linux-kernel - Re: [PATCH v8 1/2] seccomp: add a return code to trap to userspace

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Date:   Tue, 30 Oct 2018 15:54:04 -0600
From:   Tycho Andersen <tycho@...ho.ws>
To:     Kees Cook <keescook@...omium.org>
Cc:     Andy Lutomirski <luto@...capital.net>,
        Oleg Nesterov <oleg@...hat.com>,
        "Eric W . Biederman" <ebiederm@...ssion.com>,
        "Serge E . Hallyn" <serge@...lyn.com>,
        Christian Brauner <christian@...uner.io>,
        Tyler Hicks <tyhicks@...onical.com>,
        Akihiro Suda <suda.akihiro@....ntt.co.jp>,
        Aleksa Sarai <asarai@...e.de>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux Containers <containers@...ts.linux-foundation.org>,
        Linux API <linux-api@...r.kernel.org>
Subject: Re: [PATCH v8 1/2] seccomp: add a return code to trap to userspace

On Tue, Oct 30, 2018 at 02:49:21PM -0700, Kees Cook wrote:
> On Mon, Oct 29, 2018 at 3:40 PM, Tycho Andersen <tycho@...ho.ws> wrote:
> >     * switch to a flags based future-proofing mechanism for struct
> >       seccomp_notif and seccomp_notif_resp, thus avoiding version issues
> >       with structure length (Kees)
> [...]
> >
> > +struct seccomp_notif {
> > +       __u64 id;
> > +       __u32 pid;
> > +       __u32 flags;
> > +       struct seccomp_data data;
> > +};
> > +
> > +struct seccomp_notif_resp {
> > +       __u64 id;
> > +       __s64 val;
> > +       __s32 error;
> > +       __u32 flags;
> > +};
> 
> Hrm, so, what's the plan for when struct seccomp_data changes size?

I guess my plan was don't ever change the size again, just use flags
and have extra state available via ioctl().

> I'm realizing that it might be "too late" for userspace to discover
> it's running on a newer kernel. i.e. it gets a user notification, and
> discovers flags it doesn't know how to handle. Do we actually need
> both flags AND a length? Designing UAPI is frustrating! :)

:). I don't see this as such a big problem -- in fact it's better than
the length mode, where you don't know what you don't know, because it
only copied as much info as you could handle. Older userspace would
simply not use information it didn't know how to use.

> Do we need another ioctl to discover the seccomp_data size maybe?

That could be an option as well, assuming we agree that size would
work, which I thought we didn't?

Tycho