selftests/bpf: Add kprobe multi write ctx attach test

Adding test to check we can't attach kprobe multi program
that writes to the context.

It's x86_64 specific test.

Acked-by: Andrii Nakryiko <[e-mail]>
Signed-off-by: Jiri Olsa <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Alexei Starovoitov <[e-mail]>

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

···
 #include "kprobe_multi_session.skel.h"
 #include "kprobe_multi_session_cookie.skel.h"
 #include "kprobe_multi_verifier.skel.h"
+#include "kprobe_write_ctx.skel.h"
 #include "bpf/libbpf_internal.h"
 #include "bpf/hashmap.h"
 
···
 	kprobe_multi_override__destroy(skel);
 }
 
+#ifdef __x86_64__
+static void test_attach_write_ctx(void)
+{
+	struct kprobe_write_ctx *skel = NULL;
+	struct bpf_link *link = NULL;
+
+	skel = kprobe_write_ctx__open_and_load();
+	if (!ASSERT_OK_PTR(skel, "kprobe_write_ctx__open_and_load"))
+		return;
+
+	link = bpf_program__attach_kprobe_opts(skel->progs.kprobe_multi_write_ctx,
+						     "bpf_fentry_test1", NULL);
+	if (!ASSERT_ERR_PTR(link, "bpf_program__attach_kprobe_opts"))
+		bpf_link__destroy(link);
+
+	kprobe_write_ctx__destroy(skel);
+}
+#else
+static void test_attach_write_ctx(void)
+{
+	test__skip();
+}
+#endif
+
 void serial_test_kprobe_multi_bench_attach(void)
 {
 	if (test__start_subtest("kernel"))
···
 		test_session_cookie_skel_api();
 	if (test__start_subtest("unique_match"))
 		test_unique_match();
+	if (test__start_subtest("attach_write_ctx"))
+		test_attach_write_ctx();
 	RUN_TESTS(kprobe_multi_verifier);
 }

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

···
 	ctx->ax = 0;
 	return 0;
 }
+
+SEC("kprobe.multi")
+int kprobe_multi_write_ctx(struct pt_regs *ctx)
+{
+	ctx->ax = 0;
+	return 0;
+}
 #endif