selftests/bpf: Add tests for livepatch + bpf trampoline

Both livepatch and BPF trampoline use ftrace. Special attention is needed
when livepatch and fexit program touch the same function at the same
time, because livepatch updates a kernel function and the BPF trampoline
need to call into the right version of the kernel function.

Use samples/livepatch/livepatch-sample.ko for the test.

The test covers two cases:
  1) When a fentry program is loaded first. This exercises the
     modify_ftrace_direct code path.
  2) When a fentry program is loaded first. This exercises the
     register_ftrace_direct code path.

Signed-off-by: Song Liu <[email protected]>

Author: liu-song-6

tools/testing/selftests/bpf/config

@@ -50,6 +50,7 @@ CONFIG_IPV6_SIT=y
 CONFIG_IPV6_TUNNEL=y
 CONFIG_KEYS=y
 CONFIG_LIRC=y
+CONFIG_LIVEPATCH=y
 CONFIG_LWTUNNEL=y
 CONFIG_MODULE_SIG=y
 CONFIG_MODULE_SRCVERSION_ALL=y
@@ -111,6 +112,8 @@ CONFIG_IP6_NF_FILTER=y
 CONFIG_NF_NAT=y
 CONFIG_PACKET=y
 CONFIG_RC_CORE=y
+CONFIG_SAMPLES=y
+CONFIG_SAMPLE_LIVEPATCH=m
 CONFIG_SECURITY=y
 CONFIG_SECURITYFS=y
 CONFIG_SYN_COOKIES=y

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

@@ -0,0 +1,101 @@
+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (c) 2025 Meta Platforms, Inc. and affiliates. */
+
+#include <test_progs.h>
+#include "testing_helpers.h"
+#include "livepatch_trampoline.skel.h"
+
+static int load_livepatch(void)
+{
+	return load_module("../../../../samples/livepatch/livepatch-sample.ko", false);
+}
+
+static void unload_livepatch(void)
+{
+	/* Disable the livepatch before unloading the module */
+	system("echo 0 > /sys/kernel/livepatch/livepatch_sample/enabled");
+
+	unload_module("livepatch_sample", false);
+}
+
+static void read_proc_cmdline(void)
+{
+	char buf[4096];
+	int fd, ret;
+
+	fd = open("/proc/cmdline", O_RDONLY);
+	if (!ASSERT_OK_FD(fd, "open /proc/cmdline"))
+		return;
+
+	ret = read(fd, buf, sizeof(buf));
+	if (!ASSERT_GT(ret, 0, "read /proc/cmdline"))
+		goto out;
+
+	ASSERT_OK(strncmp(buf, "this has been live patched", 26), "strncmp");
+
+out:
+	close(fd);
+}
+
+static void __test_livepatch_trampoline(bool fexit_first)
+{
+	struct livepatch_trampoline *skel = NULL;
+	int err;
+
+	skel = livepatch_trampoline__open_and_load();
+	if (!ASSERT_OK_PTR(skel, "skel_open_and_load"))
+		goto out;
+
+	skel->bss->my_pid = getpid();
+
+	if (!fexit_first) {
+		/* fentry program is loaded first by default */
+		err = livepatch_trampoline__attach(skel);
+		if (!ASSERT_OK(err, "skel_attach"))
+			goto out;
+	} else {
+		/* Manually load fexit program first. */
+		skel->links.fexit_cmdline = bpf_program__attach(skel->progs.fexit_cmdline);
+		if (!ASSERT_OK_PTR(skel->links.fexit_cmdline, "attach_fexit"))
+			goto out;
+
+		skel->links.fentry_cmdline = bpf_program__attach(skel->progs.fentry_cmdline);
+		if (!ASSERT_OK_PTR(skel->links.fentry_cmdline, "attach_fentry"))
+			goto out;
+	}
+
+	read_proc_cmdline();
+
+	ASSERT_EQ(skel->bss->fentry_hit, 1, "fentry_hit");
+	ASSERT_EQ(skel->bss->fexit_hit, 1, "fexit_hit");
+out:
+	livepatch_trampoline__destroy(skel);
+}
+
+void test_livepatch_trampoline(void)
+{
+	int retry_cnt = 0;
+
+retry:
+	if (load_livepatch()) {
+		if (retry_cnt) {
+			ASSERT_OK(1, "load_livepatch");
+			goto out;
+		}
+		/*
+		 * Something else (previous run of the same test?) loaded
+		 * the KLP module. Unload the KLP module and retry.
+		 */
+		unload_livepatch();
+		retry_cnt++;
+		goto retry;
+	}
+
+	if (test__start_subtest("fentry_first"))
+		__test_livepatch_trampoline(false);
+
+	if (test__start_subtest("fexit_first"))
+		__test_livepatch_trampoline(true);
+out:
+	unload_livepatch();
+}

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

@@ -0,0 +1,30 @@
+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (c) 2025 Meta Platforms, Inc. and affiliates. */
+
+#include <linux/bpf.h>
+#include <bpf/bpf_helpers.h>
+#include <bpf/bpf_tracing.h>
+
+int fentry_hit;
+int fexit_hit;
+int my_pid;
+
+SEC("fentry/cmdline_proc_show")
+int BPF_PROG(fentry_cmdline)
+{
+	if (my_pid != (bpf_get_current_pid_tgid() >> 32))
+		return 0;
+
+	fentry_hit = 1;
+	return 0;
+}
+
+SEC("fexit/cmdline_proc_show")
+int BPF_PROG(fexit_cmdline)
+{
+	if (my_pid != (bpf_get_current_pid_tgid() >> 32))
+		return 0;
+
+	fexit_hit = 1;
+	return 0;
+}