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Date:   Mon, 14 Sep 2020 21:36:55 +0300
From:   Ilias Apalodimas <ilias.apalodimas@linaro.org>
To:     Luke Nelson <luke.r.nels@gmail.com>
Cc:     Xi Wang <xi.wang@gmail.com>,
        Jesper Dangaard Brouer <brouer@redhat.com>,
        Will Deacon <will@kernel.org>, bpf <bpf@vger.kernel.org>,
        ardb@kernel.org, naresh.kamboju@linaro.org,
        Jean-Philippe Brucker <jean-philippe@linaro.org>,
        Yauheni Kaliuta <yauheni.kaliuta@redhat.com>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Alexei Starovoitov <ast@kernel.org>,
        Zi Shen Lim <zlim.lnx@gmail.com>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Martin KaFai Lau <kafai@fb.com>,
        Song Liu <songliubraving@fb.com>, Yonghong Song <yhs@fb.com>,
        Andrii Nakryiko <andriin@fb.com>,
        John Fastabend <john.fastabend@gmail.com>,
        KP Singh <kpsingh@chromium.org>,
        "David S. Miller" <davem@davemloft.net>,
        Jakub Kicinski <kuba@kernel.org>,
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        Anders Roxell <anders.roxell@linaro.org>,
        =?iso-8859-1?Q?Bj=F6rn_T=F6pel?= <bjorn.topel@gmail.com>
Subject: Re: [PATCH] arm64: bpf: Fix branch offset in JIT
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Hi Luke, 

On Mon, Sep 14, 2020 at 11:21:58AM -0700, Luke Nelson wrote:
> On Mon, Sep 14, 2020 at 11:08 AM Xi Wang <xi.wang@gmail.com> wrote:
> > I don't think there's some consistent semantics of "offsets" across
> > the JITs of different architectures (maybe it's good to clean that
> > up).  RV64 and RV32 JITs are doing something similar to arm64 with
> > respect to offsets.  CCing Bj?rn and Luke.
> 
> As I understand it, there are two strategies JITs use to keep track of
> the ctx->offset table.
> 
> Some JITs (RV32, RV64, arm32, arm64 currently, x86-32) track the end
> of each instruction (e.g., ctx->offset[i] marks the beginning of
> instruction i + 1).
> This requires care to handle jumps to the first instruction to avoid
> using ctx->offset[-1]. The RV32 and RV64 JITs have special handling
> for this case,
> while the arm32, arm64, and x86-32 JITs appear not to. The arm32 and
> x32 probably need to be fixed for the same reason arm64 does.
> 
> The other strategy is for ctx->offset[i] to track the beginning of
> instruction i. The x86-64 JIT currently works this way.
> This can be easier to use (no need to special case -1) but looks to be
> trickier to construct. This patch changes the arm64 JIT to work this
> way.
> 
> I don't think either strategy is inherently better, both can be
> "correct" as long as the JIT uses ctx->offset in the right way.
> This might be a good opportunity to change the JITs to be consistent
> about this (especially if the arm32, arm64, and x32 JITs all need to
> be fixed anyways).
> Having all JITs agree on the meaning of ctx->offset could help future
> readers debug / understand the code, and could help to someday verify
> the
> ctx->offset construction.
> 
> Any thoughts?

The common strategy does make a lot of sense and yes, both patches will  works 
assuming the ctx->offset ends up being what the JIT engine expects it to be. 
As I mentioned earlier we did consider both, but ended up using the later, 
since as you said, removes the need for handling the special (-1) case.

Cheers
/Ilias

> 
> - Luke