Re: [PATCH v2 bpf-next 06/17] libbpf: refactor BTF map definition parsing

On Thu, Apr 22, 2021 at 8:33 AM Yonghong Song [off-list ref] wrote:

On 4/16/21 1:23 PM, Andrii Nakryiko wrote:

quoted

Refactor BTF-defined maps parsing logic to allow it to be nicely reused by BPF
static linker. Further, at least for BPF static linker, it's important to know
which attributes of a BPF map were defined explicitly, so provide a bit set
for each known portion of BTF map definition. This allows BPF static linker to
do a simple check when dealing with extern map declarations.

The same capabilities allow to distinguish attributes explicitly set to zero
(e.g., __uint(max_entries, 0)) vs the case of not specifying it at all (no
max_entries attribute at all). Libbpf is currently not utilizing that, but it
could be useful for backwards compatibility reasons later.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
---
  tools/lib/bpf/libbpf.c          | 256 ++++++++++++++++++--------------
  tools/lib/bpf/libbpf_internal.h |  32 ++++
  2 files changed, 177 insertions(+), 111 deletions(-)

diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
index a0e6d6bc47f3..f6f4126389ac 100644
--- a/tools/lib/bpf/libbpf.c
+++ b/tools/lib/bpf/libbpf.c

@@ -2025,255 +2025,262 @@ static int build_map_pin_path(struct bpf_map *map, const char *path)
      return bpf_map__set_pin_path(map, buf);
  }

-
-static int parse_btf_map_def(struct bpf_object *obj,
-                          struct bpf_map *map,
-                          const struct btf_type *def,
-                          bool strict, bool is_inner,
-                          const char *pin_root_path)
+int parse_btf_map_def(const char *map_name, struct btf *btf,
+                   const struct btf_type *def_t, bool strict,
+                   struct btf_map_def *map_def, struct btf_map_def *inner_def)
  {
      const struct btf_type *t;
      const struct btf_member *m;
+     bool is_inner = inner_def == NULL;
      int vlen, i;

-     vlen = btf_vlen(def);
-     m = btf_members(def);
+     vlen = btf_vlen(def_t);
+     m = btf_members(def_t);
      for (i = 0; i < vlen; i++, m++) {
-             const char *name = btf__name_by_offset(obj->btf, m->name_off);
+             const char *name = btf__name_by_offset(btf, m->name_off);

[...]

quoted

              }
              else if (strcmp(name, "values") == 0) {
+                     char inner_map_name[128];
                      int err;

                      if (is_inner) {
                              pr_warn("map '%s': multi-level inner maps not supported.\n",
-                                     map->name);
+                                     map_name);
                              return -ENOTSUP;
                      }
                      if (i != vlen - 1) {
                              pr_warn("map '%s': '%s' member should be last.\n",
-                                     map->name, name);
+                                     map_name, name);
                              return -EINVAL;
                      }
-                     if (!bpf_map_type__is_map_in_map(map->def.type)) {
+                     if (!bpf_map_type__is_map_in_map(map_def->map_type)) {
                              pr_warn("map '%s': should be map-in-map.\n",
-                                     map->name);
+                                     map_name);
                              return -ENOTSUP;
                      }
-                     if (map->def.value_size && map->def.value_size != 4) {
+                     if (map_def->value_size && map_def->value_size != 4) {
                              pr_warn("map '%s': conflicting value size %u != 4.\n",
-                                     map->name, map->def.value_size);
+                                     map_name, map_def->value_size);
                              return -EINVAL;
                      }
-                     map->def.value_size = 4;
-                     t = btf__type_by_id(obj->btf, m->type);
+                     map_def->value_size = 4;
+                     t = btf__type_by_id(btf, m->type);
                      if (!t) {
                              pr_warn("map '%s': map-in-map inner type [%d] not found.\n",
-                                     map->name, m->type);
+                                     map_name, m->type);
                              return -EINVAL;
                      }
                      if (!btf_is_array(t) || btf_array(t)->nelems) {
                              pr_warn("map '%s': map-in-map inner spec is not a zero-sized array.\n",
-                                     map->name);
+                                     map_name);
                              return -EINVAL;
                      }
-                     t = skip_mods_and_typedefs(obj->btf, btf_array(t)->type,
-                                                NULL);
+                     t = skip_mods_and_typedefs(btf, btf_array(t)->type, NULL);
                      if (!btf_is_ptr(t)) {
                              pr_warn("map '%s': map-in-map inner def is of unexpected kind %s.\n",
-                                     map->name, btf_kind_str(t));
+                                     map_name, btf_kind_str(t));
                              return -EINVAL;
                      }
-                     t = skip_mods_and_typedefs(obj->btf, t->type, NULL);
+                     t = skip_mods_and_typedefs(btf, t->type, NULL);
                      if (!btf_is_struct(t)) {
                              pr_warn("map '%s': map-in-map inner def is of unexpected kind %s.\n",
-                                     map->name, btf_kind_str(t));
+                                     map_name, btf_kind_str(t));
                              return -EINVAL;
                      }

-                     map->inner_map = calloc(1, sizeof(*map->inner_map));
-                     if (!map->inner_map)
-                             return -ENOMEM;
-                     map->inner_map->fd = -1;
-                     map->inner_map->sec_idx = obj->efile.btf_maps_shndx;

The refactoring didn't set map->inner_map->sec_idx. I guess since
inner_map is only used internally by libbpf, so it does not
have a user visible sec_idx and hence useless? It is worthwhile to
mention in the commit message for this difference, I think.

quoted

-                     map->inner_map->name = malloc(strlen(map->name) +
-                                                   sizeof(".inner") + 1);
-                     if (!map->inner_map->name)
-                             return -ENOMEM;
-                     sprintf(map->inner_map->name, "%s.inner", map->name);
-
-                     err = parse_btf_map_def(obj, map->inner_map, t, strict,
-                                             true /* is_inner */, NULL);
+                     snprintf(inner_map_name, sizeof(inner_map_name), "%s.inner", map_name);
+                     err = parse_btf_map_def(inner_map_name, btf, t, strict, inner_def, NULL);
                      if (err)
                              return err;
+
+                     map_def->parts |= MAP_DEF_INNER_MAP;
              } else if (strcmp(name, "pinning") == 0) {
                      __u32 val;
-                     int err;

                      if (is_inner) {
-                             pr_debug("map '%s': inner def can't be pinned.\n",
-                                      map->name);
+                             pr_warn("map '%s': inner def can't be pinned.\n", map_name);
                              return -EINVAL;
                      }
-                     if (!get_map_field_int(map->name, obj->btf, m, &val))
+                     if (!get_map_field_int(map_name, btf, m, &val))
                              return -EINVAL;
-                     pr_debug("map '%s': found pinning = %u.\n",
-                              map->name, val);
-
-                     if (val != LIBBPF_PIN_NONE &&
-                         val != LIBBPF_PIN_BY_NAME) {
+                     if (val != LIBBPF_PIN_NONE && val != LIBBPF_PIN_BY_NAME) {
                              pr_warn("map '%s': invalid pinning value %u.\n",
-                                     map->name, val);
+                                     map_name, val);
                              return -EINVAL;
                      }
-                     if (val == LIBBPF_PIN_BY_NAME) {
-                             err = build_map_pin_path(map, pin_root_path);
-                             if (err) {
-                                     pr_warn("map '%s': couldn't build pin path.\n",
-                                             map->name);
-                                     return err;
-                             }
-                     }
+                     map_def->pinning = val;
+                     map_def->parts |= MAP_DEF_PINNING;
              } else {
                      if (strict) {
-                             pr_warn("map '%s': unknown field '%s'.\n",
-                                     map->name, name);
+                             pr_warn("map '%s': unknown field '%s'.\n", map_name, name);
                              return -ENOTSUP;
                      }
-                     pr_debug("map '%s': ignoring unknown field '%s'.\n",
-                              map->name, name);
+                     pr_debug("map '%s': ignoring unknown field '%s'.\n", map_name, name);
              }
      }

-     if (map->def.type == BPF_MAP_TYPE_UNSPEC) {
-             pr_warn("map '%s': map type isn't specified.\n", map->name);
+     if (map_def->map_type == BPF_MAP_TYPE_UNSPEC) {
+             pr_warn("map '%s': map type isn't specified.\n", map_name);
              return -EINVAL;
      }

      return 0;
  }

+static void fill_map_from_def(struct bpf_map *map, const struct btf_map_def *def)
+{

[...]

quoted

+}
+
  static int bpf_object__init_user_btf_map(struct bpf_object *obj,
                                       const struct btf_type *sec,
                                       int var_idx, int sec_idx,
                                       const Elf_Data *data, bool strict,
                                       const char *pin_root_path)
  {
+     struct btf_map_def map_def = {}, inner_def = {};
      const struct btf_type *var, *def;
      const struct btf_var_secinfo *vi;
      const struct btf_var *var_extra;
      const char *map_name;
      struct bpf_map *map;
+     int err;

      vi = btf_var_secinfos(sec) + var_idx;
      var = btf__type_by_id(obj->btf, vi->type);

@@ -2327,7 +2334,34 @@ static int bpf_object__init_user_btf_map(struct bpf_object *obj,
      pr_debug("map '%s': at sec_idx %d, offset %zu.\n",
               map_name, map->sec_idx, map->sec_offset);

-     return parse_btf_map_def(obj, map, def, strict, false, pin_root_path);
+     err = parse_btf_map_def(map->name, obj->btf, def, strict, &map_def, &inner_def);
+     if (err)
+             return err;
+
+     fill_map_from_def(map, &map_def);
+
+     if (map_def.pinning == LIBBPF_PIN_BY_NAME) {
+             err = build_map_pin_path(map, pin_root_path);
+             if (err) {
+                     pr_warn("map '%s': couldn't build pin path.\n", map->name);
+                     return err;
+             }
+     }
+
+     if (map_def.parts & MAP_DEF_INNER_MAP) {
+             map->inner_map = calloc(1, sizeof(*map->inner_map));
+             if (!map->inner_map)
+                     return -ENOMEM;
+             map->inner_map->fd = -1;

missing set map->inner_map->sec_idx here.

I'll add it back here, but it was never really necessary. More for the
completeness sake. sec_idx is used only to match relo to a map, and
this inner_map is never matched and never referenced by a relocation.

quoted

+             map->inner_map->name = malloc(strlen(map_name) + sizeof(".inner") + 1);
+             if (!map->inner_map->name)
+                     return -ENOMEM;
+             sprintf(map->inner_map->name, "%s.inner", map_name);
+
+             fill_map_from_def(map->inner_map, &inner_def);
+     }
+
+     return 0;
  }

  static int bpf_object__init_user_btf_maps(struct bpf_object *obj, bool strict,

diff --git a/tools/lib/bpf/libbpf_internal.h b/tools/lib/bpf/libbpf_internal.h
index 92b7eae10c6d..17883073710c 100644
--- a/tools/lib/bpf/libbpf_internal.h
+++ b/tools/lib/bpf/libbpf_internal.h

@@ -138,6 +138,38 @@ static inline __u32 btf_type_info(int kind, int vlen, int kflag)
      return (kflag << 31) | (kind << 24) | vlen;
  }

[...]