Re: [RFC PATCH seccomp 0/2] seccomp: Add bitmap cache of arg-independent filter results that allow syscalls

[Tycho Andersen]

On Mon, Sep 21, 2020 at 10:27:56AM -0500, YiFei Zhu wrote:
> On Mon, Sep 21, 2020 at 8:51 AM Tycho Andersen <tycho@tycho.pizza> wrote:
> > One problem with a kernel config setting is that it's for all tasks.
> > While docker and systemd may make decsisions based on syscall number,
> > other applications may have more nuanced filters, and this cache would
> > yield incorrect results.
> >
> > You could work around this by making this a filter flag instead;
> > filter authors would generally know whether their filter results can
> > be cached and probably be motivated to opt in if their users are
> > complaining about slow syscall execution.
> >
> > Tycho
> 
> Yielding incorrect results should not be possible. The purpose of the
> "emulator" (for the lack of a better term) is to determine whether the
> filter reads any syscall arguments. A read from a syscall argument
> must go through the BPF_LD | BPF_ABS instruction, where the 32 bit
> multiuse field "k" is an offset to struct seccomp_data.

I see, I missed this somehow. So is there a reason to hide this behind
a config option? Isn't it just always better?

Tycho