linux-kernel - Re: [PATCH bpf-next 1/1] arm64: bpf: Add BPF exception tables

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Date:   Fri, 31 Jul 2020 00:45:29 +0200
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Jean-Philippe Brucker <jean-philippe@...aro.org>
Cc:     Qian Cai <cai@....pw>, linux-arm-kernel@...ts.infradead.org,
        bpf@...r.kernel.org, songliubraving@...com, andriin@...com,
        catalin.marinas@....com, john.fastabend@...il.com, ast@...nel.org,
        zlim.lnx@...il.com, kpsingh@...omium.org, yhs@...com,
        will@...nel.org, kafai@...com, sfr@...b.auug.org.au,
        linux-next@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH bpf-next 1/1] arm64: bpf: Add BPF exception tables

On 7/30/20 11:14 PM, Jean-Philippe Brucker wrote:
> On Thu, Jul 30, 2020 at 09:47:39PM +0200, Daniel Borkmann wrote:
>> 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@...aro.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.
> 
> Yes please squash it into the original patch, sorry for the mess

Done, thanks!