Re: [RFC PATCH seccomp 2/2] seccomp/cache: Cache filter results that allow syscalls

[Jann Horn]

On Tue, Sep 22, 2020 at 12:51 AM YiFei Zhu <zhuyifei1999@xxxxxxxxx> wrote:
> On Mon, Sep 21, 2020 at 1:09 PM Jann Horn <jannh@xxxxxxxxxx> wrote:
> >
> > On Mon, Sep 21, 2020 at 7:35 AM YiFei Zhu <zhuyifei1999@xxxxxxxxx> wrote:
> > [...]
> > > We do this by creating a per-task bitmap of permitted syscalls.
> > > If seccomp filter is invoked we check if it is cached and if so
> > > directly return allow. Else we call into the cBPF filter, and if
> > > the result is an allow then we cache the results.
> >
> > What? Why? We already have code to statically evaluate the filter for
> > all syscall numbers. We should be using the results of that instead of
> > re-running the filter and separately caching the results.
> >
> > > The cache is per-task
> >
> > Please don't. The static results are per-filter, so the bitmask(s)
> > should also be per-filter and immutable.
>
> I do agree that an immutable bitmask is faster and easier to reason
> about its correctness. However, I did not find the "code to statically
> evaluate the filter for all syscall numbers" while reading seccomp.c.
> Would you give me a pointer to that and I will see how to best make
> use of it?

I'm talking about the code you're adding in the other patch ("[RFC
PATCH seccomp 1/2] seccomp/cache: Add "emulator" to check if filter is
arg-dependent"). Sorry, that was a bit unclear.