On Fri, May 29, 2020 at 09:09:28AM -0700, Kees Cook wrote: > On Fri, May 29, 2020 at 08:43:56AM -0700, Alexei Starovoitov wrote: > > 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. So while it's not the Android measurements, here's what I'm seeing on x86_64 (this is hardly a perfect noiseless benchmark, but sampling error appears to close to 1%): net.core.bpf_jit_enable=0: Benchmarking 16777216 samples... 10.633756139 - 0.004359714 = 10629396425 getpid native: 633 ns 23.008737499 - 10.633967641 = 12374769858 getpid RET_ALLOW 1 filter: 737 ns 36.723141843 - 23.008975696 = 13714166147 getpid RET_ALLOW 2 filters: 817 ns 47.751422021 - 36.723345630 = 11028076391 getpid BPF-less allow: 657 ns Estimated total seccomp overhead for 1 filter: 104 ns Estimated total seccomp overhead for 2 filters: 184 ns Estimated seccomp per-filter overhead: 80 ns Estimated seccomp entry overhead: 24 ns Estimated BPF overhead per filter: 80 ns net.core.bpf_jit_enable=1: net.core.bpf_jit_harden=1: Benchmarking 16777216 samples... 31.939978606 - 21.275190689 = 10664787917 getpid native: 635 ns 43.324592380 - 31.940794751 = 11383797629 getpid RET_ALLOW 1 filter: 678 ns 55.001650599 - 43.326293248 = 11675357351 getpid RET_ALLOW 2 filters: 695 ns 65.986452855 - 55.002249904 = 10984202951 getpid BPF-less allow: 654 ns Estimated total seccomp overhead for 1 filter: 43 ns Estimated total seccomp overhead for 2 filters: 60 ns Estimated seccomp per-filter overhead: 17 ns Estimated seccomp entry overhead: 26 ns Estimated BPF overhead per filter: 24 ns net.core.bpf_jit_enable=1: net.core.bpf_jit_harden=0: Benchmarking 16777216 samples... 10.684681435 - 0.004198682 = 10680482753 getpid native: 636 ns 22.050823167 - 10.685571417 = 11365251750 getpid RET_ALLOW 1 filter: 677 ns 33.714134291 - 22.051100183 = 11663034108 getpid RET_ALLOW 2 filters: 695 ns 44.793312551 - 33.714383001 = 11078929550 getpid BPF-less allow: 660 ns Estimated total seccomp overhead for 1 filter: 41 ns Estimated total seccomp overhead for 2 filters: 59 ns Estimated seccomp per-filter overhead: 18 ns Estimated seccomp entry overhead: 23 ns Estimated BPF overhead per filter: 17 ns The above is from my (very dangerous!) benchmarking patch[1]. So, with the layered nature of seccomp filters there's a reasonable gain to be seen for a O(1) bitmap lookup to skip running even a single filter, even for the fastest BPF mode. Not that we need to optimize for the pathological case, but this would be especially useful for cases like systemd, which appears to be constructing seccomp filters very inefficiently maybe on a per-syscall[3] basis? For example, systemd-resolved has 32 (!) seccomp filters attached[2]: # grep ^Seccomp_filters /proc/$(pidof systemd-resolved)/status Seccomp_filters: 32 # grep SystemCall /lib/systemd/system/systemd-resolved.service SystemCallArchitectures=native SystemCallErrorNumber=EPERM SystemCallFilter=@system-service I'd like to better understand what they're doing, but haven't had time to dig in. (The systemd devel mailing list requires subscription, so I've directly CCed some systemd folks that have touched seccomp there recently. Hi! The starts of this thread is here[4].) -Kees [1] https://git.kernel.org/pub/scm/linux/kernel/git/kees/linux.git/commit/?h=seccomp/benchmark-bpf&id=20cc7d8f4238ea3bc1798f204bb865f4994cca27 [2] https://git.kernel.org/pub/scm/linux/kernel/git/kees/linux.git/commit/?h=for-next/seccomp&id=9d06f16f463cef5c445af9738efed2bfe4c64730 [3] https://www.freedesktop.org/software/systemd/man/systemd.exec.html#SystemCallFilter= [4] https://lore.kernel.org/bpf/c22a6c3cefc2412cad00ae14c1371711@xxxxxxxxxx/ -- Kees Cook
