Date:   Wed, 22 May 2019 17:58:23 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Jakub Kicinski <jakub.kicinski@...ronome.com>
Cc:     Andrii Nakryiko <andriin@...com>, Alexei Starovoitov <ast@...com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Networking <netdev@...r.kernel.org>, bpf@...r.kernel.org,
        Kernel Team <kernel-team@...com>
Subject: Re: [PATCH bpf-next 10/12] bpftool: add C output format option to btf
 dump subcommand

On Wed, May 22, 2019 at 5:25 PM Jakub Kicinski
<jakub.kicinski@...ronome.com> wrote:
>
> On Wed, 22 May 2019 12:50:51 -0700, Andrii Nakryiko wrote:
> > 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 | 63 +++++++++++++++++++++++++++++++++++++++--
> >  1 file changed, 60 insertions(+), 3 deletions(-)
> >
> > diff --git a/tools/bpf/bpftool/btf.c b/tools/bpf/bpftool/btf.c
> > index a22ef6587ebe..ed3d3221cc78 100644
> > --- a/tools/bpf/bpftool/btf.c
> > +++ b/tools/bpf/bpftool/btf.c
> > @@ -1,5 +1,12 @@
> >  // SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
> > -/* Copyright (C) 2019 Facebook */
> > +
> > +/*
> > + * BTF dumping command.
> > + * Load BTF from multiple possible sources and outptu entirety or subset of
> > + * types in either raw format or C-syntax format.
> > + *
>
> I don't think this header adds any value.  Its very unlikely people
> will remember to update it.  And it's misspelled to begin with.
> Please remove.

OK, will remove.

>
> > + * Copyright (C) 2019 Facebook
> > + */
> >
> >  #include <errno.h>
> >  #include <fcntl.h>
> > @@ -340,11 +347,48 @@ static int dump_btf_raw(const struct btf *btf,
> >       return 0;
> >  }
> >
> > +static void btf_dump_printf(void *ctx, const char *fmt, va_list args)
> > +{
> > +     vfprintf(stdout, fmt, args);
> > +}
> > +
> > +static int dump_btf_c(const struct btf *btf,
> > +                   __u32 *root_type_ids, int root_type_cnt)
>
> Please break the line after static int.

I don't mind, but it seems that prevalent formatting for such cases
(at least in bpftool code base) is aligning arguments and not break
static <return type> into separate line:

// multi-line function definitions with static on the same line
$ rg '^static \w+.*\([^\)]*$' | wc -l
45
// multi-line function definitions with static on separate line
$ rg '^static \w+[^\(\{;]*$' | wc -l
12

So I don't mind changing, but which one is canonical way of formatting?


>
> > +{
> > +     struct btf_dump *d;
> > +     int err = 0, i, id;
>
> Hmm.. why do you have both i and id here?  Maybe my eyes are failing me
> but it seems either one or the other is used in different branches of
> the main if () :)

You are right. i is used as an index into array of IDs, while for the
other branch we iterate type IDs explicitly. I thought it's less
confusing, but apparently it's the other way. I can do everything with
just 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 (id = 1; id <= cnt; id++) {
> > +                     err = btf_dump__dump_type(d, id);
> > +                     if (err)
> > +                             goto done;
> > +             }
> > +     }
> > +
> > +done:
> > +     btf_dump__free(d);
> > +     return err;
>
> What do we do for JSON output?

Still dump C syntax. What do you propose? Error out if json enabled?

>
> > +}
> > +
> >  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 +475,16 @@ static int do_dump(int argc, char **argv)
> >               goto done;
> >       }
> >
> > +     while (argc) {
> > +             if (strcmp(*argv, "c") == 0) {
> > +                     dump_c = true;
> > +                     NEXT_ARG();
> > +             } else {
> > +                     p_err("unrecognized option: '%s'", *argv);
> > +                     goto done;
> > +             }
> > +     }
>
> This code should have checked there are no arguments and return an
> error from the start :S

I might be missing your point here. Lack of extra options is not an
error, they are optional. It's just if there is an option, that we
can't recognize - then we error out.

>
> >       if (!btf) {
> >               err = btf__get_from_id(btf_id, &btf);
> >               if (err) {
> > @@ -444,7 +498,10 @@ static int do_dump(int argc, char **argv)
> >               }
> >       }
> >
> > -     dump_btf_raw(btf, root_type_ids, root_type_cnt);
> > +     if (dump_c)
> > +             dump_btf_c(btf, root_type_ids, root_type_cnt);
> > +     else
> > +             dump_btf_raw(btf, root_type_ids, root_type_cnt);
> >
> >  done:
> >       close(fd);
> > @@ -460,7 +517,7 @@ static int do_help(int argc, char **argv)
> >       }
> >
> >       fprintf(stderr,
> > -             "Usage: %s btf dump BTF_SRC\n"
> > +             "Usage: %s btf dump BTF_SRC [c]\n"
>
> bpftool generally uses <key value> formats.  So perhaps we could do
> something like "[format raw|c]" here for consistency, defaulting to raw?

That's not true for options, though. I see that at cgroup, prog, and
some map subcommands (haven't checked all other) just accept a list of
options without extra identifying key.

>
> >               "       %s btf help\n"
> >               "\n"
> >               "       BTF_SRC := { id BTF_ID | prog PROG | map MAP [{key | value | kv | all}] | file FILE }\n"
>

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