selftests/bpf: Add __ksym extern selftest

Validate libbpf is able to handle weak and strong kernel symbol externs in BPF
code correctly.
Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Reviewed-by: Hao Luo <haoluo@google.com>
Link: https://lore.kernel.org/bpf/20200619231703.738941-4-andriin@fb.com

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

+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (c) 2019 Facebook */
+
+#include <test_progs.h>
+#include "test_ksyms.skel.h"
+#include <sys/stat.h>
+
+static int duration;
+
+static __u64 kallsyms_find(const char *sym)
+{
+	char type, name[500];
+	__u64 addr, res = 0;
+	FILE *f;
+
+	f = fopen("/proc/kallsyms", "r");
+	if (CHECK(!f, "kallsyms_fopen", "failed to open: %d\n", errno))
+		return 0;
+
+	while (fscanf(f, "%llx %c %499s%*[^\n]\n", &addr, &type, name) > 0) {
+		if (strcmp(name, sym) == 0) {
+			res = addr;
+			goto out;
+		}
+	}
+
+	CHECK(false, "not_found", "symbol %s not found\n", sym);
+out:
+	fclose(f);
+	return res;
+}
+
+void test_ksyms(void)
+{
+	__u64 link_fops_addr = kallsyms_find("bpf_link_fops");
+	const char *btf_path = "/sys/kernel/btf/vmlinux";
+	struct test_ksyms *skel;
+	struct test_ksyms__data *data;
+	struct stat st;
+	__u64 btf_size;
+	int err;
+
+	if (CHECK(stat(btf_path, &st), "stat_btf", "err %d\n", errno))
+		return;
+	btf_size = st.st_size;
+
+	skel = test_ksyms__open_and_load();
+	if (CHECK(!skel, "skel_open", "failed to open and load skeleton\n"))
+		return;
+
+	err = test_ksyms__attach(skel);
+	if (CHECK(err, "skel_attach", "skeleton attach failed: %d\n", err))
+		goto cleanup;
+
+	/* trigger tracepoint */
+	usleep(1);
+
+	data = skel->data;
+	CHECK(data->out__bpf_link_fops != link_fops_addr, "bpf_link_fops",
+	      "got 0x%llx, exp 0x%llx\n",
+	      data->out__bpf_link_fops, link_fops_addr);
+	CHECK(data->out__bpf_link_fops1 != 0, "bpf_link_fops1",
+	      "got %llu, exp %llu\n", data->out__bpf_link_fops1, (__u64)0);
+	CHECK(data->out__btf_size != btf_size, "btf_size",
+	      "got %llu, exp %llu\n", data->out__btf_size, btf_size);
+	CHECK(data->out__per_cpu_start != 0, "__per_cpu_start",
+	      "got %llu, exp %llu\n", data->out__per_cpu_start, (__u64)0);
+
+cleanup:
+	test_ksyms__destroy(skel);
+}

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

+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (c) 2019 Facebook */
+
+#include <stdbool.h>
+#include <linux/bpf.h>
+#include <bpf/bpf_helpers.h>
+
+__u64 out__bpf_link_fops = -1;
+__u64 out__bpf_link_fops1 = -1;
+__u64 out__btf_size = -1;
+__u64 out__per_cpu_start = -1;
+
+extern const void bpf_link_fops __ksym;
+extern const void __start_BTF __ksym;
+extern const void __stop_BTF __ksym;
+extern const void __per_cpu_start __ksym;
+/* non-existing symbol, weak, default to zero */
+extern const void bpf_link_fops1 __ksym __weak;
+
+SEC("raw_tp/sys_enter")
+int handler(const void *ctx)
+{
+	out__bpf_link_fops = (__u64)&bpf_link_fops;
+	out__btf_size = (__u64)(&__stop_BTF - &__start_BTF);
+	out__per_cpu_start = (__u64)&__per_cpu_start;
+
+	out__bpf_link_fops1 = (__u64)&bpf_link_fops1;
+
+	return 0;
+}
+
+char _license[] SEC("license") = "GPL";