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

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Message-ID: <15487721-b3de-73c7-5ef3-614c6da2f8cd@iogearbox.net>
Date:   Thu, 28 Oct 2021 01:11:03 +0200
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Björn Töpel <bjorn.topel@...il.com>,
        Tong Tiangen <tongtiangen@...wei.com>
Cc:     Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Palmer Dabbelt <palmerdabbelt@...gle.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        Luke Nelson <luke.r.nels@...il.com>,
        Xi Wang <xi.wang@...il.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>,
        linux-riscv <linux-riscv@...ts.infradead.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Netdev <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>
Subject: Re: [PATCH bpf-next,v3] riscv, bpf: Add BPF exception tables

On 10/27/21 6:55 PM, Björn Töpel wrote:
> On Wed, 27 Oct 2021 at 13:03, Tong Tiangen <tongtiangen@...wei.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@...wei.com>
>> Tested-by: Pu Lehui <pulehui@...wei.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@...nel.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,
Daniel