new seccomp mode aims to improve performance

[Lennart Poettering]

On Mo, 01.06.20 08:32, Paul Moore (paul at paul-moore.com) wrote:

> In situations where the calling application creates multiple per-ABI
> filters, the seccomp_merge(3) function can be used to merge the
> filters into one.  There are some limitations (same byte ordering,
> filter attributes, etc.) but in general it should work without problem
> when merging x86_64, x32, and x86.

Hmm, so we currently only use seccomp_rule_add_exact() to build an
individual filter and finally seccomp_load() to install it. Which
tells us exactly what works and what does not.

If we now would use seccomp_rule_add_exact() to build the filters, but
then use seccomp_merge() to merge them all, and then only do a single
seccomp_load(), will this give us the same info? i.e. will
seccomp_merge() give us the same errors seccomp_load() currently gives
us when something cannot work?

> > If we wanted to optimize that in userspace, then libseccomp would have
> > to be improved quite substantially to let us know exactly what works
> > and what doesn't, and to have sane fallback both when building
> > whitelists and blacklists.
>
> It has been quite a while since we last talked about systemd's use of
> libseccomp, but the upcoming v2.5.0 release (no date set yet, but
> think weeks not months) finally takes a first step towards defining
> proper return values on error for the API, no more "negative values on
> error".  I'm sure there are other things, but I recall this as being
> one of the bigger systemd wants.

Yes, we care about error codes a lot.

> As an aside, it is always going to be difficult to allow fine grained
> control when you have a single libseccomp filter that includes
> multiple ABIs; the different ABI oddities are just too great (see
> comments above).  If you need exacting control of the filter, or ABI
> specific handling, then the recommended way is to create those filters
> independently and merge them together before loading them into the
> kernel or applying any common rules.

Hmm, so not sure I got this. But are you saying that when using
seccomp_merge() am I supposed to merge filters for different archs
into one megafilter, or are you saying the opposite: am I supposed not
to do that?

I.e. in an ideal world, should we come to a situation where per
service on x86-64 we will have exactly one filter installed, or should
we come to a situation where we'll have exactly three installed, once
for each ABI?

Lennart

--
Lennart Poettering, Berlin