Re: [PATCH bpf-next v2 2/9] bpftool: Honor BPF_CORE_TYPE_MATCHES relocation

["Daniel Müller" <deso@posteo.net>]

On Fri, Jun 24, 2022 at 02:25:50PM -0700, Andrii Nakryiko wrote:
> On Thu, Jun 23, 2022 at 2:22 PM Daniel Müller <deso@posteo.net> wrote:
> >
> > bpftool needs to know about the newly introduced BPF_CORE_TYPE_MATCHES
> > relocation for its 'gen min_core_btf' command to work properly in the
> > present of this relocation.
> > Specifically, we need to make sure to mark types and fields so that they
> > are present in the minimized BTF for "type match" checks to work out.
> > However, contrary to the existing btfgen_record_field_relo, we need to
> > rely on the BTF -- and not the spec -- to find fields. With this change
> > we handle this new variant correctly. The functionality will be tested
> > with follow on changes to BPF selftests, which already run against a
> > minimized BTF created with bpftool.
> >
> > Cc: Quentin Monnet <quentin@isovalent.com>
> > Signed-off-by: Daniel Müller <deso@posteo.net>
> > ---
> >  tools/bpf/bpftool/gen.c | 107 ++++++++++++++++++++++++++++++++++++++++
> >  1 file changed, 107 insertions(+)
> >
> > diff --git a/tools/bpf/bpftool/gen.c b/tools/bpf/bpftool/gen.c
> > index 480cbd8..6cd0ed 100644
> > --- a/tools/bpf/bpftool/gen.c
> > +++ b/tools/bpf/bpftool/gen.c
> > @@ -1856,6 +1856,111 @@ static int btfgen_record_field_relo(struct btfgen_info *info, struct bpf_core_sp
> >         return 0;
> >  }
> >
> > +/* Mark types, members, and member types. Compared to btfgen_record_field_relo,
> > + * this function does not rely on the target spec for inferring members, but
> > + * uses the associated BTF.
> > + *
> > + * The `behind_ptr` argument is used to stop marking of composite types reached
> > + * through a pointer. This way, we keep can keep BTF size in check while
> > + * providing reasonable match semantics.
> > + */
> > +static int btfgen_mark_types_match(struct btfgen_info *info, __u32 type_id, bool behind_ptr)
> > +{
> > +       const struct btf_type *btf_type;
> > +       struct btf *btf = info->src_btf;
> > +       struct btf_type *cloned_type;
> > +       int i, err;
> > +
> > +       if (type_id == 0)
> > +               return 0;
> > +
> > +       btf_type = btf__type_by_id(btf, type_id);
> > +       /* mark type on cloned BTF as used */
> > +       cloned_type = (struct btf_type *)btf__type_by_id(info->marked_btf, type_id);
> > +       cloned_type->name_off = MARKED;
> > +
> > +       switch (btf_kind(btf_type)) {
> > +       case BTF_KIND_UNKN:
> > +       case BTF_KIND_INT:
> > +       case BTF_KIND_FLOAT:
> > +       case BTF_KIND_ENUM:
> > +       case BTF_KIND_ENUM64:
> > +               break;
> > +       case BTF_KIND_STRUCT:
> > +       case BTF_KIND_UNION: {
> > +               struct btf_member *m = btf_members(btf_type);
> > +               __u16 vlen = btf_vlen(btf_type);
> > +
> > +               if (behind_ptr)
> > +                       break;
> > +
> > +               for (i = 0; i < vlen; i++, m++) {
> > +                       /* mark member */
> > +                       btfgen_mark_member(info, type_id, i);
> > +
> > +                       /* mark member's type */
> > +                       err = btfgen_mark_types_match(info, m->type, false);
> > +                       if (err)
> > +                               return err;
> > +               }
> > +               break;
> > +       }
> > +       case BTF_KIND_CONST:
> > +       case BTF_KIND_FWD:
> > +       case BTF_KIND_VOLATILE:
> > +       case BTF_KIND_TYPEDEF:
> 
> BTF_KIND_RESTRICT is missing?

Good catch. It's missing in btfgen_mark_type as well. Will add it.

> > +               return btfgen_mark_types_match(info, btf_type->type, false);
> > +       case BTF_KIND_PTR:
> > +               return btfgen_mark_types_match(info, btf_type->type, true);
> > +       case BTF_KIND_ARRAY: {
> > +               struct btf_array *array;
> > +
> > +               array = btf_array(btf_type);
> > +               /* mark array type */
> > +               err = btfgen_mark_types_match(info, array->type, false);
> > +               /* mark array's index type */
> > +               err = err ? : btfgen_mark_types_match(info, array->index_type, false);
> > +               if (err)
> > +                       return err;
> > +               break;
> > +       }
> 
> [...]