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

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