linux-kernel - Re: [PATCH bpf-next 08/10] riscv, bpf: Add ex_insn_off and ex_jmp

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Message-ID: <3e0c7e92-9158-4dd4-8169-e72dad01fd16@meta.com>
Date: Mon, 15 Sep 2025 10:23:39 -0400
From: Chris Mason <clm@...a.com>
To: Pu Lehui <pulehui@...weicloud.com>
Cc: bpf@...r.kernel.org, linux-riscv@...ts.infradead.org,
        linux-kernel@...r.kernel.org,
        Björn Töpel
 <bjorn@...nel.org>,
        Puranjay Mohan <puranjay@...nel.org>,
        Palmer Dabbelt <palmer@...belt.com>, Alexandre Ghiti <alex@...ti.fr>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau
 <martin.lau@...ux.dev>,
        Eduard Zingerman <eddyz87@...il.com>, Song Liu <song@...nel.org>,
        Yonghong Song <yonghong.song@...ux.dev>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>, Stanislav Fomichev <sdf@...ichev.me>,
        Hao Luo <haoluo@...gle.com>, Jiri Olsa <jolsa@...nel.org>,
        Pu Lehui <pulehui@...wei.com>
Subject: Re: [PATCH bpf-next 08/10] riscv, bpf: Add ex_insn_off and ex_jmp_off
 for exception table handling



On 9/15/25 5:39 AM, Pu Lehui wrote:
> 
> 
> On 2025/9/13 23:51, Chris Mason wrote:
>> On Sat, 19 Jul 2025 09:17:28 +0000 Pu Lehui <pulehui@...weicloud.com> wrote:
>>
>>> From: Pu Lehui <pulehui@...wei.com>
>>>
>>> Add ex_insn_off and ex_jmp_off fields to struct rv_jit_context so that
>>> add_exception_handler() does not need to be immediately followed by the
>>> instruction to add the exception table. ex_insn_off indicates the offset
>>> of the instruction to add the exception table, and ex_jmp_off indicates
>>> the offset to jump over the faulting instruction. This is to prepare for
>>> adding the exception table to atomic instructions later, because some
>>> atomic instructions need to perform zext or other operations.
>>>
>>
>> Hi everyone,
>>
>> I've been working on some patch review automation, and I recently ran it on
>> the bpf-next branch.  I don't know the verifier well enough to decide if this
>> is a false positive, but Alexei asked me to kick off discussion, so:
>>
>>> diff --git a/arch/riscv/net/bpf_jit_comp64.c b/arch/riscv/net/bpf_jit_comp64.c
>>> index 8e813809d3054..56b592af53a64 100644
>>> --- a/arch/riscv/net/bpf_jit_comp64.c
>>> +++ b/arch/riscv/net/bpf_jit_comp64.c
>>
>> [ ... ]
>>
>>> -static int emit_stx(u8 rd, s16 off, u8 rs, u8 size, struct rv_jit_context *ctx)
>>> +static void emit_stx(u8 rd, s16 off, u8 rs, u8 size, struct rv_jit_context *ctx)
>>>   {
>>> -	int insns_start;
>>> -
>>>   	if (is_12b_int(off)) {
>>> -		insns_start = ctx->ninsns;
>>> +		ctx->ex_insn_off = ctx->ninsns;
>>>   		emit_stx_insn(rd, off, rs, size, ctx);
>>> -		return ctx->ninsns - insns_start;
>>> +		ctx->ex_jmp_off = ctx->ex_jmp_off;
> 
> Hi Chris,
> 
> I'm not sure if I have misunderstood your intention, maybe just for 
> talking about reviewing automation? But the code I checked in the 
> bpf-next branch is inconsistent with yours. The code here in bpf-next is:
> 
> `ctx->ex_jmp_off = ctx->ninsns;`
> 
> https://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git/tree/arch/riscv/net/bpf_jit_comp64.c?h=next-20250912#n553 

You're right, the review tooling got confused because it only indexed
the x86 version.  Sorry about that, thanks for double checking.

-chris