The BPF trampoline preserves only 8 bytes of the target's return value
(R0), so attaching an fexit/fmod_ret/fsession program to a function that
returns a >8 byte value is now rejected by the verifier.

Add a bpf_testmod function returning __int128 and an fexit program that
targets it. The program is expected to fail to load with the
"with a >8 byte return value is not supported for this attach type"
message.

__int128 is only available on 64-bit targets (where the compiler defines
__SIZEOF_INT128__). Guard the testmod function and the test with
__SIZEOF_INT128__ so a 32-bit build still compiles; on such builds
the subtest is simply not run.

Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
---
 .../selftests/bpf/prog_tests/tracing_failure.c      | 12 ++++++++++++
 tools/testing/selftests/bpf/progs/tracing_failure.c |  6 ++++++
 .../testing/selftests/bpf/test_kmods/bpf_testmod.c  | 13 +++++++++++++
 3 files changed, 31 insertions(+)

diff --git a/tools/testing/selftests/bpf/prog_tests/tracing_failure.c b/tools/testing/selftests/bpf/prog_tests/tracing_failure.c
index f9f9e1cb87bf..6c199f5ff4db 100644
--- a/tools/testing/selftests/bpf/prog_tests/tracing_failure.c
+++ b/tools/testing/selftests/bpf/prog_tests/tracing_failure.c
@@ -76,6 +76,14 @@ static void test_fexit_noreturns(void)
 			       "Attaching fexit/fsession/fmod_ret to __noreturn function 'do_exit' is rejected.");
 }
 
+#ifdef __SIZEOF_INT128__
+static void test_fexit_int128_ret(void)
+{
+	test_tracing_fail_prog("fexit_int128_ret",
+			       "with a >8 byte return value is not supported for this attach type");
+}
+#endif
+
 void test_tracing_failure(void)
 {
 	if (test__start_subtest("bpf_spin_lock"))
@@ -86,4 +94,8 @@ void test_tracing_failure(void)
 		test_tracing_deny();
 	if (test__start_subtest("fexit_noreturns"))
 		test_fexit_noreturns();
+#ifdef __SIZEOF_INT128__
+	if (test__start_subtest("fexit_int128_ret"))
+		test_fexit_int128_ret();
+#endif
 }
diff --git a/tools/testing/selftests/bpf/progs/tracing_failure.c b/tools/testing/selftests/bpf/progs/tracing_failure.c
index 65e485c4468c..f7a095767679 100644
--- a/tools/testing/selftests/bpf/progs/tracing_failure.c
+++ b/tools/testing/selftests/bpf/progs/tracing_failure.c
@@ -30,3 +30,9 @@ int BPF_PROG(fexit_noreturns)
 {
 	return 0;
 }
+
+SEC("?fexit/bpf_testmod_test_int128_ret")
+int BPF_PROG(fexit_int128_ret)
+{
+	return 0;
+}
diff --git a/tools/testing/selftests/bpf/test_kmods/bpf_testmod.c b/tools/testing/selftests/bpf/test_kmods/bpf_testmod.c
index 30f1cd23093c..902dbf658250 100644
--- a/tools/testing/selftests/bpf/test_kmods/bpf_testmod.c
+++ b/tools/testing/selftests/bpf/test_kmods/bpf_testmod.c
@@ -161,6 +161,15 @@ bpf_testmod_test_arg_ptr_to_struct(struct bpf_testmod_struct_arg_1 *a) {
 	return bpf_testmod_test_struct_arg_result;
 }
 
+#ifdef __SIZEOF_INT128__
+noinline __int128
+bpf_testmod_test_int128_ret(int a)
+{
+	bpf_testmod_test_struct_arg_result = a;
+	return (__int128)a;
+}
+#endif
+
 __weak noinline void bpf_testmod_looooooooooooooooooooooooooooooong_name(void)
 {
 }
@@ -514,6 +523,10 @@ bpf_testmod_test_read(struct file *file, struct kobject *kobj,
 
 	(void)bpf_testmod_test_arg_ptr_to_struct(&struct_arg1_2);
 
+#ifdef __SIZEOF_INT128__
+	(void)bpf_testmod_test_int128_ret(i);
+#endif
+
 	(void)trace_bpf_testmod_test_raw_tp_null_tp(NULL);
 
 	bpf_testmod_test_struct_ops3();
-- 
2.53.0-Meta