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Subject: Re: [PATCH bpf-next 1/1] arm64: bpf: Add BPF exception tables
To:     Jean-Philippe Brucker <jean-philippe@linaro.org>,
        Qian Cai <cai@lca.pw>
Cc:     linux-arm-kernel@lists.infradead.org, bpf@vger.kernel.org,
        songliubraving@fb.com, andriin@fb.com, catalin.marinas@arm.com,
        john.fastabend@gmail.com, ast@kernel.org, zlim.lnx@gmail.com,
        kpsingh@chromium.org, yhs@fb.com, will@kernel.org, kafai@fb.com,
        sfr@canb.auug.org.au, linux-next@vger.kernel.org,
        linux-kernel@vger.kernel.org
References: <20200728152122.1292756-1-jean-philippe@linaro.org>
 <20200728152122.1292756-2-jean-philippe@linaro.org>
 <20200730122855.GA3773@lca.pw> <20200730142213.GB1529030@myrica>
From:   Daniel Borkmann <daniel@iogearbox.net>
Message-ID: <f2f05f41-ccf9-e693-85bf-59ebbf8dadfe@iogearbox.net>
Date:   Thu, 30 Jul 2020 21:47:39 +0200
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On 7/30/20 4:22 PM, Jean-Philippe Brucker wrote:
> On Thu, Jul 30, 2020 at 08:28:56AM -0400, Qian Cai wrote:
>> On Tue, Jul 28, 2020 at 05:21:26PM +0200, Jean-Philippe Brucker wrote:
>>> When a tracing BPF program attempts to read memory without using the
>>> bpf_probe_read() helper, the verifier marks the load instruction with
>>> the BPF_PROBE_MEM flag. Since the arm64 JIT does not currently recognize
>>> this flag it falls back to the interpreter.
>>>
>>> Add support for BPF_PROBE_MEM, by appending an exception table to the
>>> BPF program. If the load instruction causes a data abort, the fixup
>>> infrastructure finds the exception table and fixes up the fault, by
>>> clearing the destination register and jumping over the faulting
>>> instruction.
>>>
>>> To keep the compact exception table entry format, inspect the pc in
>>> fixup_exception(). A more generic solution would add a "handler" field
>>> to the table entry, like on x86 and s390.
>>>
>>> Signed-off-by: Jean-Philippe Brucker <jean-philippe@linaro.org>
>>
>> This will fail to compile on arm64,
>>
>> https://gitlab.com/cailca/linux-mm/-/blob/master/arm64.config
>>
>> arch/arm64/mm/extable.o: In function `fixup_exception':
>> arch/arm64/mm/extable.c:19: undefined reference to `arm64_bpf_fixup_exception'
> 
> Thanks for the report, I attached a fix. Daniel, can I squash it and
> resend as v2 or is it too late?

If you want I can squash your attached snippet into the original patch of
yours. If you want to send a v2 that is fine as well of course. Let me know.

Thanks,
Daniel