Date:   Fri, 24 May 2019 10:14:35 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Quentin Monnet <quentin.monnet@...ronome.com>
Cc:     Andrii Nakryiko <andriin@...com>,
        Networking <netdev@...r.kernel.org>, bpf@...r.kernel.org,
        Alexei Starovoitov <ast@...com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Kernel Team <kernel-team@...com>
Subject: Re: [PATCH v2 bpf-next 10/12] bpftool: add C output format option to
 btf dump subcommand

On Fri, May 24, 2019 at 2:14 AM Quentin Monnet
<quentin.monnet@...ronome.com> wrote:
>
> Hi Andrii,
>
> Some nits inline, nothing blocking though.
>
> 2019-05-23 13:42 UTC-0700 ~ Andrii Nakryiko <andriin@...com>
> > Utilize new libbpf's btf_dump API to emit BTF as a C definitions.
> >
> > Signed-off-by: Andrii Nakryiko <andriin@...com>
> > ---
> >  tools/bpf/bpftool/btf.c | 74 +++++++++++++++++++++++++++++++++++++++--
> >  1 file changed, 72 insertions(+), 2 deletions(-)
> >
> > diff --git a/tools/bpf/bpftool/btf.c b/tools/bpf/bpftool/btf.c
> > index a22ef6587ebe..1cdbfad42b38 100644
> > --- a/tools/bpf/bpftool/btf.c
> > +++ b/tools/bpf/bpftool/btf.c
> > @@ -340,11 +340,48 @@ static int dump_btf_raw(const struct btf *btf,
> >       return 0;
> >  }
> >
> > +static void btf_dump_printf(void *ctx, const char *fmt, va_list args)
>
> Nit: This function could have a printf attribute ("__printf(2, 0)").

added, though I don't think it matters as it's only used as a callback function.

>
> > +{
> > +     vfprintf(stdout, fmt, args);
> > +}
> > +
> > +static int dump_btf_c(const struct btf *btf,
> > +                   __u32 *root_type_ids, int root_type_cnt)
> > +{
> > +     struct btf_dump *d;
> > +     int err = 0, i;
> > +
> > +     d = btf_dump__new(btf, NULL, NULL, btf_dump_printf);
> > +     if (IS_ERR(d))
> > +             return PTR_ERR(d);
> > +
> > +     if (root_type_cnt) {
> > +             for (i = 0; i < root_type_cnt; i++) {
> > +                     err = btf_dump__dump_type(d, root_type_ids[i]);
> > +                     if (err)
> > +                             goto done;
> > +             }
> > +     } else {
> > +             int cnt = btf__get_nr_types(btf);
> > +
> > +             for (i = 1; i <= cnt; i++) {
> > +                     err = btf_dump__dump_type(d, i);
> > +                     if (err)
> > +                             goto done;
> > +             }
> > +     }
> > +
> > +done:
> > +     btf_dump__free(d);
> > +     return err;
> > +}
> > +
> >  static int do_dump(int argc, char **argv)
> >  {
> >       struct btf *btf = NULL;
> >       __u32 root_type_ids[2];
> >       int root_type_cnt = 0;
> > +     bool dump_c = false;
> >       __u32 btf_id = -1;
> >       const char *src;
> >       int fd = -1;
> > @@ -431,6 +468,29 @@ static int do_dump(int argc, char **argv)
> >               goto done;
> >       }
> >
> > +     while (argc) {
> > +             if (is_prefix(*argv, "format")) {
> > +                     NEXT_ARG();
> > +                     if (argc < 1) {
> > +                             p_err("expecting value for 'format' option\n");
> > +                             goto done;
> > +                     }
> > +                     if (strcmp(*argv, "c") == 0) {
> > +                             dump_c = true;
> > +                     } else if (strcmp(*argv, "raw") == 0) {
>
> Do you think we could use is_prefix() instead of strcmp() here?

So I considered it, and then decided against it, though I can still be
convinced otherwise. Right now we have raw and c, but let's say we add
rust as an option. r will become ambiguous, but actually will be
resolved to whatever we check first: either raw or rust, which is not
great. So given that those format specifiers will tend to be short, I
decided it's ok to require to specify them fully. Does it make sense?

>
> > +                             dump_c = false;
> > +                     } else {
> > +                             p_err("unrecognized format specifier: '%s'",
> > +                                   *argv);
>
> Would it be worth reminding the user about the valid specifiers in that
> message? (But then we already have it in do_help(), so maybe not.)

Added possible options to the message.


>
> > +                             goto done;
> > +                     }
> > +                     NEXT_ARG();
> > +             } else {
> > +                     p_err("unrecognized option: '%s'", *argv);
> > +                     goto done;
> > +             }
> > +     }
> > +
> >       if (!btf) {
> >               err = btf__get_from_id(btf_id, &btf);
> >               if (err) {
> > @@ -444,7 +504,16 @@ static int do_dump(int argc, char **argv)
> >               }
> >       }
> >
> > -     dump_btf_raw(btf, root_type_ids, root_type_cnt);
> > +     if (dump_c) {
> > +             if (json_output) {
> > +                     p_err("JSON output for C-syntax dump is not supported");
> > +                     err = -ENOTSUP;
> > +                     goto done;
> > +             }
> > +             err = dump_btf_c(btf, root_type_ids, root_type_cnt);
> > +     } else {
> > +             err = dump_btf_raw(btf, root_type_ids, root_type_cnt);
> > +     }
> >
> >  done:
> >       close(fd);
> > @@ -460,10 +529,11 @@ static int do_help(int argc, char **argv)
> >       }
> >
> >       fprintf(stderr,
> > -             "Usage: %s btf dump BTF_SRC\n"
> > +             "Usage: %s btf dump BTF_SRC [format FORMAT]\n"
> >               "       %s btf help\n"
> >               "\n"
> >               "       BTF_SRC := { id BTF_ID | prog PROG | map MAP [{key | value | kv | all}] | file FILE }\n"
> > +             "       FORMAT  := { raw | c }\n"
> >               "       " HELP_SPEC_MAP "\n"
> >               "       " HELP_SPEC_PROGRAM "\n"
> >               "       " HELP_SPEC_OPTIONS "\n"
> >
>

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