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From:   Anton Protopopov <a.s.protopopov@gmail.com>
Date:   Tue, 17 Mar 2020 21:11:57 -0400
Message-ID: <CAGn_itz7jgoP5J1pjJ7BLaeh4my=JY2yQ7T8ssoYrqPOWvwKug@mail.gmail.com>
Subject: Re: [PATCH] seccomp: allow BPF_MOD ALU instructions
To:     Kees Cook <keescook@chromium.org>
Cc:     Andy Lutomirski <luto@amacapital.net>,
        Will Drewry <wad@chromium.org>,
        open list <linux-kernel@vger.kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        bpf <bpf@vger.kernel.org>
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=D0=B2=D1=82, 17 =D0=BC=D0=B0=D1=80. 2020 =D0=B3. =D0=B2 16:21, Kees Cook <=
keescook@chromium.org>:
>
> On Mon, Mar 16, 2020 at 06:17:34PM -0400, Anton Protopopov wrote:
> > and in every case to walk only a corresponding factor-list. In my case
> > I had a list of ~40 syscall numbers and after this change filter
> > executed in 17.25 instructions on average per syscall vs. 45
> > instructions for the linear filter (so this removes about 30
> > instructions penalty per every syscall). To replace "mod #4" I
> > actually used "and #3", but this obviously doesn't work for
> > non-power-of-two divisors. If I would use "mod 5", then it would give
> > me about 15.5 instructions on average.
>
> Gotcha. My real concern is with breaking the ABI here -- using BPF_MOD
> would mean a process couldn't run on older kernels without some tricks
> on the seccomp side.

Yes, I understood. Could you tell what would you do exactly if there
was a real need in a new instruction?

> Since the syscall list is static for a given filter, why not arrange it
> as a binary search? That should get even better average instructions
> as O(log n) instead of O(n).

Right, thanks! This saves about 4 more instructions for my case and
works 1-2 ns faster.

> Though frankly I've also been considering an ABI version bump for adding
> a syscall bitmap feature: the vast majority of seccomp filters are just
> binary yes/no across a list of syscalls. Only the special cases need
> special handling (arg inspection, fd notification, etc). Then these
> kinds of filters could run as O(1).
>
> --
> Kees Cook

Thanks,
Anton