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

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Date:   Mon, 9 Dec 2019 13:08:34 -0800
From:   Luke Nelson <lukenels@...washington.edu>
To:     Björn Töpel <bjorn.topel@...il.com>
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, 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>