selftests/bpf: Test kernel module ksym externs

Add per-CPU variable to bpf_testmod.ko and use those from new selftest to
validate it works end-to-end.
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Yonghong Song <yhs@fb.com>
Acked-by: Hao Luo <haoluo@google.com>
Link: https://lore.kernel.org/bpf/20210112075520.4103414-8-andrii@kernel.org

tools/testing/selftests/bpf/bpf_testmod/bpf_testmod.c

@@ -3,6 +3,7 @@
 #include <linux/error-injection.h>
 #include <linux/init.h>
 #include <linux/module.h>
+#include <linux/percpu-defs.h>
 #include <linux/sysfs.h>
 #include <linux/tracepoint.h>
 #include "bpf_testmod.h"
@@ -10,6 +11,8 @@
 #define CREATE_TRACE_POINTS
 #include "bpf_testmod-events.h"
 
+DEFINE_PER_CPU(int, bpf_testmod_ksym_percpu) = 123;
+
 noinline ssize_t
 bpf_testmod_test_read(struct file *file, struct kobject *kobj,
 		      struct bin_attribute *bin_attr,

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

+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (c) 2021 Facebook */
+
+#include <test_progs.h>
+#include <bpf/libbpf.h>
+#include <bpf/btf.h>
+#include "test_ksyms_module.skel.h"
+
+static int duration;
+
+void test_ksyms_module(void)
+{
+	struct test_ksyms_module* skel;
+	int err;
+
+	skel = test_ksyms_module__open_and_load();
+	if (CHECK(!skel, "skel_open", "failed to open skeleton\n"))
+		return;
+
+	err = test_ksyms_module__attach(skel);
+	if (CHECK(err, "skel_attach", "skeleton attach failed: %d\n", err))
+		goto cleanup;
+
+	usleep(1);
+
+	ASSERT_EQ(skel->bss->triggered, true, "triggered");
+	ASSERT_EQ(skel->bss->out_mod_ksym_global, 123, "global_ksym_val");
+
+cleanup:
+	test_ksyms_module__destroy(skel);
+}

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

+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (c) 2021 Facebook */
+
+#include "vmlinux.h"
+
+#include <bpf/bpf_helpers.h>
+
+extern const int bpf_testmod_ksym_percpu __ksym;
+
+int out_mod_ksym_global = 0;
+bool triggered = false;
+
+SEC("raw_tp/sys_enter")
+int handler(const void *ctx)
+{
+	int *val;
+	__u32 cpu;
+
+	val = (int *)bpf_this_cpu_ptr(&bpf_testmod_ksym_percpu);
+	out_mod_ksym_global = *val;
+	triggered = true;
+
+	return 0;
+}
+
+char LICENSE[] SEC("license") = "GPL";