[PATCH v5 bpf-next 2/2] selftests/bpf: Add a kernel flag test for LSM bpf hook

[Blaise Boscaccy <bboscaccy@linux.microsoft.com>]

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>
---
 .../selftests/bpf/prog_tests/kernel_flag.c    | 43 +++++++++++++++++++
 .../selftests/bpf/progs/test_kernel_flag.c    | 32 ++++++++++++++
 2 files changed, 75 insertions(+)
 create mode 100644 tools/testing/selftests/bpf/prog_tests/kernel_flag.c
 create mode 100644 tools/testing/selftests/bpf/progs/test_kernel_flag.c

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 0000000000000..479ad5de3737e
--- /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;
+
+	ret = test_kernel_flag__attach(lsm_skel);
+	if (!ASSERT_OK(ret, "test_kernel_flag__attach"))
+		goto close_prog;
+
+	lsm_skel->bss->monitored_pid = getpid();
+
+	/* 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_pid = 0;
+	test_kernel_flag__destroy(lsm_skel);
+}
diff --git a/tools/testing/selftests/bpf/progs/test_kernel_flag.c b/tools/testing/selftests/bpf/progs/test_kernel_flag.c
new file mode 100644
index 0000000000000..1e0320a1d2f6b
--- /dev/null
+++ b/tools/testing/selftests/bpf/progs/test_kernel_flag.c
@@ -0,0 +1,32 @@
+// SPDX-License-Identifier: GPL-2.0
+
+/*
+ * Copyright (C) 2025 Microsoft Corporation
+ *
+ * Author: Blaise Boscaccy <bboscaccy@linux.microsoft.com>
+ */
+
+#include "vmlinux.h"
+#include <errno.h>
+#include <bpf/bpf_helpers.h>
+#include <bpf/bpf_tracing.h>
+
+char _license[] SEC("license") = "GPL";
+
+__u32 monitored_pid;
+
+SEC("lsm.s/bpf")
+int BPF_PROG(bpf, int cmd, union bpf_attr *attr, unsigned int size, bool kernel)
+{
+	__u32 pid;
+
+	pid = bpf_get_current_pid_tgid() >> 32;
+
+	if (pid != monitored_pid)
+		return 0;
+
+	if (kernel)
+		return -EINVAL;
+	else
+		return 0;
+}
-- 
2.48.1