Re: [PATCH bpf-next v4 1/2] bpf: support bpf_get_func_arg() for BPF_TRACE_RAW_TP

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

On 2026/1/21 08:38 Andrii Nakryiko <andrii.nakryiko@gmail.com> write:
> On Mon, Jan 19, 2026 at 11:31 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>
> > ---
> > v4:
> > - fix the error of using bpf_get_func_arg() for BPF_TRACE_ITER
> >
> > 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, 32 insertions(+), 4 deletions(-)
> >
> 
> other than stylistical choices, looks good to me
> 
> Acked-by: Andrii Nakryiko <andrii@kernel.org>
> 
> > diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> > index 9de0ec0c3ed9..0b281b7c41eb 100644
> > --- a/kernel/bpf/verifier.c
> > +++ b/kernel/bpf/verifier.c
> > @@ -23323,8 +23323,20 @@ 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);
> > @@ -23376,8 +23388,20 @@ 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 */
> > +                               insn_buf[0] = BPF_MOV64_IMM(BPF_REG_0, nr_args);
> 
> nit: isn't this just a very verbose way of writing:
> 
> int nr_args = btf_type_vlen(prog->aux->attach_func_proto);
> /* skip 'void *__data' in btf_trace_##name() and save to reg0 */
> insn_buf[0] = BPF_MOV64_IMM(BPF_REG_0, nr_args - 1);
> 
> even if you want to preserve nr_args-- for clarity, at least make that
> 4-line comment into a single-line one, please
> 
> > +                       } 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 f73e08c223b5..0efdad3adcce 100644
> > --- a/kernel/trace/bpf_trace.c
> > +++ b/kernel/trace/bpf_trace.c
> > @@ -1734,10 +1734,14 @@ 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:
> > +               if (prog->expected_attach_type == BPF_TRACE_RAW_TP)
> > +                       return &bpf_get_func_arg_proto;
> >                 return bpf_prog_has_trampoline(prog) ? &bpf_get_func_arg_proto : NULL;
> >         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:
> > +               if (prog->expected_attach_type == BPF_TRACE_RAW_TP)
> > +                       return &bpf_get_func_arg_cnt_proto;
> >                 return bpf_prog_has_trampoline(prog) ? &bpf_get_func_arg_cnt_proto : NULL;
> 
> hm, wouldn't "has trampoline or is raw_tp" a more logical grouping?
> 
> if (bpf_prog_has_trampoline(prog) || prog->expected_attach_type ==
> BPF_TRACE_RAW_TP)
>     return &bpf_get_func_arg_cnt_proto;
> return NULL;
> 
> maybe you'll need to wrap that condition, but still, at least no one
> has to double check that we return exactly the same prototype in both
> cases, no?

Yeah, it looks better. I tried to write it as:
     return (bpf_prog_has_trampoline(prog) || prog->expected_attach_type ==  BPF_TRACE_RAW_TP) ? &bpf_get_func_arg_cnt_proto : NULL;

but found it ugly.

The way you mentioned looks nice ;)
I'll do it next version.

Thanks!
Menglong Dong

> 
> 
> >         case BPF_FUNC_get_attach_cookie:
> >                 if (prog->type == BPF_PROG_TYPE_TRACING &&
> > --
> > 2.52.0
> >
>