new seccomp mode aims to improve performance

[Kees Cook]

On Fri, May 29, 2020 at 08:43:56AM -0700, Alexei Starovoitov wrote:
> On Fri, May 29, 2020 at 5:50 AM zhujianwei (C) <zhujianwei7 at huawei.com> wrote:
> >
> > Hi, all
> >
> > 　　We're using seccomp to increase container security, but bpf rules filter causes performance to deteriorate. So, is there a good solution to improve performance, or can we add a simplified seccomp mode to improve performance?

Yes, there are already plans for a simple syscall bitmap[1] seccomp feature.

> I don't think your hunch at where cpu is spending cycles is correct.
> Could you please do two experiments:
> 1. try trivial seccomp bpf prog that simply returns 'allow'
> 2. replace bpf_prog_run_pin_on_cpu() in seccomp.c with C code
>   that returns 'allow' and make sure it's noinline or in a different C file,
>   so that compiler doesn't optimize the whole seccomp_run_filters() into a nop.
> 
> Then measure performance of both.
> I bet you'll see exactly the same numbers.

Android has already done this, it appeared to not be the same. Calling
into a SECCOMP_RET_ALLOW filter had a surprisingly high cost. I'll see
if I can get you the numbers. I was frankly quite surprised -- I
understood the bulk of the seccomp overhead to be in taking the TIF_WORK
path, copying arguments, etc, but something else is going on there.

-Kees

[1] https://lore.kernel.org/lkml/202005181120.971232B7B@keescook/

-- 
Kees Cook