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Date:   Fri, 1 Mar 2019 07:53:41 -0700
From:   Tycho Andersen <tycho@tycho.ws>
To:     "Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com>
Cc:     "Serge E. Hallyn" <serge@hallyn.com>, linux-man@vger.kernel.org,
        Kees Cook <keescook@chromium.org>,
        Linux API <linux-api@vger.kernel.org>,
        lkml <linux-kernel@vger.kernel.org>,
        Andy Lutomirski <luto@amacapital.net>,
        Jann Horn <jann@thejh.net>, Oleg Nesterov <oleg@redhat.com>,
        Christian Brauner <christian@brauner.io>,
        "Eric W. Biederman" <ebiederm@xmission.com>,
        Containers <containers@lists.linux-foundation.org>,
        Aleksa Sarai <asarai@suse.de>,
        Tyler Hicks <tyhicks@canonical.com>,
        Akihiro Suda <suda.akihiro@lab.ntt.co.jp>
Subject: Re: [PATCH 2/2] seccomp.2: document userspace notification
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On Thu, Feb 28, 2019 at 02:25:55PM +0100, Michael Kerrisk (man-pages) wrote:
> > 7. The monitoring process can use the information in the
> >    'struct seccomp_notif' to make a determination about the
> >    system call being made by the target process. This
> >    structure includes a 'data' field that is the same
> >    'struct seccomp_data' that is passed to a BPF filter.
> > 
> >    In addition, the monitoring process may make use of other 
> >    information that is available from user space. For example, 
> >    it may inspect the memory of the target process (whose PID
> >    is provided in the 'struct seccomp_notif') using
> >    /proc/PID/mem, which includes inspecting the values
> >    pointed to by system call arguments (whose location is
> >    available 'seccomp_notif.data.args). However, when using
> >    the target process PID in this way, one must guard against
> >    PID re-use race conditions using the seccomp()
> >    SECCOMP_IOCTL_NOTIF_ID_VALID operation.
> > 
> > 8. Having arrived at a decision about the target process's
> >    system call, the monitoring process can inform the kernel
> >    of its decision using the operation
> > 
> >        ioctl(listenfd, SECCOMP_IOCTL_NOTIF_SEND, respptr)
> > 
> >    where the third argument is a pointer to a
> >    'struct seccomp_notif_resp'. [Some more details
> >    needed here, but I still don't yet understand fully
> >    the semantics of the 'error' and 'val' fields.]
> 
> So clearly, I misunderstood these last two steps.
> 
> (7) is something like: discover information in userspace
> as required; perform userspace actions if appropriate
> (perhaps doing the system call operation "on behalf of" the
> target process).
> 
> 
> (8) is something like:
>    set 'error' and 'val' to return info to the target process:
>     * error != 0 ==> make it look like the syscall failed,
>       with 'errno' set to that value
>     * error == 0 ==> make it look like the syscall succeeded 
>       and returned 'val'
> 
> Right?

Yep, exactly.

Tycho