Re: [PATCH bpf-next v2 1/2] bpf: support bpf_get_func_arg() for BPF_TRACE_RAW_TP

[Menglong Dong <menglong.dong@linux.dev>]

On 2026/1/17 07:32, Andrii Nakryiko wrote:
> On Thu, Jan 15, 2026 at 11:18 PM Menglong Dong <menglong8.dong@gmail.com> 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>
> > ---
> > v2:
> > - for nr_args, skip first 'void *__data' argument in btf_trace_##name
> >   typedef
> > ---
> >  kernel/bpf/verifier.c    | 36 ++++++++++++++++++++++++++++++++----
> >  kernel/trace/bpf_trace.c |  4 ++--
> >  2 files changed, 34 insertions(+), 6 deletions(-)
> >
> > diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> > index faa1ecc1fe9d..422d35c100ff 100644
> > --- a/kernel/bpf/verifier.c
> > +++ b/kernel/bpf/verifier.c
> > @@ -23316,8 +23316,22 @@ 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;
> > +
> > +                               if (!prog->aux->attach_func_proto)
> > +                                       return -EINVAL;
> 
> can this happen? can we have tp_btf program without attach_func_proto
> properly set?

I saw it can be NULL in some case, such as bpf2bpf. Maybe it can't
happen for tp_btf, and I'll do further analysis on this point.

Thanks!
Menglong Dong


> 
> > +                               /*
> > +                                * skip first 'void *__data' argument in btf_trace_##name
> > +                                * typedef
> > +                                */
> > +                               nr_args = btf_type_vlen(prog->aux->attach_func_proto) - 1;
> > +                               /* 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 +23383,22 @@ 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;
> > +
> > +                               if (!prog->aux->attach_func_proto)
> > +                                       return -EINVAL;
> > +                               /*
> > +                                * skip first 'void *__data' argument in btf_trace_##name
> > +                                * typedef
> > +                                */
> > +                               nr_args = btf_type_vlen(prog->aux->attach_func_proto) - 1;
> > +                               /* 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);
> > +                       }
> >
> >                         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
> >
> 
>