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

From: Daniel Borkmann <daniel@iogearbox.net>
To: Song Liu <song@kernel.org>,
	Jean-Philippe Brucker <jean-philippe@linaro.org>
Cc: linux-arm-kernel@lists.infradead.org, bpf <bpf@vger.kernel.org>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Will Deacon <will@kernel.org>,
	Alexei Starovoitov <ast@kernel.org>,
	zlim.lnx@gmail.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>
Subject: Re: [PATCH bpf-next 1/1] arm64: bpf: Add BPF exception tables
Date: Wed, 29 Jul 2020 23:29:43 +0200	[thread overview]
Message-ID: <4791872a-9f7e-1c1c-392c-8b68a13091e3@iogearbox.net> (raw)
In-Reply-To: <CAPhsuW5CmQzELjc8+tQVWZStjPxENhGB7066YJLp=ANs8BYiHA@mail.gmail.com>

On 7/29/20 7:28 PM, Song Liu wrote:
> On Tue, Jul 28, 2020 at 8:37 AM Jean-Philippe Brucker
> <jean-philippe@linaro.org> 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 patch looks good to me.
> 
> Acked-by: Song Liu <songliubraving@fb.com>

+1, applied, thanks a lot!

> It is possible to add a selftest for this? I thought about this a
> little bit, but
> didn't get a good idea.

Why not adding a test_verifier.c test case which calls into bpf_get_current_task()
to fetch pointer to current and then read out some field via BPF_PROBE_MEM which
should then succeed on x86/s390x/arm64 but be skipped on the other archs? Jean-Philippe,
could you look into following up with such test case(s)?

Thanks,
Daniel