Re: [RFC 0/3] seccomp trap to userspace

[Andy Lutomirski]

On Thu, Mar 15, 2018 at 4:09 PM, Christian Brauner
<christian.brauner@xxxxxxxxxxxxx> wrote:
> On Sun, Feb 04, 2018 at 11:49:43AM +0100, Tycho Andersen wrote:
>> Several months ago at Linux Plumber's, we had a discussion about adding a
>> feature to seccomp which would allow seccomp to trigger a notification for some
>> other process. Here's a draft of that feature.
>>
>> Patch 1 contains the bulk of it, patches 2 & 3 offer an alternative way to
>> acquire the fd that receives notifications via ptrace (the method in patch 1
>> poses some problems). Other suggestions for how to acquire one of these fds
>> would be welcome.
>>
>> Take a close look at the synchronization. I think I've got it right, but I
>> probably don't :)
>>
>> Thanks!
>>
>> Tycho Andersen (3):
>>   seccomp: add a return code to trap to userspace
>>   seccomp: hoist out filter resolving logic
>>   seccomp: add a way to get a listener fd from ptrace
>>
>>  arch/Kconfig                                  |   7 +
>>  include/linux/seccomp.h                       |  14 +-
>>  include/uapi/linux/ptrace.h                   |   1 +
>>  include/uapi/linux/seccomp.h                  |  18 +-
>>  kernel/ptrace.c                               |   4 +
>>  kernel/seccomp.c                              | 467 ++++++++++++++++++++++++--
>>  tools/testing/selftests/seccomp/seccomp_bpf.c | 180 +++++++++-
>>  7 files changed, 653 insertions(+), 38 deletions(-)
>
> Hey,
>
> So, I've been following the discussion silently in the background and I
> see that it got sidetracked into seccomp + ebpf. While I can see that
> there is value in adding epbf support to seccomp I'd really like to see
> this decoupled from this patchset. Afaict, this patchset would just work
> fine without the ebpf portion (but I might be just have missed the
> point). So if possible I would like to see a second version of this with
> the comments accounted for and - if possible - have this up for merging
> independent of the ebpf patchset that's floating around.
>

The issue is that it might be (and, then again, might not be) nicer to
to *synchronously* call out to the monitor in the filter.  eBPF can do
that very cleanly, whereas classic BPF can't.