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 14:49:21 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Tycho Andersen <tycho@...ho.ws>
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 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'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! :)

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

-- 
Kees Cook