selftests/bpf: Add a kernel flag test for LSM bpf hook

This test exercises the kernel flag added to security_bpf by
effectively blocking light-skeletons from loading while allowing
normal skeletons to function as-is. Since this should work with any
arbitrary BPF program, an existing program from LSKELS_EXTRA was
used as a test payload.

Signed-off-by: Blaise Boscaccy <bboscaccy@linux.microsoft.com>
Acked-by: Song Liu <song@kernel.org>
Link: https://lore.kernel.org/r/20250310221737.821889-3-bboscaccy@linux.microsoft.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

1 files changed, 43 insertions, 0 deletions

diff --git a/tools/testing/selftests/bpf/prog_tests/kernel_flag.c b/tools/testing/selftests/bpf/prog_tests/kernel_flag.c
new file mode 100644
index 000000000000..a133354ac9bc
--- /dev/null
+++ b/tools/testing/selftests/bpf/prog_tests/kernel_flag.c
@@ -0,0 +1,43 @@
+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (c) 2025 Microsoft */
+#include <test_progs.h>
+#include "kfunc_call_test.skel.h"
+#include "kfunc_call_test.lskel.h"
+#include "test_kernel_flag.skel.h"
+
+void test_kernel_flag(void)
+{
+	struct test_kernel_flag *lsm_skel;
+	struct kfunc_call_test *skel = NULL;
+	struct kfunc_call_test_lskel *lskel = NULL;
+	int ret;
+
+	lsm_skel = test_kernel_flag__open_and_load();
+	if (!ASSERT_OK_PTR(lsm_skel, "lsm_skel"))
+		return;
+
+	lsm_skel->bss->monitored_tid = gettid();
+
+	ret = test_kernel_flag__attach(lsm_skel);
+	if (!ASSERT_OK(ret, "test_kernel_flag__attach"))
+		goto close_prog;
+
+	/* Test with skel. This should pass the gatekeeper */
+	skel = kfunc_call_test__open_and_load();
+	if (!ASSERT_OK_PTR(skel, "skel"))
+		goto close_prog;
+
+	/* Test with lskel. This should fail due to blocking kernel-based bpf() invocations */
+	lskel = kfunc_call_test_lskel__open_and_load();
+	if (!ASSERT_ERR_PTR(lskel, "lskel"))
+		goto close_prog;
+
+close_prog:
+	if (skel)
+		kfunc_call_test__destroy(skel);
+	if (lskel)
+		kfunc_call_test_lskel__destroy(lskel);
+
+	lsm_skel->bss->monitored_tid = 0;
+	test_kernel_flag__destroy(lsm_skel);
+}