Date:   Thu, 13 Jun 2019 14:50:15 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Stanislav Fomichev <sdf@...ichev.me>
Cc:     Alexei Starovoitov <alexei.starovoitov@...il.com>,
        Stanislav Fomichev <sdf@...gle.com>,
        Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Martin Lau <kafai@...com>, Andrii Nakryiko <andriin@...com>
Subject: Re: [PATCH bpf-next v5 1/8] bpf: implement getsockopt and setsockopt hooks

On Thu, Jun 13, 2019 at 2:20 PM Stanislav Fomichev <sdf@...ichev.me> wrote:
>
> On 06/13, Alexei Starovoitov wrote:
> > C based example doesn't use ret=1.
> > imo that's a sign that something is odd in the api.
> I decided not to test ret=1 it because there are tests in the test_sockopt.c
> for ret=1 usecase. But I can certainly extend C based test to cover
> ret=1 as well. I agree that C based test can be used as an example,
> will extend that to cover ret=0/1/2.
>
> > In particular ret=1 doesn't prohibit bpf prog to modify the optval.
> That's a good point, Martin brought that up as well. We were trying
> to remedy it by doing copy_to_user only if any program returned 2 ("BPF
> handled that, bypass the kernel"). But I agree, the fact that the prog in
> the chain can modify optval and return 1 is suboptimal. Especially if
> the previous one filled in some valid data and returned 2.
>
> > Multiple progs can overwrite it and still return 1.
> > But that optval is not going to be processed by the kernel.
> > Should we do copy_to_user(optval, ctx.optval, ctx.optlen) here
> > and let kernel pick it up from there?
> I was thinking initially about that, that kernel can "transparently"
> modify user buffer and then kernel (or next BPF program in the chain)
> can run standard getsockopt on that.
>
> But it sounds a bit complicated and I don't really have a good use case
> for that.
>
> > Should bpf prog be allowed to change optlen as well?
> > ret=1 would mean that bpf prog did something and needs kernel
> > to continue.
> >
> > Now consider a sequence of bpf progs.
> > Some are doing ret=1. Some others are doing ret=2
> > ret=2 will supersede.
> > If first executed prog (child in cgroup) did ret=2
> > the parent has no way to tell kernel to handle it.
> > Even if parent does ret=1, it's effectively ignored.
> > Parent can enforce rejection with ret=0, but it's a weird
> > discrepancy.
> > The rule for cgroup progs was 'all yes is yes, any no is no'.
> My canonical example when reasoning about multiple progs was that each one
> of them would implement handling for a particular level+optname. So only
> a single one form the chain would return 2 or 0, the rest would return 1
> without touching the buffer. I can't come up with a good use-case where
> two programs in the chain can both return 2 and fill out the buffer.
> The majority of the sockopts would still be handled by the kernel,
> we'd have only a handful of bpf progs that handle a tiny subset
> and delegate the rest to the kernel.
>
> How about we stop further processing as soon as some program in the chain
> returned 2? I think that would address most of the concerns?

What about a case of passive "auditing" BPF programs, that are not
modifying anything, but want to capture every single
getsockopt/setsockopt call? This premature stop would render that
whole approach broken.

> Maybe, in this case, also stop further processing as soon as
> we get ret=0 (EPERM) for consistency?
>
> > So if ret=1 means 'kernel handles it'. Should it be almost
> > as strong as 'reject it': any prog doing ret=1 means 'kernel does it'
> > (unless some prog did ret=0. then reject it) ?
> > if ret=1 means 'bpf did some and needs kernel to continue' that's
> > another story.
> > For ret=2 being 'bpf handled it completely', should parent overwrite it?
> See above, I was thinking the opposite. Treat ret=1 from the BPF
> program as "I'm not interested in this level+optname, other BPF
> program or kernel should do the job". Essentially, as soon as bpf program
> returns 2, that means BPF had consumed the request and no further processing
> from neither BPF, nor kernel is requred; we can return to userspace.
>
> There is a problem that some prog in the chain might do some
> "background" work and still return 1, but so far I don't see why
> that can be useful. The pattern should be: filter the option
> you want, handle it, otherwise return 1 to let the other progs/kernel
> run.
>
> That BPF_F_ALLOW_MULTI use-case probably deserves another selftest :-/
>
> > May be retval from child prog should be seen by parent prog?
> >
> > In some sense kernel can be seen as another bpf prog in a sequence.
> >
> > Whatever new behavior is with 3 values it needs to be
> > documented in uapi/bpf.h
> > We were sloppy with such docs in the past, but that's not
> > a reason to continue.
> Good point on documenting that, I was trying to document everything
> in Documentation/bpf/prog_cgroup_sockopt.rst, uapi/bpf.h seems too
> constrained (I didn't find a good place to put that ret 1 vs 2 info).
> Do you think having a file under Documentation/ with all the details
> is not enough? Where can I put this ret=0/1/2 handing info in the
> uapi/bpf.h?

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