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Subject: Re: [PATCH 2/2] seccomp.2: document userspace notification
To:     Tycho Andersen <tycho@tycho.ws>,
        "Serge E. Hallyn" <serge@hallyn.com>
References: <20181213001106.15268-1-tycho@tycho.ws>
 <20181213001106.15268-3-tycho@tycho.ws>
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From:   "Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com>
Message-ID: <e7a5abd9-f7b7-e077-221a-4c3ef7b818df@gmail.com>
Date:   Thu, 28 Feb 2019 14:25:55 +0100
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> 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?

Cheers,

Michael

-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/