[PATCH bpf-next v1 7/8] resolve_btfids: Discover kfuncs from BTF ID sets

[Ihor Solodrai <ihor.solodrai@linux.dev>]

collect_kfuncs() currently uses bpf_kfunc decl tags to identify the
list of kfuncs. The decl tags are generated by pahole, which makes
current implementation implicitly rely on those tags being generated.

The authoritative source, used by the the BPF verifier for kfunc
registration, of functions being BPF kfuncs are
BTF_KFUNCS_START()/END() declarations. These are BTF_ID_SET8 under the
hood. Currently resolve_btfids reads kfunc flags from these sets, and
populates them with BTF IDs.

Implement kfunc discovery from BTF_ID_SET8 symbols in resolve_btfids,
removing the dependency on pahole's emmission of decl tags.

Walk BTF_ID_KIND_SET8 sets, and use the address-to-symbol index to
look up set entry's BTF_ID symbol name (before .BTF_ids is patched),
recording the paired flags directly. This makes find_kfunc_flags()
helper unnecessary, so it's removed.

Signed-off-by: Ihor Solodrai <ihor.solodrai@linux.dev>
---
 tools/bpf/resolve_btfids/main.c | 89 +++++++++++----------------------
 1 file changed, 29 insertions(+), 60 deletions(-)

diff --git a/tools/bpf/resolve_btfids/main.c b/tools/bpf/resolve_btfids/main.c
index ab3ab3045592..338d0c0a8e58 100644
--- a/tools/bpf/resolve_btfids/main.c
+++ b/tools/bpf/resolve_btfids/main.c
@@ -1039,19 +1039,18 @@ static int collect_decl_tags(struct btf2btf_context *ctx)
 	return 0;
 }
 
-/*
- * To find kfunc flags, scan BTF_SET8_KFUNCS entries and use the entry
- * address to recover the corresponding BTF_ID symbol name.
- * Return 0 (no flags!) if not found.
- */
-static u32 find_kfunc_flags(struct object *obj, struct btf_id *kfunc_id)
+static int collect_kfuncs(struct object *obj, struct btf2btf_context *ctx)
 {
 	Elf_Data *idlist = obj->efile.idlist;
-	struct btf_id *set_id;
+	struct btf *btf = ctx->btf;
 	struct rb_node *next;
 
+	if (!idlist || !idlist->d_buf)
+		return 0;
+
 	for (next = rb_first(&obj->sets); next; next = rb_next(next)) {
 		struct btf_id_set8 *set8;
+		struct btf_id *set_id;
 		u64 set_addr;
 
 		set_id = rb_entry(next, struct btf_id, rb_node);
@@ -1066,64 +1065,34 @@ static u32 find_kfunc_flags(struct object *obj, struct btf_id *kfunc_id)
 		for (u32 i = 0; i < set_id->cnt; i++) {
 			size_t off = (char *)&set8->pairs[i] - (char *)set8;
 			const char *name = find_name_by_addr(obj, set_addr + off);
+			struct kfunc kfunc;
+			s32 func_id;
+			int err;
 
-			if (!name || strcmp(name, kfunc_id->name) != 0)
+			if (!name) {
+				pr_err("WARN: resolve_btfids: no BTF ID symbol for %s entry %u\n",
+				       set_id->name, i);
+				warnings++;
 				continue;
+			}
 
-			pr_debug("found kfunc %s in BTF_ID_FLAGS %s\n",
-				 kfunc_id->name, set_id->name);
-
-			return set8->pairs[i].flags;
-		}
-	}
-
-	return 0;
-}
-
-static int collect_kfuncs(struct object *obj, struct btf2btf_context *ctx)
-{
-	const char *tag_name, *func_name;
-	struct btf *btf = ctx->btf;
-	const struct btf_type *t;
-	u32 flags, func_id;
-	struct kfunc kfunc;
-	struct btf_id *id;
-	int err;
-
-	if (ctx->nr_decl_tags == 0)
-		return 0;
-
-	for (u32 i = 0; i < ctx->nr_decl_tags; i++) {
-		t = btf__type_by_id(btf, ctx->decl_tags[i]);
-		if (btf_kflag(t) || btf_decl_tag(t)->component_idx != -1)
-			continue;
-
-		tag_name = btf__name_by_offset(btf, t->name_off);
-		if (strcmp(tag_name, "bpf_kfunc") != 0)
-			continue;
-
-		func_id = t->type;
-		t = btf__type_by_id(btf, func_id);
-		if (!btf_is_func(t))
-			continue;
-
-		func_name = btf__name_by_offset(btf, t->name_off);
-		if (!func_name)
-			continue;
-
-		id = btf_id__find(&obj->funcs, func_name);
-		if (!id || id->kind != BTF_ID_KIND_SYM)
-			continue;
-
-		flags = find_kfunc_flags(obj, id);
+			func_id = btf__find_by_name_kind_own(btf, name, BTF_KIND_FUNC);
+			if (func_id < 0) {
+				pr_err("WARN: resolve_btfids: no BTF func for kfunc %s in %s\n",
+				       name, set_id->name);
+				warnings++;
+				continue;
+			}
 
-		kfunc.name = id->name;
-		kfunc.btf_id = func_id;
-		kfunc.flags = flags;
+			pr_debug("found kfunc %s in %s\n", name, set_id->name);
 
-		err = push_kfunc(ctx, &kfunc);
-		if (err)
-			return err;
+			kfunc.name = name;
+			kfunc.btf_id = func_id;
+			kfunc.flags = set8->pairs[i].flags;
+			err = push_kfunc(ctx, &kfunc);
+			if (err)
+				return err;
+		}
 	}
 
 	return 0;
-- 
2.55.0