netdev - Re: [PATCH v3 bpf-next 1/4] bpftool: Print the enum's name instead of value

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Date:   Wed, 18 Mar 2020 10:10:28 -0700
From:   Martin KaFai Lau <kafai@...com>
To:     Andrii Nakryiko <andrii.nakryiko@...il.com>
CC:     bpf <bpf@...r.kernel.org>, Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Kernel Team <kernel-team@...com>,
        Networking <netdev@...r.kernel.org>
Subject: Re: [PATCH v3 bpf-next 1/4] bpftool: Print the enum's name instead
 of value

On Tue, Mar 17, 2020 at 11:09:24PM -0700, Andrii Nakryiko wrote:
> On Tue, Mar 17, 2020 at 8:15 PM Martin KaFai Lau <kafai@...com> wrote:
> >
> > This patch prints the enum's name if there is one found in
> > the array of btf_enum.
> >
> > The commit 9eea98497951 ("bpf: fix BTF verification of enums")
> > has details about an enum could have any power-of-2 size (up to 8 bytes).
> > This patch also takes this chance to accommodate these non 4 byte
> > enums.
> >
> > Acked-by: Quentin Monnet <quentin@...valent.com>
> > Signed-off-by: Martin KaFai Lau <kafai@...com>
> > ---
> >  tools/bpf/bpftool/btf_dumper.c | 39 +++++++++++++++++++++++++++++++---
> >  1 file changed, 36 insertions(+), 3 deletions(-)
> >
> > diff --git a/tools/bpf/bpftool/btf_dumper.c b/tools/bpf/bpftool/btf_dumper.c
> > index 01cc52b834fa..079f9171b1a3 100644
> > --- a/tools/bpf/bpftool/btf_dumper.c
> > +++ b/tools/bpf/bpftool/btf_dumper.c
> > @@ -43,9 +43,42 @@ static int btf_dumper_modifier(const struct btf_dumper *d, __u32 type_id,
> >         return btf_dumper_do_type(d, actual_type_id, bit_offset, data);
> >  }
> >
> > -static void btf_dumper_enum(const void *data, json_writer_t *jw)
> > +static void btf_dumper_enum(const struct btf_dumper *d,
> > +                           const struct btf_type *t,
> > +                           const void *data)
> >  {
> > -       jsonw_printf(jw, "%d", *(int *)data);
> > +       const struct btf_enum *enums = btf_enum(t);
> > +       __s64 value;
> > +       __u16 i;
> > +
> > +       switch (t->size) {
> > +       case 8:
> > +               value = *(__s64 *)data;
> > +               break;
> > +       case 4:
> > +               value = *(__s32 *)data;
> > +               break;
> > +       case 2:
> > +               value = *(__s16 *)data;
> > +               break;
> > +       case 1:
> > +               value = *(__s8 *)data;
> > +               break;
> > +       default:
> > +               jsonw_string(d->jw, "<invalid_enum_size>");
> 
> Why not return error and let it propagate, similar to how
> btf_dumper_array() can return an error? BTF is malformed if this
> happened, so there is no point in continuing dumping, it's most
> probably going to be a garbage.
I can send v4 to return -EINVAL here.

However, the caller of btf_dump*() is pretty loose on checking it.
It won't be difficult to find other existing codes that will
continue on btf_type's related error cases.  I also don't
think fixing all these error checking/returning is the
right answer here

The proper place to check malformed BTF is in btf__new().
Check it once there like how the kernel does.
[ btw, the data and btf here are obtained from the kernel
  which has verified it ].

> 
> > +               return;
> > +       }
> > +
> > +       for (i = 0; i < btf_vlen(t); i++) {
> > +               if (value == enums[i].val) {
> > +                       jsonw_string(d->jw,
> > +                                    btf__name_by_offset(d->btf,
> > +                                                        enums[i].name_off));
> 
> nit: local variable will make it cleaner
I prefer to keep it as is.  There are many other uses like this.

> 
> > +                       return;
> > +               }
> > +       }
> > +
> > +       jsonw_int(d->jw, value);
> >  }
> >
> >  static int btf_dumper_array(const struct btf_dumper *d, __u32 type_id,
> > @@ -366,7 +399,7 @@ static int btf_dumper_do_type(const struct btf_dumper *d, __u32 type_id,
> >         case BTF_KIND_ARRAY:
> >                 return btf_dumper_array(d, type_id, data);
> >         case BTF_KIND_ENUM:
> > -               btf_dumper_enum(data, d->jw);
> > +               btf_dumper_enum(d, t, data);
> >                 return 0;
> >         case BTF_KIND_PTR:
> >                 btf_dumper_ptr(data, d->jw, d->is_plain_text);
> > --
> > 2.17.1
> >