Re: [RFC] libbpf,selftests: Question on btf_dump__emit_type_decl for BTF_KIND_FUNC

[Andrii Nakryiko <andrii.nakryiko@gmail.com>]

On Tue, Mar 3, 2020 at 6:12 AM Jiri Olsa <jolsa@kernel.org> wrote:
>
> hi,
> for bpftrace I'd like to print BTF functions (BTF_KIND_FUNC)
> declarations together with their names.
>
> I saw we have btf_dump__emit_type_decl and added BTF_KIND_FUNC,
> where it seemed to be missing, so it prints out something now
> (not sure it's the right fix though).
>
> Anyway, would you be ok with adding some flag/bool to struct
> btf_dump_emit_type_decl_opts, so I could get output like:
>
>   kfunc:ksys_readahead(int fd, long long int offset, long unsigned int count) = ssize_t
>   kfunc:ksys_read(unsigned int fd, char buf, long unsigned int count) = size_t
>
> ... to be able to the arguments and return type separated,
> so I could easily get to something like above?
>
> Current interface is just vfprintf callback and I'm not sure
> I can rely that it will allywas be called with same arguments,
> like having separated calls for parsed atoms like 'return type',
> '(', ')', '(', 'arg type', 'arg name', ...
>
> I'm open to any suggestion ;-)

Hey Jiri!

Can you please elaborate on the use case and problem you are trying to solve?

I think we can (and probably even should) add such option and support
to dump functions, but whatever we do it should be a valid C syntax
and should be compilable.
Example above:

kfunc:ksys_read(unsigned int fd, char buf, long unsigned int count) = size_t

Is this really the syntax you need to get? I think btf_dump, when
(optionally) emitting function declaration, will have to emit that
particular one as:

size_t ksys_read(unsigned int fd, char buf, long unsigned int count);

But I'd like to hear the use case before we add this. Thanks!

>
> thanks,
> jirka
>
>
> ---
>  tools/lib/bpf/btf_dump.c                      |  4 ++++
>  .../selftests/bpf/prog_tests/btf_dump.c       | 21 +++++++++++++++++++
>  .../bpf/progs/btf_dump_test_case_bitfields.c  | 10 +++++++++
>  .../bpf/progs/btf_dump_test_case_multidim.c   |  3 +++
>  .../progs/btf_dump_test_case_namespacing.c    | 19 +++++++++++++++++
>  .../bpf/progs/btf_dump_test_case_ordering.c   |  3 +++
>  .../bpf/progs/btf_dump_test_case_packing.c    | 16 ++++++++++++++
>  .../bpf/progs/btf_dump_test_case_padding.c    | 15 +++++++++++++
>  .../bpf/progs/btf_dump_test_case_syntax.c     |  3 +++
>  9 files changed, 94 insertions(+)
>

[...]

>
> +/*
> + *int ()(struct {
> + *     struct packed_trailing_space _1;
> + *     short: 16;
> + *     struct non_packed_trailing_space _2;
> + *     struct packed_fields _3;
> + *     short: 16;
> + *     struct non_packed_fields _4;
> + *     struct nested_packed _5;
> + *     short: 16;
> + *     union union_is_never_packed _6;
> + *     union union_does_not_need_packing _7;
> + *     union jump_code_union _8;
> + *     int: 24;
> + *} __attribute__((packed)) *_)
> + */

This clearly isn't compilable, right?

>  /*------ END-EXPECTED-OUTPUT ------ */
>
>  int f(struct {
> diff --git a/tools/testing/selftests/bpf/progs/btf_dump_test_case_padding.c b/tools/testing/selftests/bpf/progs/btf_dump_test_case_padding.c
> index 35c512818a56..581349bb0c2f 100644
> --- a/tools/testing/selftests/bpf/progs/btf_dump_test_case_padding.c
> +++ b/tools/testing/selftests/bpf/progs/btf_dump_test_case_padding.c
> @@ -102,6 +102,21 @@ struct zone {
>         struct zone_padding __pad__;
>  };
>

[...]