Re: [PATCH bpf-next,v3] riscv, bpf: Add BPF exception tables

From: tongtiangen <tongtiangen@huawei.com>
To: "Daniel Borkmann" <daniel@iogearbox.net>,
	"Björn Töpel" <bjorn.topel@gmail.com>
Cc: Paul Walmsley <paul.walmsley@sifive.com>,
	Palmer Dabbelt <palmer@dabbelt.com>,
	Palmer Dabbelt <palmerdabbelt@google.com>,
	Albert Ou <aou@eecs.berkeley.edu>,
	Luke Nelson <luke.r.nels@gmail.com>, Xi Wang <xi.wang@gmail.com>,
	Alexei Starovoitov <ast@kernel.org>,
	Andrii Nakryiko <andrii@kernel.org>,
	Martin KaFai Lau <kafai@fb.com>, Song Liu <songliubraving@fb.com>,
	Yonghong Song <yhs@fb.com>,
	John Fastabend <john.fastabend@gmail.com>,
	KP Singh <kpsingh@kernel.org>,
	linux-riscv <linux-riscv@lists.infradead.org>,
	LKML <linux-kernel@vger.kernel.org>,
	Netdev <netdev@vger.kernel.org>, bpf <bpf@vger.kernel.org>
Subject: Re: [PATCH bpf-next,v3] riscv, bpf: Add BPF exception tables
Date: Thu, 28 Oct 2021 09:01:54 +0800	[thread overview]
Message-ID: <5c38b790-65e9-6ba2-75bd-121d6d51ab34@huawei.com> (raw)
In-Reply-To: <15487721-b3de-73c7-5ef3-614c6da2f8cd@iogearbox.net>


On 2021/10/28 7:11, Daniel Borkmann wrote:
> On 10/27/21 6:55 PM, Björn Töpel wrote:
>> On Wed, 27 Oct 2021 at 13:03, Tong Tiangen <tongtiangen@huawei.com> 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 riscv 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.
>>>
>>> A more generic solution would add a "handler" field to the table entry,
>>> like on x86 and s390.
>>>
>>> The same issue in ARM64 is fixed in:
>>> commit 800834285361 ("bpf, arm64: Add BPF exception tables")
>>>
>>> Signed-off-by: Tong Tiangen <tongtiangen@huawei.com>
>>> Tested-by: Pu Lehui <pulehui@huawei.com>
>>> ---
>>> v3:
>>> Modify according to Björn's comments, mainly code optimization.
>>
>> Thank you!
>>
>> I ran this patch against the test_bpf.ko, and selftests/bpf -- no
>> regressions, and after the patch is applied more tests passes. Yay!
>>
>> On a related note. The RISC-V selftests/bpf is in a pretty lousy
>> state. I'll send a cleanup patch for them soonish. E.g.:
>
> Thanks for testing!
>
>> * RISC-V is missing in bpf_tracing.h (libbpf)
>> * Some programs don't converge in 16 steps, I had to increase it to ~32
>> * The selftest/bpf Makefile needed some RV specific changes
>> * ...a lot of tests still don't pass, and needs to be looked in to
>
> Sounds good, please ship them. ;)
>
>> Feel free to add:
>>
>> Acked-by: Björn Töpel <bjorn@kernel.org>
>
> Applied, thanks! Tong, if you have a chance, please follow up with Mark's
> suggestion to align the extable infra to arm64/x86.

Thanks, Mark's suggestion is good. I will improve this part if I have the opportunity.
>
> Thanks,
> Daniel
> .
>