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
