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Date:   Wed, 22 May 2019 21:43:43 -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 6:23 PM Jakub Kicinski
<jakub.kicinski@...ronome.com> wrote:
>
> On Wed, 22 May 2019 17:58:23 -0700, Andrii Nakryiko wrote:
> > On Wed, May 22, 2019 at 5:25 PM Jakub Kicinski wrote:
> > > On Wed, 22 May 2019 12:50:51 -0700, Andrii Nakryiko wrote:
> > > > + * 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?
>
> Not really, just my preference :)

I'll stick to majority :) I feel like it's also a preferred style in
libbpf, so I'd rather converge to that.

>
> In my experience having the return type on a separate line if its
> longer than a few chars is the simplest rule for consistent and good
> looking code.
>
> > > > +     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?
>
> I wonder.  Letting it just print C is going to confuse anything that
> just feeds the output into a JSON parser.  I'd err on the side of
> returning an error, we can always relax that later if we find a use
> case of returning C syntax via JSON.

Ok, I'll emit error (seems like pr_err automatically handles JSON
output, which is very nice).

>
> > > > +}
> > > > +
> > > >  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.
>
> Oh, I was just remarking that before your patch bpftool would not error
> if garbage options were passed, it'd be better if we errored from the
> start.  But too late now, your code is good
>

Ah, I see, yeah, that was my original omission, you are right.


> > > >       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.
>
> Yeah, we weren't 100% enforcing this rule and it's a bit messy now :/

Unless you feel very strongly about this, it seems ok to me to allow
"boolean options" (similarly to boolean --flag args) as a stand-alone
set of tags. bpftool invocations are already very verbose, no need to
add to that. Plus it also makes bash-completion simpler, it's always
good not to complicate bash script unnecessarily :)

>
> > > >               "       %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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