[PATCH 2/2] seccomp.2: document userspace notification

Michael Kerrisk (man-pages) mtk.manpages at gmail.com
Fri Mar 1 15:13:46 UTC 2019

On 3/1/19 3:53 PM, Tycho Andersen wrote:
> 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()
>>> 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

That piece should be amended:
error < 0 ==> make it look like syscall failed.
error > 0 ==> make it look like the syscall succeeded 
       and returned 'error'

Is that really supposed to happen?

>>     * error == 0 ==> make it look like the syscall succeeded 
>>       and returned 'val'



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

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