Re: [PATCH bpf-next v4 2/2] selftests/bpf: check whether s32 is sufficient for kfunc offset

In add_kfunc_call(), bpf_kfunc_desc->imm with type s32 is used to
represent the offset of called kfunc from __bpf_call_base, so
add a test to ensure that the offset will not be overflowed.

Signed-off-by: Hou Tao <redacted>
---
  .../selftests/bpf/prog_tests/ksyms_module.c   | 42 +++++++++++++++++++
  1 file changed, 42 insertions(+)

diff --git a/tools/testing/selftests/bpf/prog_tests/ksyms_module.c b/tools/testing/selftests/bpf/prog_tests/ksyms_module.c
index a1ebac70ec29..8055fbbf720b 100644
--- a/tools/testing/selftests/bpf/prog_tests/ksyms_module.c
+++ b/tools/testing/selftests/bpf/prog_tests/ksyms_module.c

@@ -3,9 +3,49 @@
  
  #include <test_progs.h>
  #include <network_helpers.h>
+#include <trace_helpers.h>
  #include "test_ksyms_module.lskel.h"
  #include "test_ksyms_module.skel.h"
  
+/*
+ * Check whether or not s32 in bpf_kfunc_desc is sufficient
+ * to represent the offset between bpf_testmod_test_mod_kfunc
+ * and __bpf_call_base.
+ */
+static void test_ksyms_module_valid_offset(void)
+{
+	struct test_ksyms_module *skel;
+	unsigned long long kfunc_addr;
+	unsigned long long base_addr;
+	long long actual_offset;
+	int used_offset;
+	int err;
+
+	if (!env.has_testmod) {
+		test__skip();
+		return;
+	}
+
+	/* Ensure kfunc call is supported */
+	skel = test_ksyms_module__open_and_load();
+	if (!ASSERT_OK_PTR(skel, "test_ksyms_module__open"))
+		return;
+
+	err = kallsyms_find("bpf_testmod_test_mod_kfunc", &kfunc_addr);
+	if (!ASSERT_OK(err, "find kfunc addr"))
+		goto cleanup;
+
+	err = kallsyms_find("__bpf_call_base", &base_addr);
+	if (!ASSERT_OK(err, "find base addr"))
+		goto cleanup;
+
+	used_offset = kfunc_addr - base_addr;
+	actual_offset = kfunc_addr - base_addr;
+	ASSERT_EQ((long long)used_offset, actual_offset, "kfunc offset overflowed");

+cleanup:
+	test_ksyms_module__destroy(skel);
+}
+
  static void test_ksyms_module_lskel(void)
  {
  	struct test_ksyms_module_lskel *skel;

@@ -62,6 +102,8 @@ static void test_ksyms_module_libbpf(void)
  
  void test_ksyms_module(void)
  {
+	if (test__start_subtest("valid_offset"))
+		test_ksyms_module_valid_offset();
  	if (test__start_subtest("lskel"))
  		test_ksyms_module_lskel();
  	if (test__start_subtest("libbpf"))