netdev - Re: [PATCH] arm64: bpf: Fix branch offset in JIT

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Date:   Mon, 14 Sep 2020 22:24:07 +0300
From:   Ilias Apalodimas <ilias.apalodimas@...aro.org>
To:     Xi Wang <xi.wang@...il.com>
Cc:     Jesper Dangaard Brouer <brouer@...hat.com>,
        Will Deacon <will@...nel.org>, bpf@...r.kernel.org,
        ardb@...nel.org, naresh.kamboju@...aro.org,
        Jean-Philippe Brucker <jean-philippe@...aro.org>,
        Yauheni Kaliuta <yauheni.kaliuta@...hat.com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Alexei Starovoitov <ast@...nel.org>,
        Zi Shen Lim <zlim.lnx@...il.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        Andrii Nakryiko <andriin@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...omium.org>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Jesper Dangaard Brouer <hawk@...nel.org>,
        netdev@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Anders Roxell <anders.roxell@...aro.org>,
        Björn Töpel <bjorn.topel@...il.com>,
        Luke Nelson <luke.r.nels@...il.com>
Subject: Re: [PATCH] arm64: bpf: Fix branch offset in JIT

On Mon, Sep 14, 2020 at 11:52:16AM -0700, Xi Wang wrote:
> On Mon, Sep 14, 2020 at 11:28 AM Ilias Apalodimas
> <ilias.apalodimas@...aro.org> wrote:
> > Even if that's true, is any reason at all why we should skip the first element
> > of the array, that's now needed since 7c2e988f400 to jump back to the first
> > instruction?
> > Introducing 2 extra if conditions and hotfix the array on the fly (and for
> > every future invocation of that), seems better to you?
> 
> My point was that there's no inherently correct/wrong way to construct
> offsets.  As Luke explained in his email, 1) there are two different
> strategies used by the JITs and 2) there are likely similar bugs
> beyond arm64.
> 
> Each strategy has pros and cons, and I'm fine with either.  I like the
> strategy used in your patch because it's more intuitive (offset[i] is
> the start of the emitted instructions for BPF instruction i, rather
> than the end), though the changes to the construction process are
> trickier.
> 

Well the arm64 was literally a 'save the idx before building the instruction',
and add another element on the array.  So it's not that trickier, especially
if we document it properly.

I haven't checked the rest of the architectures tbh (apart from x86). 
I assumed the tracking used in arm64 at that point, was a result of how 
eBPF worked before bounded loops were introduced. Maybe I was wrong.
It felt a bit more natural to track the beginning of the emitted 
instructions rather than the end.

> If we decide to patch the arm64 JIT the way you proposed, we should
> consider whether to change other JITs consistently.

I think this is a good idea. Following the code is not exactly a stroll in the
park, so we can at least make it consistent across architectures.

Thanks
/Ilias