вт, 17 мар. 2020 г. в 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
