netdev - Re: [PATCH bpf-next v11 02/12] bpf: use the least significant byte for the nr

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Message-Id: <d91544bd4b513290bf468386c0261f913b59b761c34405839c424f8eaf8b1d04@mail.kernel.org>
Date: Fri, 23 Jan 2026 08:07:45 +0000 (UTC)
From: bot+bpf-ci@...nel.org
To: menglong8.dong@...il.com,andrii@...nel.org
Cc: daniel@...earbox.net,martin.lau@...ux.dev,eddyz87@...il.com,song@...nel.org,yonghong.song@...ux.dev,john.fastabend@...il.com,kpsingh@...nel.org,sdf@...ichev.me,haoluo@...gle.com,jolsa@...nel.org,davem@...emloft.net,dsahern@...nel.org,tglx@...utronix.de,mingo@...hat.com,jiang.biao@...ux.dev,bp@...en8.de,dave.hansen@...ux.intel.com,x86@...nel.org,hpa@...or.com,bpf@...r.kernel.org,netdev@...r.kernel.org,linux-kernel@...r.kernel.org,ast@...nel.org,andrii@...nel.org,daniel@...earbox.net,martin.lau@...nel.org,eddyz87@...il.com,yonghong.song@...ux.dev,clm@...a.com,ihor.solodrai@...ux.dev
Subject: Re: [PATCH bpf-next v11 02/12] bpf: use the least significant byte for the nr_args in trampoline

> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index 41bbed6418b5..b3f912ad18ba 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c

[ ... ]

> @@ -23779,12 +23781,13 @@ static int do_misc_fixups(struct bpf_verifier_env *env)
>  			    eatype == BPF_MODIFY_RETURN) {
>  				/* Load nr_args from ctx - 8 */
>  				insn_buf[0] = BPF_LDX_MEM(BPF_DW, BPF_REG_0, BPF_REG_1, -8);
> -				insn_buf[1] = BPF_ALU64_IMM(BPF_LSH, BPF_REG_0, 3);
> +				insn_buf[1] = BPF_ALU64_IMM(BPF_AND, BPF_REG_0, 0xFF);
> +				insn_buf[2] = BPF_ALU64_IMM(BPF_LSH, BPF_REG_0, 3);

[ ... ]

> -				cnt = 6;
> +				cnt = 7;
>  			} else {
>  				insn_buf[0] = BPF_MOV64_IMM(BPF_REG_0, -EOPNOTSUPP);
>  				cnt = 1;
>  			}
>
>  			new_prog = bpf_patch_insn_data(env, i + delta, insn_buf, cnt);

[ ... ]

> 			delta    += cnt - 1;

Note that the bpf_get_func_ret block above correctly updates delta after
patching.

> @@ -23808,12 +23811,15 @@ static int do_misc_fixups(struct bpf_verifier_env *env)
>
>  				/* skip 'void *__data' in btf_trace_##name() and save to reg0 */
>  				insn_buf[0] = BPF_MOV64_IMM(BPF_REG_0, nr_args - 1);
> +				cnt = 1;
>  			} else {
>  				/* Load nr_args from ctx - 8 */
>  				insn_buf[0] = BPF_LDX_MEM(BPF_DW, BPF_REG_0, BPF_REG_1, -8);
> +				insn_buf[1] = BPF_ALU64_IMM(BPF_AND, BPF_REG_0, 0xFF);
> +				cnt = 2;
>  			}
>
> -			new_prog = bpf_patch_insn_data(env, i + delta, insn_buf, 1);
> +			new_prog = bpf_patch_insn_data(env, i + delta, insn_buf, cnt);
>  			if (!new_prog)
>  				return -ENOMEM;
>
>  			env->prog = prog = new_prog;
>  			insn      = new_prog->insnsi + i + delta;
>  			goto next_insn;
>  		}

Should there be a "delta += cnt - 1;" before assigning env->prog in this
bpf_get_func_arg_cnt block? When cnt is 2 (non-RAW_TP path), the program
grows by one instruction, but delta is not updated. This could cause
subsequent bpf_patch_insn_data() calls to use incorrect offsets.

The bpf_get_func_arg block at the beginning of this patch has:

    delta    += cnt - 1;
    env->prog = prog = new_prog;

And the bpf_get_func_ret block similarly has delta updated. But this
bpf_get_func_arg_cnt block goes directly to env->prog assignment without
updating delta.


---
AI reviewed your patch. Please fix the bug or email reply why it's not a bug.
See: https://github.com/kernel-patches/vmtest/blob/master/ci/claude/README.md

CI run summary: https://github.com/kernel-patches/bpf/actions/runs/21278745581