[PATCH bpf-next 6/8] selftests/bpf: Add a test to verify kprobe_write_ctx compatibility enforcement

[Leon Hwang <leon.hwang@linux.dev>]

Add prog_array coverage for kprobe programs with and without
kprobe_write_ctx requirements.

The test verifies one-way enforcement: !kprobe_write_ctx programs
cannot tail-call kprobe_write_ctx programs.

Assisted-by: Codex:gpt-5.3-codex
Signed-off-by: Leon Hwang <leon.hwang@linux.dev>
---
 .../selftests/bpf/prog_tests/attach_probe.c   | 44 +++++++++++++++++++
 .../selftests/bpf/progs/kprobe_write_ctx.c    |  6 +++
 2 files changed, 50 insertions(+)

diff --git a/tools/testing/selftests/bpf/prog_tests/attach_probe.c b/tools/testing/selftests/bpf/prog_tests/attach_probe.c
index 9e77e5da7097..a9444db92246 100644
--- a/tools/testing/selftests/bpf/prog_tests/attach_probe.c
+++ b/tools/testing/selftests/bpf/prog_tests/attach_probe.c
@@ -220,11 +220,53 @@ static void test_attach_kprobe_write_ctx(void)
 
 	kprobe_write_ctx__destroy(skel);
 }
+
+static void test_kprobe_write_ctx_prog_array_compat(void)
+{
+	__u32 key = 0, read_prog_fd, write_prog_fd;
+	struct kprobe_write_ctx *skel = NULL;
+	int map_fd = -1, err;
+
+	skel = kprobe_write_ctx__open_and_load();
+	if (!ASSERT_OK_PTR(skel, "kprobe_write_ctx__open_and_load"))
+		return;
+
+	map_fd = bpf_map_create(BPF_MAP_TYPE_PROG_ARRAY, NULL, sizeof(key),
+				sizeof(__u32), 1, NULL);
+	if (!ASSERT_GE(map_fd, 0, "bpf_map_create"))
+		goto cleanup;
+
+	read_prog_fd = bpf_program__fd(skel->progs.kprobe_read_ctx);
+	write_prog_fd = bpf_program__fd(skel->progs.kprobe_write_ctx);
+	if (!ASSERT_GE(read_prog_fd, 0, "read_prog_fd"))
+		goto cleanup;
+	if (!ASSERT_GE(write_prog_fd, 0, "write_prog_fd"))
+		goto cleanup;
+
+	err = bpf_map_update_elem(map_fd, &key, &read_prog_fd, BPF_ANY);
+	if (!ASSERT_OK(err, "bpf_map_update_elem success"))
+		goto cleanup;
+
+	err = bpf_map_update_elem(map_fd, &key, &write_prog_fd, BPF_ANY);
+	if (!ASSERT_ERR(err, "bpf_map_update_elem failure"))
+		goto cleanup;
+	ASSERT_EQ(errno, EINVAL, "bpf_map_update_elem errno");
+
+cleanup:
+	if (map_fd >= 0)
+		close(map_fd);
+	kprobe_write_ctx__destroy(skel);
+}
 #else
 static void test_attach_kprobe_write_ctx(void)
 {
 	test__skip();
 }
+
+static void test_kprobe_write_ctx_prog_array_compat(void)
+{
+	test__skip();
+}
 #endif
 
 static void test_attach_probe_auto(struct test_attach_probe *skel)
@@ -434,6 +476,8 @@ void test_attach_probe(void)
 		test_attach_kprobe_long_event_name();
 	if (test__start_subtest("kprobe-write-ctx"))
 		test_attach_kprobe_write_ctx();
+	if (test__start_subtest("kprobe-write-ctx-prog-array-compat"))
+		test_kprobe_write_ctx_prog_array_compat();
 
 cleanup:
 	test_attach_probe__destroy(skel);
diff --git a/tools/testing/selftests/bpf/progs/kprobe_write_ctx.c b/tools/testing/selftests/bpf/progs/kprobe_write_ctx.c
index f77aef0474d3..09e0c487bde5 100644
--- a/tools/testing/selftests/bpf/progs/kprobe_write_ctx.c
+++ b/tools/testing/selftests/bpf/progs/kprobe_write_ctx.c
@@ -13,6 +13,12 @@ int kprobe_write_ctx(struct pt_regs *ctx)
 	return 0;
 }
 
+SEC("kprobe")
+int kprobe_read_ctx(struct pt_regs *ctx)
+{
+	return 0;
+}
+
 SEC("kprobe.multi")
 int kprobe_multi_write_ctx(struct pt_regs *ctx)
 {
-- 
2.52.0