selftests/bpf: Use bpf_num_possible_cpus() in per-cpu map allocations

bpf_map_value_size() uses num_possible_cpus() to determine map size, but
some of the tests only allocate enough memory for online cpus. This
results in out-of-bound writes in userspace during bpf(BPF_MAP_LOOKUP_ELEM)
syscalls in cases when number of online cpus is lower than the number of
possible cpus. Fix by switching from get_nprocs_conf() to
bpf_num_possible_cpus() when determining the number of processors in
these tests (test_progs/netcnt and test_cgroup_storage).

Signed-off-by: Artem Savkov <asavkov@redhat.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20220406085408.339336-1-asavkov@redhat.com

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

@@ -25,7 +25,7 @@ void serial_test_netcnt(void)
 	if (!ASSERT_OK_PTR(skel, "netcnt_prog__open_and_load"))
 		return;
 
-	nproc = get_nprocs_conf();
+	nproc = bpf_num_possible_cpus();
 	percpu_netcnt = malloc(sizeof(*percpu_netcnt) * nproc);
 	if (!ASSERT_OK_PTR(percpu_netcnt, "malloc(percpu_netcnt)"))
 		goto err;

tools/testing/selftests/bpf/test_cgroup_storage.c

@@ -7,6 +7,7 @@
 #include <sys/sysinfo.h>
 
 #include "bpf_rlimit.h"
+#include "bpf_util.h"
 #include "cgroup_helpers.h"
 #include "testing_helpers.h"
 
@@ -44,7 +45,7 @@ int main(int argc, char **argv)
 	unsigned long long *percpu_value;
 	int cpu, nproc;
 
-	nproc = get_nprocs_conf();
+	nproc = bpf_num_possible_cpus();
 	percpu_value = malloc(sizeof(*percpu_value) * nproc);
 	if (!percpu_value) {
 		printf("Not enough memory for per-cpu area (%d cpus)\n", nproc);