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Date:   Mon, 14 Sep 2020 19:02:00 +0300
From:   Ilias Apalodimas <ilias.apalodimas@linaro.org>
To:     Will Deacon <will@kernel.org>
Cc:     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>,
        Jesper Dangaard Brouer <hawk@kernel.org>,
        netdev@vger.kernel.org, linux-arm-kernel@lists.infradead.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH] arm64: bpf: Fix branch offset in JIT
Message-ID: <20200914160200.GA19026@apalos.home>
References: <20200914083622.116554-1-ilias.apalodimas@linaro.org>
 <20200914122042.GA24441@willie-the-truck>
 <20200914123504.GA124316@apalos.home>
 <20200914132350.GA126552@apalos.home>
 <20200914140114.GG24441@willie-the-truck>
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Hi Will,

On Mon, Sep 14, 2020 at 03:01:15PM +0100, Will Deacon wrote:
> Hi Ilias,
> 

[...]

> > > > 
> > > > No Fixes: tag?
> > > 
> > > I'll re-spin and apply one 
> > > 
> > Any suggestion on any Fixes I should apply? The original code was 'correct' and
> > broke only when bounded loops and their self-tests were introduced.
> 
> Ouch, that's pretty bad as it means nobody is regression testing BPF on
> arm64 with mainline. Damn.

That might not be entirely true. Since offset is a pointer, there's a chance
(and a pretty high one according to my reproducer) that the offset[-1] value 
happens to be 0. In that case the tests will pass fine. I can reproduce the bug
approximately 1 every 6-7 passes here.

I'll send a v2 shortly fixing the tags and adding a few comments on the code,
which will hopefully make future reading easier.

Cheers
/Ilias