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

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Date:   Wed, 27 Oct 2021 18:55:10 +0200
From:   Björn Töpel <bjorn.topel@...il.com>
To:     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>,
        Daniel Borkmann <daniel@...earbox.net>,
        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 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.:

* 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

Feel free to add:

Acked-by: Björn Töpel <bjorn@...nel.org>

> v2:
> Modify according to Björn's comments, mainly removes redundant head files
> extable.h and some code style issues.
>
>  arch/riscv/mm/extable.c         |  19 +++-
>  arch/riscv/net/bpf_jit.h        |   1 +
>  arch/riscv/net/bpf_jit_comp64.c | 185 +++++++++++++++++++++++++-------
>  arch/riscv/net/bpf_jit_core.c   |  19 ++--
>  4 files changed, 177 insertions(+), 47 deletions(-)
>

[...]

Björn