Re: [PATCH] seccomp: allow BPF_MOD ALU instructions

[Anton Protopopov]

ÐÑ, 17 ÐÐÑ. 2020 Ð. Ð 16:21, Kees Cook <keescook@xxxxxxxxxxxx>:
>
> 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