Re: [PATCH] seccomp: allow BPF_MOD ALU instructions

[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.

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).

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