netdev - Re: [PATCH v3 bpf-next 0/9] bpf: bounded loops and other features

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Date:   Mon, 17 Jun 2019 12:06:42 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Alexei Starovoitov <ast@...com>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Daniel Borkmann <daniel@...earbox.net>,
        Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
        Kernel Team <Kernel-team@...com>
Subject: Re: [PATCH v3 bpf-next 0/9] bpf: bounded loops and other features

On Mon, Jun 17, 2019 at 11:58 AM Alexei Starovoitov <ast@...com> wrote:
>
> On 6/17/19 9:39 AM, Andrii Nakryiko wrote:
> > On Sat, Jun 15, 2019 at 12:12 PM Alexei Starovoitov <ast@...nel.org> wrote:
> >>
> >> v2->v3: fixed issues in backtracking pointed out by Andrii.
> >> The next step is to add a lot more tests for backtracking.
> >>
> >
> > Tests would be great, verifier complexity is at the level, where it's
> > very easy to miss issues.
> >
> > Was fuzzying approach ever discussed for BPF verifier? I.e., have a
> > fuzzer to generate both legal and illegal random small programs. Then
> > re-implement verifier as user-level program with straightforward
> > recursive exhaustive verification (so no state pruning logic, no
> > precise/coarse, etc, just register/stack state tracking) of all
> > possible branches. If kernel verifier's verdict differs from
> > user-level verifier's verdict - flag that as a test case and figure
> > out why they differ. Obviously that would work well only for small
> > programs, but that should be a good first step already.
> >
> > In addition, if this is done, that user-land verifier can be a HUGE
> > help to BPF application developers, as libbpf would (potentially) be
> > able to generate better error messages using it as well.
>
> In theory that sounds good, but doesn't work in practice.
> The kernel verifier keeps changing faster than user space can catch up.
> It's also relying on loaded maps and all sorts of callbacks that
> check context, allowed helpers, maps, combinations of them from all
> over the kernel.
> The last effort to build kernel verifier as-is into .o and link
> with kmalloc/map wrappers in user space was here:
> https://github.com/iovisor/bpf-fuzzer
> It was fuzzing the verifier and was able to find few minor bugs.
> But it quickly bit rotted.
>
> Folks brought up in the past the idea to collect user space
> verifiers from different kernels, so that user space tooling can
> check whether particular program will load on a set of kernels
> without need to run them in VMs.
> Even if such feature existed today it won't really solve this production
> headache, since all kernels prior to today will not be covered.
>
> I think syzbot is still generating bpf programs. iirc it found
> one bug in the past in the verifier core.
> I think the only way to make verifier more robust is to keep
> adding new test cases manually.
> Most interesting bugs we found by humans.
>
> Another approach to 'better error message' that was considered
> in the past was to teach llvm to recognize things that verifier
> will reject and let llvm warn on them.
> But it's also not practical. We had llvm error on calls.
> Then we added them to the verifier and had to change llvm.
> If we had llvm error on loops, now we'd need to change it.
> imo it's better to let llvm handle everything.

That all makes sense. Thanks for elaborate explanation!