > be detected right away.
> > >
> > > Unfortunately Docker/Moby does not include unshare in the seccomp.json
> > > list unless CAP_SYS_ADMIN is given. Other seccomp.json lists always
> > > include unshare (e.g. podman is unaffected):
> > >
> > https://raw.githubusercontent.com/seccomp/containers-golang/master/seccomp.json
> > >
> > > Use "docker run --security-opt seccomp=path/to/seccomp.json ..." if the
> > > default seccomp.json is missing unshare.
> >
> > Hi, sorry for a bit late.
> >
> > unshare() was added to fix xattr problem:
> >
> > https://github.com/qemu/qemu/commit/bdfd66788349acc43cd3f1298718ad491663cfcc#
> > In theory we don't need to call unshare if xattr is disabled, but it is
> > hard to get to know
> > if xattr is enabled or disabled in fv_queue_worker(), right?
> >
> >
> In kubevirt we want to run virtiofsd in containers. We would already not
> have xattr support for e.g. overlayfs in the VM after this patch series (an
> acceptable con at least for us right now).
> If we can get rid of the unshare (and potentially of needing root) that
> would be great. We always assume that everything which we run in containers
> should work for cri-o and docker.

Root is required to access files with any uid/gid.

Dave Gilbert is working on xattr support without CAP_SYS_ADMIN. He may
be able to find a way to drop unshare (at least in containers).

> "Just" pointing docker to a different seccomp.json file is something which
> k8s users/admin in many cases can't do.

There is a Moby PR to change the default seccomp.json file here but it's
unclear if it will be merged:
https://github.com/moby/moby/pull/41244

Stefan

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