bpf: selftest: Ensure the return value of the bpf_per_cpu_ptr() must be checked

This patch tests all pointers returned by bpf_per_cpu_ptr() must be
tested for NULL first before it can be accessed.

This patch adds a subtest "null_check", so it moves the ".data..percpu"
existence check to the very beginning and before doing any subtest.

Signed-off-by: Martin KaFai Lau <[e-mail]>
Signed-off-by: Alexei Starovoitov <[e-mail]>
Link: https://lore.kernel.org/bpf/[e-mail]

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

···
 #include <bpf/libbpf.h>
 #include <bpf/btf.h>
 #include "test_ksyms_btf.skel.h"
+#include "test_ksyms_btf_null_check.skel.h"
 
 static int duration;
 
-void test_ksyms_btf(void)
+static void test_basic(void)
 {
 	__u64 runqueues_addr, bpf_prog_active_addr;
 	__u32 this_rq_cpu;
 	int this_bpf_prog_active;
 	struct test_ksyms_btf *skel = NULL;
 	struct test_ksyms_btf__data *data;
-	struct btf *btf;
-	int percpu_datasec;
 	int err;
 
 	err = kallsyms_find("runqueues", &runqueues_addr);
···
 		return;
 	if (CHECK(err == -ENOENT, "ksym_find", "symbol 'bpf_prog_active' not found\n"))
 		return;
-
-	btf = libbpf_find_kernel_btf();
-	if (CHECK(IS_ERR(btf), "btf_exists", "failed to load kernel BTF: %ld\n",
-		  PTR_ERR(btf)))
-		return;
-
-	percpu_datasec = btf__find_by_name_kind(btf, ".data..percpu",
-						BTF_KIND_DATASEC);
-	if (percpu_datasec < 0) {
-		printf("%s:SKIP:no PERCPU DATASEC in kernel btf\n",
-		       __func__);
-		test__skip();
-		goto cleanup;
-	}
 
 	skel = test_ksyms_btf__open_and_load();
 	if (CHECK(!skel, "skel_open", "failed to open and load skeleton\n"))
···
 	      data->out__bpf_prog_active);
 
 cleanup:
-	btf__free(btf);
 	test_ksyms_btf__destroy(skel);
+}
+
+static void test_null_check(void)
+{
+	struct test_ksyms_btf_null_check *skel;
+
+	skel = test_ksyms_btf_null_check__open_and_load();
+	CHECK(skel, "skel_open", "unexpected load of a prog missing null check\n");
+
+	test_ksyms_btf_null_check__destroy(skel);
+}
+
+void test_ksyms_btf(void)
+{
+	int percpu_datasec;
+	struct btf *btf;
+
+	btf = libbpf_find_kernel_btf();
+	if (CHECK(IS_ERR(btf), "btf_exists", "failed to load kernel BTF: %ld\n",
+		  PTR_ERR(btf)))
+		return;
+
+	percpu_datasec = btf__find_by_name_kind(btf, ".data..percpu",
+						BTF_KIND_DATASEC);
+	btf__free(btf);
+	if (percpu_datasec < 0) {
+		printf("%s:SKIP:no PERCPU DATASEC in kernel btf\n",
+		       __func__);
+		test__skip();
+		return;
+	}
+
+	if (test__start_subtest("basic"))
+		test_basic();
+
+	if (test__start_subtest("null_check"))
+		test_null_check();
 }

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

···
+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (c) 2020 Facebook */
+
+#include "vmlinux.h"
+
+#include <bpf/bpf_helpers.h>
+
+extern const struct rq runqueues __ksym; /* struct type global var. */
+extern const int bpf_prog_active __ksym; /* int type global var. */
+
+SEC("raw_tp/sys_enter")
+int handler(const void *ctx)
+{
+	struct rq *rq;
+	int *active;
+	__u32 cpu;
+
+	cpu = bpf_get_smp_processor_id();
+	rq = (struct rq *)bpf_per_cpu_ptr(&runqueues, cpu);
+	active = (int *)bpf_per_cpu_ptr(&bpf_prog_active, cpu);
+	if (active) {
+		/* READ_ONCE */
+		*(volatile int *)active;
+		/* !rq has not been tested, so verifier should reject. */
+		*(volatile int *)(&rq->cpu);
+	}
+
+	return 0;
+}
+
+char _license[] SEC("license") = "GPL";