netdev - Re: [PATCH bpf-next 2/8] riscv, bpf: add support for far branching

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Message-ID: <CAJ+HfNi4Ht_+a7+-NWE0LLfGRXJDq1g0cpuyshHY=BJ-+UX4ig@mail.gmail.com>
Date:   Tue, 10 Dec 2019 06:32:45 +0100
From:   Björn Töpel <bjorn.topel@...il.com>
To:     Luke Nelson <lukenels@...washington.edu>
Cc:     Daniel Borkmann <daniel@...earbox.net>,
        Alexei Starovoitov <ast@...nel.org>,
        Netdev <netdev@...r.kernel.org>, linux-riscv@...ts.infradead.org,
        bpf <bpf@...r.kernel.org>, Xi Wang <xi.wang@...il.com>
Subject: Re: [PATCH bpf-next 2/8] riscv, bpf: add support for far branching

On Mon, 9 Dec 2019 at 22:08, Luke Nelson <lukenels@...washington.edu> wrote:
>
> On Mon, Dec 9, 2019 at 9:32 AM Björn Töpel <bjorn.topel@...il.com> wrote:
> >
> > This commit adds branch relaxation to the BPF JIT, and with that
> > support for far (offset greater than 12b) branching.
> >
> > The branch relaxation requires more than two passes to converge. For
> > most programs it is three passes, but for larger programs it can be
> > more.
> >
> > Signed-off-by: Björn Töpel <bjorn.topel@...il.com>
>
> We have been developing a formal verification tool for BPF JIT
> compilers, which we have used in the past to find bugs in the RV64
> and x32 BPF JITs:
>
> https://unsat.cs.washington.edu/projects/serval/
>
> Recently I added support for verifying the JIT for branch and jump
> instructions, and thought it a good opportunity to verify these
> patches that add support for far jumps and branching.
>
> I ported these patches to our tool and ran verification, which
> didn't find any bugs according to our specification of BPF and
> RISC-V.
>
> The tool and code are publicly available, and you can read a more
> detailed writeup of the results here:
>
> https://github.com/uw-unsat/bpf-jit-verif/tree/far-jump-review
>
> Currently the tool works on a manually translated version of the
> JIT from C to Rosette, but we are experimenting with ways of making
> this process more automated.
>
>
> Reviewed-by: Luke Nelson <lukenels@...washington.edu>
> Cc: Xi Wang <xi.wang@...il.com>

Wow! Very cool! Thanks a bunch for this!