selftests/bpf: Add set_attach_target() API selftest for module target

Add test for bpf_program__set_attach_target() API, validating it can find
kernel module fentry target.
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20201211215825.3646154-3-andrii@kernel.org

tools/testing/selftests/bpf/prog_tests/module_attach.c

@@ -28,10 +28,18 @@ void test_module_attach(void)
 	struct test_module_attach__bss *bss;
 	int err;
 
-	skel = test_module_attach__open_and_load();
+	skel = test_module_attach__open();
 	if (CHECK(!skel, "skel_open", "failed to open skeleton\n"))
 		return;
 
+	err = bpf_program__set_attach_target(skel->progs.handle_fentry_manual,
+					     0, "bpf_testmod_test_read");
+	ASSERT_OK(err, "set_attach_target");
+
+	err = test_module_attach__load(skel);
+	if (CHECK(err, "skel_load", "failed to load skeleton\n"))
+		return;
+
 	bss = skel->bss;
 
 	err = test_module_attach__attach(skel);
@@ -44,6 +52,7 @@ void test_module_attach(void)
 	ASSERT_EQ(bss->raw_tp_read_sz, READ_SZ, "raw_tp");
 	ASSERT_EQ(bss->tp_btf_read_sz, READ_SZ, "tp_btf");
 	ASSERT_EQ(bss->fentry_read_sz, READ_SZ, "fentry");
+	ASSERT_EQ(bss->fentry_manual_read_sz, READ_SZ, "fentry_manual");
 	ASSERT_EQ(bss->fexit_read_sz, READ_SZ, "fexit");
 	ASSERT_EQ(bss->fexit_ret, -EIO, "fexit_tet");
 	ASSERT_EQ(bss->fmod_ret_read_sz, READ_SZ, "fmod_ret");

tools/testing/selftests/bpf/progs/test_module_attach.c

@@ -38,6 +38,17 @@ int BPF_PROG(handle_fentry,
 	return 0;
 }
 
+__u32 fentry_manual_read_sz = 0;
+
+SEC("fentry/placeholder")
+int BPF_PROG(handle_fentry_manual,
+	     struct file *file, struct kobject *kobj,
+	     struct bin_attribute *bin_attr, char *buf, loff_t off, size_t len)
+{
+	fentry_manual_read_sz = len;
+	return 0;
+}
+
 __u32 fexit_read_sz = 0;
 int fexit_ret = 0;