Re: [PATCH bpf-next v5 7/9] bpftool: Implement btfgen_get_btf()

[Andrii Nakryiko <andrii.nakryiko@gmail.com>]

On Thu, Feb 3, 2022 at 8:10 AM Mauricio Vásquez Bernal
<mauricio@kinvolk.io> wrote:
>
> On Wed, Feb 2, 2022 at 2:37 PM Andrii Nakryiko
> <andrii.nakryiko@gmail.com> wrote:
> >
> > On Fri, Jan 28, 2022 at 2:33 PM Mauricio Vásquez <mauricio@kinvolk.io> wrote:
> > >
> > > The last part of the BTFGen algorithm is to create a new BTF object with
> > > all the types that were recorded in the previous steps.
> > >
> > > This function performs two different steps:
> > > 1. Add the types to the new BTF object by using btf__add_type(). Some
> > > special logic around struct and unions is implemented to only add the
> > > members that are really used in the field-based relocations. The type
> > > ID on the new and old BTF objects is stored on a map.
> > > 2. Fix all the type IDs on the new BTF object by using the IDs saved in
> > > the previous step.
> > >
> > > Signed-off-by: Mauricio Vásquez <mauricio@kinvolk.io>
> > > Signed-off-by: Rafael David Tinoco <rafael.tinoco@aquasec.com>
> > > Signed-off-by: Lorenzo Fontana <lorenzo.fontana@elastic.co>
> > > Signed-off-by: Leonardo Di Donato <leonardo.didonato@elastic.co>
> > > ---
> > >  tools/bpf/bpftool/gen.c | 158 +++++++++++++++++++++++++++++++++++++++-
> > >  1 file changed, 157 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/tools/bpf/bpftool/gen.c b/tools/bpf/bpftool/gen.c
> > > index 7413ec808a80..55e6f640cbbb 100644
> > > --- a/tools/bpf/bpftool/gen.c
> > > +++ b/tools/bpf/bpftool/gen.c
> > > @@ -1599,10 +1599,166 @@ static int btfgen_record_obj(struct btfgen_info *info, const char *obj_path)
> > >         return err;
> > >  }
> > >
> > > +static unsigned int btfgen_get_id(struct hashmap *ids, unsigned int old)
> > > +{
> > > +       uintptr_t new = 0;
> > > +
> > > +       /* deal with BTF_KIND_VOID */
> > > +       if (old == 0)
> > > +               return 0;
> > > +
> > > +       if (!hashmap__find(ids, uint_as_hash_key(old), (void **)&new)) {
> > > +               /* return id for void as it's possible that the ID we're looking for is
> > > +                * the type of a pointer that we're not adding.
> > > +                */
> > > +               return 0;
> > > +       }
> > > +
> > > +       return (unsigned int)(uintptr_t)new;
> > > +}
> > > +
> > > +static int btfgen_add_id(struct hashmap *ids, unsigned int old, unsigned int new)
> > > +{
> > > +       return hashmap__add(ids, uint_as_hash_key(old), uint_as_hash_key(new));
> > > +}
> > > +
> > >  /* Generate BTF from relocation information previously recorded */
> > >  static struct btf *btfgen_get_btf(struct btfgen_info *info)
> > >  {
> > > -       return ERR_PTR(-EOPNOTSUPP);
> > > +       struct hashmap_entry *entry;
> > > +       struct btf *btf_new = NULL;
> > > +       struct hashmap *ids = NULL;
> > > +       size_t bkt;
> > > +       int err = 0;
> > > +
> > > +       btf_new = btf__new_empty();
> > > +       if (libbpf_get_error(btf_new))
> > > +               goto err_out;
> > > +
> > > +       ids = hashmap__new(btfgen_hash_fn, btfgen_equal_fn, NULL);
> > > +       if (IS_ERR(ids)) {
> > > +               errno = -PTR_ERR(ids);
> > > +               goto err_out;
> > > +       }
> > > +
> > > +       /* first pass: add all types and add their new ids to the ids map */
> > > +       hashmap__for_each_entry(info->types, entry, bkt) {
> > > +               struct btfgen_type *btfgen_type = entry->value;
> > > +               struct btf_type *btf_type = btfgen_type->type;
> > > +               int new_id;
> > > +
> > > +               /* we're adding BTF_KIND_VOID to the list but it can't be added to
> > > +                * the generated BTF object, hence we skip it here.
> > > +                */
> > > +               if (btfgen_type->id == 0)
> > > +                       continue;
> > > +
> > > +               /* add members for struct and union */
> > > +               if (btf_is_struct(btf_type) || btf_is_union(btf_type)) {
> > > +                       struct hashmap_entry *member_entry;
> > > +                       struct btf_type *btf_type_cpy;
> > > +                       int nmembers, index;
> > > +                       size_t new_size;
> > > +
> > > +                       nmembers = btfgen_type->members ? hashmap__size(btfgen_type->members) : 0;
> > > +                       new_size = sizeof(struct btf_type) + nmembers * sizeof(struct btf_member);
> > > +
> > > +                       btf_type_cpy = malloc(new_size);
> > > +                       if (!btf_type_cpy)
> > > +                               goto err_out;
> > > +
> > > +                       /* copy header */
> > > +                       memcpy(btf_type_cpy, btf_type, sizeof(*btf_type_cpy));
> > > +
> > > +                       /* copy only members that are needed */
> > > +                       index = 0;
> > > +                       if (nmembers > 0) {
> > > +                               size_t bkt2;
> > > +
> > > +                               hashmap__for_each_entry(btfgen_type->members, member_entry, bkt2) {
> > > +                                       struct btfgen_member *btfgen_member;
> > > +                                       struct btf_member *btf_member;
> > > +
> > > +                                       btfgen_member = member_entry->value;
> > > +                                       btf_member = btf_members(btf_type) + btfgen_member->idx;
> > > +
> > > +                                       memcpy(btf_members(btf_type_cpy) + index, btf_member,
> > > +                                              sizeof(struct btf_member));
> >
> > you know that you are working with btf_type and btf_member, each have
> > just a few well known fields, why memcpy instead of just setting each
> > field individually? I think that would make code much easier to follow
> > and understand what transformations it's doing (and what it doesn't do
> > either).
> >
>
> Removing those memcpy() helps a lot! Do you think doing a structure
> assignment would be enough? Or do you prefer to copy field by field?
>

I think I'd prefer explicit field assignments, it's more obvious, but
yes, copying struct would work as well.

> > > +
> > > +                                       index++;
> > > +                               }
> > > +                       }
> > > +
> > > +                       /* set new vlen */
> > > +                       btf_type_cpy->info = btf_type_info(btf_kind(btf_type_cpy), nmembers,
> > > +                                                          btf_kflag(btf_type_cpy));
> > > +
> > > +                       err = btf__add_type(btf_new, info->src_btf, btf_type_cpy);
> > > +                       free(btf_type_cpy);
> > > +               } else {
> > > +                       err = btf__add_type(btf_new, info->src_btf, btf_type);
> > > +               }
> > > +
> > > +               if (err < 0)
> > > +                       goto err_out;
> > > +
> > > +               new_id = err;
> > > +
> > > +               /* add ID mapping */
> > > +               err = btfgen_add_id(ids, btfgen_type->id, new_id);
> > > +               if (err)
> > > +                       goto err_out;
> > > +       }
> > > +
> > > +       /* second pass: fix up type ids */
> > > +       for (unsigned int i = 0; i < btf__type_cnt(btf_new); i++) {
> > > +               struct btf_member *btf_member;
> > > +               struct btf_type *btf_type;
> > > +               struct btf_param *params;
> > > +               struct btf_array *array;
> > > +
> > > +               btf_type = (struct btf_type *) btf__type_by_id(btf_new, i);
> > > +
> > > +               switch (btf_kind(btf_type)) {
> > > +               case BTF_KIND_STRUCT:
> > > +               case BTF_KIND_UNION:
> > > +                       for (unsigned short j = 0; j < btf_vlen(btf_type); j++) {
> > > +                               btf_member = btf_members(btf_type) + j;
> > > +                               btf_member->type = btfgen_get_id(ids, btf_member->type);
> > > +                       }
> > > +                       break;
> > > +               case BTF_KIND_PTR:
> > > +               case BTF_KIND_TYPEDEF:
> > > +               case BTF_KIND_VOLATILE:
> > > +               case BTF_KIND_CONST:
> > > +               case BTF_KIND_RESTRICT:
> > > +               case BTF_KIND_FUNC:
> > > +               case BTF_KIND_VAR:
> > > +                       btf_type->type = btfgen_get_id(ids, btf_type->type);
> > > +                       break;
> > > +               case BTF_KIND_ARRAY:
> > > +                       array = btf_array(btf_type);
> > > +                       array->index_type = btfgen_get_id(ids, array->index_type);
> > > +                       array->type = btfgen_get_id(ids, array->type);
> > > +                       break;
> > > +               case BTF_KIND_FUNC_PROTO:
> > > +                       btf_type->type = btfgen_get_id(ids, btf_type->type);
> > > +                       params = btf_params(btf_type);
> > > +                       for (unsigned short j = 0; j < btf_vlen(btf_type); j++)
> > > +                               params[j].type = btfgen_get_id(ids, params[j].type);
> > > +                       break;
> >
> > did you try using btf_type_visit_type_ids() instead of this entire loop?
> >
>
> ah, that helped a lot, thanks!

great, you are welcome

>
> > > +               default:
> > > +                       break;
> > > +               }
> > > +       }
> > > +
> > > +       hashmap__free(ids);
> > > +       return btf_new;
> > > +
> > > +err_out:
> > > +       btf__free(btf_new);
> > > +       hashmap__free(ids);
> > > +       return NULL;
> > >  }
> > >
> > >  /* Create BTF file for a set of BPF objects.
> > > --
> > > 2.25.1
> > >