Re: [PATCH bpf-next v3 1/2] bpf: support bpf_get_func_arg() for BPF_TRACE_RAW_TP

[Jiri Olsa <olsajiri@gmail.com>]

On Mon, Jan 19, 2026 at 10:37:31AM +0800, Menglong Dong wrote:
> For now, bpf_get_func_arg() and bpf_get_func_arg_cnt() is not supported by
> the BPF_TRACE_RAW_TP, which is not convenient to get the argument of the
> tracepoint, especially for the case that the position of the arguments in
> a tracepoint can change.
> 
> The target tracepoint BTF type id is specified during loading time,
> therefore we can get the function argument count from the function
> prototype instead of the stack.
> 
> Signed-off-by: Menglong Dong <dongml2@chinatelecom.cn>
> ---
> v3:
> - remove unnecessary NULL checking for prog->aux->attach_func_proto
> 
> v2:
> - for nr_args, skip first 'void *__data' argument in btf_trace_##name
>   typedef
> ---
>  kernel/bpf/verifier.c    | 32 ++++++++++++++++++++++++++++----
>  kernel/trace/bpf_trace.c |  4 ++--
>  2 files changed, 30 insertions(+), 6 deletions(-)
> 
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index faa1ecc1fe9d..4f52342573f0 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -23316,8 +23316,20 @@ static int do_misc_fixups(struct bpf_verifier_env *env)
>  		/* Implement bpf_get_func_arg inline. */
>  		if (prog_type == BPF_PROG_TYPE_TRACING &&
>  		    insn->imm == BPF_FUNC_get_func_arg) {
> -			/* Load nr_args from ctx - 8 */
> -			insn_buf[0] = BPF_LDX_MEM(BPF_DW, BPF_REG_0, BPF_REG_1, -8);
> +			if (eatype == BPF_TRACE_RAW_TP) {
> +				int nr_args = btf_type_vlen(prog->aux->attach_func_proto);
> +
> +				/*
> +				 * skip first 'void *__data' argument in btf_trace_##name
> +				 * typedef
> +				 */
> +				nr_args--;
> +				/* Save nr_args to reg0 */
> +				insn_buf[0] = BPF_MOV64_IMM(BPF_REG_0, nr_args);
> +			} 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_JMP32_REG(BPF_JGE, BPF_REG_2, BPF_REG_0, 6);
>  			insn_buf[2] = BPF_ALU64_IMM(BPF_LSH, BPF_REG_2, 3);
>  			insn_buf[3] = BPF_ALU64_REG(BPF_ADD, BPF_REG_2, BPF_REG_1);
> @@ -23369,8 +23381,20 @@ static int do_misc_fixups(struct bpf_verifier_env *env)
>  		/* Implement get_func_arg_cnt inline. */
>  		if (prog_type == BPF_PROG_TYPE_TRACING &&
>  		    insn->imm == BPF_FUNC_get_func_arg_cnt) {
> -			/* Load nr_args from ctx - 8 */
> -			insn_buf[0] = BPF_LDX_MEM(BPF_DW, BPF_REG_0, BPF_REG_1, -8);
> +			if (eatype == BPF_TRACE_RAW_TP) {
> +				int nr_args = btf_type_vlen(prog->aux->attach_func_proto);
> +
> +				/*
> +				 * skip first 'void *__data' argument in btf_trace_##name
> +				 * typedef
> +				 */
> +				nr_args--;
> +				/* Save nr_args to reg0 */

I think we can attach single bpf program to multiple rawtp tracepoints,
in which case this would not work properly for such program links on
tracepoints with different nr_args, right?

jirka


> +				insn_buf[0] = BPF_MOV64_IMM(BPF_REG_0, nr_args);
> +			} else {
> +				/* Load nr_args from ctx - 8 */
> +				insn_buf[0] = BPF_LDX_MEM(BPF_DW, BPF_REG_0, BPF_REG_1, -8);
> +			}
>  
>  			new_prog = bpf_patch_insn_data(env, i + delta, insn_buf, 1);
>  			if (!new_prog)
> diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
> index 6e076485bf70..9b1b56851d26 100644
> --- a/kernel/trace/bpf_trace.c
> +++ b/kernel/trace/bpf_trace.c
> @@ -1734,11 +1734,11 @@ tracing_prog_func_proto(enum bpf_func_id func_id, const struct bpf_prog *prog)
>  	case BPF_FUNC_d_path:
>  		return &bpf_d_path_proto;
>  	case BPF_FUNC_get_func_arg:
> -		return bpf_prog_has_trampoline(prog) ? &bpf_get_func_arg_proto : NULL;
> +		return &bpf_get_func_arg_proto;
>  	case BPF_FUNC_get_func_ret:
>  		return bpf_prog_has_trampoline(prog) ? &bpf_get_func_ret_proto : NULL;
>  	case BPF_FUNC_get_func_arg_cnt:
> -		return bpf_prog_has_trampoline(prog) ? &bpf_get_func_arg_cnt_proto : NULL;
> +		return &bpf_get_func_arg_cnt_proto;
>  	case BPF_FUNC_get_attach_cookie:
>  		if (prog->type == BPF_PROG_TYPE_TRACING &&
>  		    prog->expected_attach_type == BPF_TRACE_RAW_TP)
> -- 
> 2.52.0
>