Extend the resolve_btfids selftest to cover kfunc sets defined with
BTF_KFUNCS_START/BTF_KFUNCS_END.

The test verifies that resolve_btfids correctly processes BTF_ID_FLAGS,
resolves function IDs, and checks the kfunc set is sorted.

Signed-off-by: Ihor Solodrai <ihor.solodrai@linux.dev>
---
 .../selftests/bpf/prog_tests/resolve_btfids.c | 56 +++++++++++++++++++
 tools/testing/selftests/bpf/progs/btf_data.c  | 10 ++++
 2 files changed, 66 insertions(+)

diff --git a/tools/testing/selftests/bpf/prog_tests/resolve_btfids.c b/tools/testing/selftests/bpf/prog_tests/resolve_btfids.c
index d0e51979d455..f6fd79b9dd23 100644
--- a/tools/testing/selftests/bpf/prog_tests/resolve_btfids.c
+++ b/tools/testing/selftests/bpf/prog_tests/resolve_btfids.c
@@ -10,6 +10,10 @@
 #include <linux/btf_ids.h>
 #include "test_progs.h"
 
+#ifndef KF_FASTCALL
+#define KF_FASTCALL (1 << 12)
+#endif
+
 struct symbol {
 	const char	*name;
 	int		 type;
@@ -26,6 +30,17 @@ struct symbol test_symbols[] = {
 	{ "func",    BTF_KIND_FUNC,    -1 },
 };
 
+struct kfunc_symbol {
+	const char	*name;
+	s32		 id;
+	u32		 flags;
+};
+
+static struct kfunc_symbol kfunc_symbols[] = {
+	{ "kfunc_a", -1, 0 },
+	{ "kfunc_b", -1, KF_FASTCALL },
+};
+
 /* Align the .BTF_ids section to 4 bytes */
 asm (
 ".pushsection " BTF_IDS_SECTION " ,\"a\"; \n"
@@ -77,7 +92,13 @@ BTF_ID(union,   U)
 BTF_ID(func,    func)
 BTF_SET_END(test_set)
 
+BTF_KFUNCS_START(test_kfunc_set)
+BTF_ID_FLAGS(func, kfunc_a)
+BTF_ID_FLAGS(func, kfunc_b, KF_FASTCALL)
+BTF_KFUNCS_END(test_kfunc_set)
+
 #pragma GCC visibility pop
+
 static int
 __resolve_symbol(struct btf *btf, int type_id)
 {
@@ -108,6 +129,18 @@ __resolve_symbol(struct btf *btf, int type_id)
 			test_symbols[i].id = type_id;
 	}
 
+	if (BTF_INFO_KIND(type->info) == BTF_KIND_FUNC) {
+		str = btf__name_by_offset(btf, type->name_off);
+		if (str) {
+			for (i = 0; i < ARRAY_SIZE(kfunc_symbols); i++) {
+				if (kfunc_symbols[i].id >= 0)
+					continue;
+				if (!strcmp(str, kfunc_symbols[i].name))
+					kfunc_symbols[i].id = type_id;
+			}
+		}
+	}
+
 	return 0;
 }
 
@@ -162,6 +195,29 @@ void test_resolve_btfids(void)
 			ASSERT_LE(test_set.ids[i - 1], test_set.ids[i], "sort_check");
 	}
 
+	/* Check BTF_KFUNCS_START(test_kfunc_set) */
+	ASSERT_EQ(test_kfunc_set.flags, BTF_SET8_KFUNCS, "kfunc_set_flags");
+	ASSERT_EQ(test_kfunc_set.cnt, ARRAY_SIZE(kfunc_symbols), "kfunc_set_cnt");
+
+	for (i = 0; i < test_kfunc_set.cnt; i++) {
+		bool found = false;
+
+		for (j = 0; j < ARRAY_SIZE(kfunc_symbols); j++) {
+			if (kfunc_symbols[j].id != (s32)test_kfunc_set.pairs[i].id)
+				continue;
+			found = true;
+			ASSERT_EQ(test_kfunc_set.pairs[i].flags,
+				  kfunc_symbols[j].flags, "kfunc_flags_check");
+			break;
+		}
+
+		ASSERT_TRUE(found, "kfunc_id_found");
+
+		if (i > 0)
+			ASSERT_LE(test_kfunc_set.pairs[i - 1].id,
+				  test_kfunc_set.pairs[i].id, "kfunc_sort_check");
+	}
+
 out:
 	btf__free(btf);
 }
diff --git a/tools/testing/selftests/bpf/progs/btf_data.c b/tools/testing/selftests/bpf/progs/btf_data.c
index baa525275bde..8587658012c3 100644
--- a/tools/testing/selftests/bpf/progs/btf_data.c
+++ b/tools/testing/selftests/bpf/progs/btf_data.c
@@ -48,3 +48,13 @@ int func(struct root_struct *root)
 {
 	return 0;
 }
+
+int kfunc_a(struct root_struct *root)
+{
+	return 0;
+}
+
+int kfunc_b(struct root_struct *root)
+{
+	return 0;
+}
-- 
2.54.0