bpf/selftests: Test for bpf_per_cpu_ptr() and bpf_this_cpu_ptr()

Test bpf_per_cpu_ptr() and bpf_this_cpu_ptr(). Test two paths in the
kernel. If the base pointer points to a struct, the returned reg is
of type PTR_TO_BTF_ID. Direct pointer dereference can be applied on
the returned variable. If the base pointer isn't a struct, the
returned reg is of type PTR_TO_MEM, which also supports direct pointer
dereference.

Signed-off-by: Hao Luo <[e-mail]>
Signed-off-by: Alexei Starovoitov <[e-mail]>
Acked-by: Andrii Nakryiko <[e-mail]>
Link: https://lore.kernel.org/bpf/[e-mail]

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

···
 void test_ksyms_btf(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;
···
 	      "got %llu, exp %llu\n",
 	      (unsigned long long)data->out__bpf_prog_active_addr,
 	      (unsigned long long)bpf_prog_active_addr);
+
+	CHECK(data->out__rq_cpu == -1, "rq_cpu",
+	      "got %u, exp != -1\n", data->out__rq_cpu);
+	CHECK(data->out__bpf_prog_active < 0, "bpf_prog_active",
+	      "got %d, exp >= 0\n", data->out__bpf_prog_active);
+	CHECK(data->out__cpu_0_rq_cpu != 0, "cpu_rq(0)->cpu",
+	      "got %u, exp 0\n", data->out__cpu_0_rq_cpu);
+
+	this_rq_cpu = data->out__this_rq_cpu;
+	CHECK(this_rq_cpu != data->out__rq_cpu, "this_rq_cpu",
+	      "got %u, exp %u\n", this_rq_cpu, data->out__rq_cpu);
+
+	this_bpf_prog_active = data->out__this_bpf_prog_active;
+	CHECK(this_bpf_prog_active != data->out__bpf_prog_active, "this_bpf_prog_active",
+	      "got %d, exp %d\n", this_bpf_prog_active,
+	      data->out__bpf_prog_active);
 
 cleanup:
 	btf__free(btf);

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

···
 __u64 out__runqueues_addr = -1;
 __u64 out__bpf_prog_active_addr = -1;
 
+__u32 out__rq_cpu = -1; /* percpu struct fields */
+int out__bpf_prog_active = -1; /* percpu int */
+
+__u32 out__this_rq_cpu = -1;
+int out__this_bpf_prog_active = -1;
+
+__u32 out__cpu_0_rq_cpu = -1; /* cpu_rq(0)->cpu */
+
 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;
+
 	out__runqueues_addr = (__u64)&runqueues;
 	out__bpf_prog_active_addr = (__u64)&bpf_prog_active;
+
+	cpu = bpf_get_smp_processor_id();
+
+	/* test bpf_per_cpu_ptr() */
+	rq = (struct rq *)bpf_per_cpu_ptr(&runqueues, cpu);
+	if (rq)
+		out__rq_cpu = rq->cpu;
+	active = (int *)bpf_per_cpu_ptr(&bpf_prog_active, cpu);
+	if (active)
+		out__bpf_prog_active = *active;
+
+	rq = (struct rq *)bpf_per_cpu_ptr(&runqueues, 0);
+	if (rq) /* should always be valid, but we can't spare the check. */
+		out__cpu_0_rq_cpu = rq->cpu;
+
+	/* test bpf_this_cpu_ptr */
+	rq = (struct rq *)bpf_this_cpu_ptr(&runqueues);
+	out__this_rq_cpu = rq->cpu;
+	active = (int *)bpf_this_cpu_ptr(&bpf_prog_active);
+	out__this_bpf_prog_active = *active;
 
 	return 0;
 }