Re: [PATCH bpf-next v6 4/7] bpftool: Implement minimize_btf() and relocations recording for BTFGen

["Mauricio Vásquez Bernal" <mauricio@kinvolk.io>]

On Fri, Feb 11, 2022 at 7:42 PM Andrii Nakryiko
<andrii.nakryiko@gmail.com> wrote:
>
> On Wed, Feb 9, 2022 at 2:27 PM Mauricio Vásquez <mauricio@kinvolk.io> wrote:
> >
>
> It would be good to shorten the subject line, it's very long.
>

Will do.

> > minimize_btf() receives the path of a source and destination BTF files
> > and a list of BPF objects. This function records the relocations for
> > all objects and then generates the BTF file by calling btfgen_get_btf()
> > (implemented in the following commit).
> >
> > btfgen_record_obj() loads the BTF and BTF.ext sections of the BPF
> > objects and loops through all CO-RE relocations. It uses
> > bpf_core_calc_relo_insn() from libbpf and passes the target spec to
> > btfgen_record_reloc(), that calls one of the following functions
> > depending on the relocation kind.
> >
> > btfgen_record_field_relo() uses the target specification to mark all the
> > types that are involved in a field-based CO-RE relocation. In this case
> > types resolved and marked recursively using btfgen_mark_type().
> > Only the struct and union members (and their types) involved in the
> > relocation are marked to optimize the size of the generated BTF file.
> >
> > btfgen_record_type_relo() marks the types involved in a type-based
> > CO-RE relocation. In this case no members for the struct and union
> > types are marked as libbpf doesn't use them while performing this kind
> > of relocation. Pointed types are marked as they are used by libbpf in
> > this case.
> >
> > btfgen_record_enumval_relo() marks the whole enum type for enum-based
> > relocations.
>
> It should be enough to leave only used enumerators, but I suppose it
> doesn't take much space to record all. We can adjust that later, if
> necessary.
>

I think the overhead is really minimal and we can improve later on if we want.

> >
> > 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/Makefile |   8 +-
> >  tools/bpf/bpftool/gen.c    | 452 ++++++++++++++++++++++++++++++++++++-
> >  2 files changed, 454 insertions(+), 6 deletions(-)
> >
>
> Looks good, few nits and concerns, but it feels like it's really close
> to being ready.
>
> [...]
>
> > +}
> > +
> > +struct btfgen_info {
> > +       struct btf *src_btf;
> > +       struct btf *marked_btf; // btf structure used to mark used types
>
> C++ comment, please use /* */
>
> > +};
> > +
> > +static size_t btfgen_hash_fn(const void *key, void *ctx)
> > +{
> > +       return (size_t)key;
> > +}
> > +
> > +static bool btfgen_equal_fn(const void *k1, const void *k2, void *ctx)
> > +{
> > +       return k1 == k2;
> > +}
> > +
> > +static void *uint_as_hash_key(int x)
> > +{
> > +       return (void *)(uintptr_t)x;
> > +}
> > +
> > +static void *u32_as_hash_key(__u32 x)
> > +{
> > +       return (void *)(uintptr_t)x;
> > +}
> > +
> > +static void btfgen_free_info(struct btfgen_info *info)
> > +{
> > +       if (!info)
> > +               return;
> > +
> > +       btf__free(info->src_btf);
> > +       btf__free(info->marked_btf);
> > +
> > +       free(info);
> > +}
> > +
> > +static struct btfgen_info *
> > +btfgen_new_info(const char *targ_btf_path)
> > +{
> > +       struct btfgen_info *info;
> > +       int err;
> > +
> > +       info = calloc(1, sizeof(*info));
> > +       if (!info)
> > +               return NULL;
> > +
> > +       info->src_btf = btf__parse(targ_btf_path, NULL);
> > +       if (!info->src_btf) {
> > +               p_err("failed parsing '%s' BTF file: %s", targ_btf_path, strerror(errno));
> > +               err = -errno;
>
> save errno before p_err, it can clobber errno otherwise
>
> > +               goto err_out;
> > +       }
> > +
> > +       info->marked_btf = btf__parse(targ_btf_path, NULL);
> > +       if (!info->marked_btf) {
> > +               p_err("failed parsing '%s' BTF file: %s", targ_btf_path, strerror(errno));
> > +               err = -errno;
>
> same, always save errno first before any non-trivial function/macro call
>

oh right, thanks!

>
> > +               goto err_out;
> > +       }
> > +
> > +       return info;
> > +
> > +err_out:
> > +       btfgen_free_info(info);
> > +       errno = -err;
> > +       return NULL;
> > +}
> > +
> > +#define MARKED UINT32_MAX
> > +
> > +static void btfgen_mark_member(struct btfgen_info *info, int type_id, int idx)
> > +{
> > +       const struct btf_type *t = btf__type_by_id(info->marked_btf, type_id);
> > +       struct btf_member *m = btf_members(t) + idx;
> > +
> > +       m->name_off = MARKED;
> > +}
> > +
> > +static int
> > +btfgen_mark_type(struct btfgen_info *info, unsigned int id, bool follow_pointers)
>
> id is type_id or could be some other id? It's best to be consistent in
> naming to avoid second guessing like in this case.

It's always type_id. Renamed it.

>
> > +{
> > +       const struct btf_type *btf_type = btf__type_by_id(info->src_btf, id);
> > +       struct btf_type *cloned_type;
> > +       struct btf_param *param;
> > +       struct btf_array *array;
> > +       int err, i;
>
> [...]
>
> > +       /* tells if some other type needs to be handled */
> > +       default:
> > +               p_err("unsupported kind: %s (%d)", btf_kind_str(btf_type), id);
> > +               return -EINVAL;
> > +       }
> > +
> > +       return 0;
> > +}
> > +
> > +static int btfgen_record_field_relo(struct btfgen_info *info, struct bpf_core_spec *targ_spec)
> > +{
> > +       struct btf *btf = (struct btf *) info->src_btf;
>
> why the cast?
>

No reason. Will remove it.

> > +       const struct btf_type *btf_type;
> > +       struct btf_member *btf_member;
> > +       struct btf_array *array;
> > +       unsigned int id = targ_spec->root_type_id;
> > +       int idx, err;
> > +
> > +       /* mark root type */
> > +       btf_type = btf__type_by_id(btf, id);
> > +       err = btfgen_mark_type(info, id, false);
> > +       if (err)
> > +               return err;
> > +
> > +       /* mark types for complex types (arrays, unions, structures) */
> > +       for (int i = 1; i < targ_spec->raw_len; i++) {
> > +               /* skip typedefs and mods */
> > +               while (btf_is_mod(btf_type) || btf_is_typedef(btf_type)) {
> > +                       id = btf_type->type;
> > +                       btf_type = btf__type_by_id(btf, id);
> > +               }
> > +
> > +               switch (btf_kind(btf_type)) {
> > +               case BTF_KIND_STRUCT:
> > +               case BTF_KIND_UNION:
> > +                       idx = targ_spec->raw_spec[i];
> > +                       btf_member = btf_members(btf_type) + idx;
> > +
> > +                       /* mark member */
> > +                       btfgen_mark_member(info, id, idx);
> > +
> > +                       /* mark member's type */
> > +                       id = btf_member->type;
> > +                       btf_type = btf__type_by_id(btf, id);
> > +                       err = btfgen_mark_type(info, id, false);
>
> why would it not follow the pointer? E.g., if I have a field defined as
>
> struct blah ***my_field;
>
> You at the very least would need either an empty struct blah or FWD
> for struct blah, no?
>

It's an optimization we do, we don't follow the pointer here because
it is possible that the definition of the pointed type is not needed.
For instance, a relocation like:

BPF_CORE_READ(task, nsproxy);

will generate this:

[1] STRUCT 'task_struct' size=9472 vlen=1
    'nsproxy' type_id=2 bits_offset=23040
[2] PTR '(anon)' type_id=0

struct nsproxy is not really accessed, so we don't need it's
definition. On the other hand, something like

BPF_CORE_READ(task, nsproxy, count);

has two relocations, and nsproxy is actually accessed, so in this case
the generated BTF includes a nsproxy struct:

[1] STRUCT '(anon)' size=4 vlen=0
[2] TYPEDEF 'atomic_t' type_id=1
[3] STRUCT 'task_struct' size=9472 vlen=1
    'nsproxy' type_id=4 bits_offset=23040
[4] PTR '(anon)' type_id=5
[5] STRUCT 'nsproxy' size=72 vlen=1
    'count' type_id=2 bits_offset=0

> > +                       if (err)
> > +                               return err;
> > +                       break;
> > +               case BTF_KIND_ARRAY:
> > +                       array = btf_array(btf_type);
> > +                       id = array->type;
> > +                       btf_type = btf__type_by_id(btf, id);
> > +                       break;
>
> [...]
>
> > +err_out:
> > +       bpf_core_free_cands(cands);
> > +       errno = -err;
> > +       return NULL;
> > +}
> > +
> > +/* Record relocation information for a single BPF object*/
>
> nit: missing space before */
>
> > +static int btfgen_record_obj(struct btfgen_info *info, const char *obj_path)
> > +{
> > +       const struct btf_ext_info_sec *sec;
> > +       const struct bpf_core_relo *relo;
> > +       const struct btf_ext_info *seg;
> > +       struct hashmap_entry *entry;
> > +       struct hashmap *cand_cache = NULL;
> > +       struct btf_ext *btf_ext = NULL;
> > +       unsigned int relo_idx;
> > +       struct btf *btf = NULL;
> > +       size_t i;
> > +       int err;
> > +
> > +       btf = btf__parse(obj_path, &btf_ext);
> > +       if (!btf) {
> > +               p_err("failed to parse BPF object '%s': %s", obj_path, strerror(errno));
> > +               return -errno;
> > +       }
>
> check that btf_ext is not NULL?
>

Done.


> > +
> > +       if (btf_ext->core_relo_info.len == 0) {
> > +               err = 0;
> > +               goto out;
> > +       }
> > +
>
> [...]