Re: [PATCH v4 seccomp 1/5] seccomp/cache: Lookup syscall allowlist bitmap for fast path

[Jann Horn]

On Fri, Oct 9, 2020 at 7:15 PM YiFei Zhu <zhuyifei1999@xxxxxxxxx> wrote:
> The overhead of running Seccomp filters has been part of some past
> discussions [1][2][3]. Oftentimes, the filters have a large number
> of instructions that check syscall numbers one by one and jump based
> on that. Some users chain BPF filters which further enlarge the
> overhead. A recent work [6] comprehensively measures the Seccomp
> overhead and shows that the overhead is non-negligible and has a
> non-trivial impact on application performance.
>
> We observed some common filters, such as docker's [4] or
> systemd's [5], will make most decisions based only on the syscall
> numbers, and as past discussions considered, a bitmap where each bit
> represents a syscall makes most sense for these filters.
>
> The fast (common) path for seccomp should be that the filter permits
> the syscall to pass through, and failing seccomp is expected to be
> an exceptional case; it is not expected for userspace to call a
> denylisted syscall over and over.
>
> When it can be concluded that an allow must occur for the given
> architecture and syscall pair (this determination is introduced in
> the next commit), seccomp will immediately allow the syscall,
> bypassing further BPF execution.
>
> Each architecture number has its own bitmap. The architecture
> number in seccomp_data is checked against the defined architecture
> number constant before proceeding to test the bit against the
> bitmap with the syscall number as the index of the bit in the
> bitmap, and if the bit is set, seccomp returns allow. The bitmaps
> are all clear in this patch and will be initialized in the next
> commit.
[...]
> Co-developed-by: Dimitrios Skarlatos <dskarlat@xxxxxxxxxx>
> Signed-off-by: Dimitrios Skarlatos <dskarlat@xxxxxxxxxx>
> Signed-off-by: YiFei Zhu <yifeifz2@xxxxxxxxxxxx>

Reviewed-by: Jann Horn <jannh@xxxxxxxxxx>