bpf/selftests: Test using global cpumask kptr with RCU

Now that struct bpf_cpumask * is considered an RCU-safe type according
to the verifier, we should add tests that validate its common usages.
This patch adds those tests to the cpumask test suite. A subsequent
changes will remove bpf_cpumask_kptr_get(), and will adjust the selftest
and BPF documentation accordingly.

Signed-off-by: David Vernet <void@manifault.com>
Link: https://lore.kernel.org/r/20230316054028.88924-4-void@manifault.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

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

@@ -17,6 +17,7 @@ static const char * const cpumask_success_testcases[] = {
 	"test_insert_leave",
 	"test_insert_remove_release",
 	"test_insert_kptr_get_release",
+	"test_global_mask_rcu",
 };
 
 static void verify_success(const char *prog_name)

tools/testing/selftests/bpf/progs/cpumask_common.h

@@ -9,6 +9,9 @@
 
 int err;
 
+#define private(name) SEC(".bss." #name) __hidden __attribute__((aligned(8)))
+private(MASK) static struct bpf_cpumask __kptr * global_mask;
+
 struct __cpumask_map_value {
 	struct bpf_cpumask __kptr * cpumask;
 };
@@ -51,6 +54,9 @@ void bpf_cpumask_copy(struct bpf_cpumask *dst, const struct cpumask *src) __ksym
 u32 bpf_cpumask_any(const struct cpumask *src) __ksym;
 u32 bpf_cpumask_any_and(const struct cpumask *src1, const struct cpumask *src2) __ksym;
 
+void bpf_rcu_read_lock(void) __ksym;
+void bpf_rcu_read_unlock(void) __ksym;
+
 static inline const struct cpumask *cast(struct bpf_cpumask *cpumask)
 {
 	return (const struct cpumask *)cpumask;

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

@@ -127,3 +127,65 @@ int BPF_PROG(test_cpumask_null, struct task_struct *task, u64 clone_flags)
 
 	return 0;
 }
+
+SEC("tp_btf/task_newtask")
+__failure __msg("R2 must be a rcu pointer")
+int BPF_PROG(test_global_mask_out_of_rcu, struct task_struct *task, u64 clone_flags)
+{
+	struct bpf_cpumask *local, *prev;
+
+	local = create_cpumask();
+	if (!local)
+		return 0;
+
+	prev = bpf_kptr_xchg(&global_mask, local);
+	if (prev) {
+		bpf_cpumask_release(prev);
+		err = 3;
+		return 0;
+	}
+
+	bpf_rcu_read_lock();
+	local = global_mask;
+	if (!local) {
+		err = 4;
+		bpf_rcu_read_unlock();
+		return 0;
+	}
+
+	bpf_rcu_read_unlock();
+
+	/* RCU region is exited before calling KF_RCU kfunc. */
+
+	bpf_cpumask_test_cpu(0, (const struct cpumask *)local);
+
+	return 0;
+}
+
+SEC("tp_btf/task_newtask")
+__failure __msg("NULL pointer passed to trusted arg1")
+int BPF_PROG(test_global_mask_no_null_check, struct task_struct *task, u64 clone_flags)
+{
+	struct bpf_cpumask *local, *prev;
+
+	local = create_cpumask();
+	if (!local)
+		return 0;
+
+	prev = bpf_kptr_xchg(&global_mask, local);
+	if (prev) {
+		bpf_cpumask_release(prev);
+		err = 3;
+		return 0;
+	}
+
+	bpf_rcu_read_lock();
+	local = global_mask;
+
+	/* No NULL check is performed on global cpumask kptr. */
+	bpf_cpumask_test_cpu(0, (const struct cpumask *)local);
+
+	bpf_rcu_read_unlock();
+
+	return 0;
+}

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

@@ -423,3 +423,36 @@ int BPF_PROG(test_insert_kptr_get_release, struct task_struct *task, u64 clone_f
 
 	return 0;
 }
+
+SEC("tp_btf/task_newtask")
+int BPF_PROG(test_global_mask_rcu, struct task_struct *task, u64 clone_flags)
+{
+	struct bpf_cpumask *local, *prev;
+
+	if (!is_test_task())
+		return 0;
+
+	local = create_cpumask();
+	if (!local)
+		return 0;
+
+	prev = bpf_kptr_xchg(&global_mask, local);
+	if (prev) {
+		bpf_cpumask_release(prev);
+		err = 3;
+		return 0;
+	}
+
+	bpf_rcu_read_lock();
+	local = global_mask;
+	if (!local) {
+		err = 4;
+		bpf_rcu_read_unlock();
+		return 0;
+	}
+
+	bpf_cpumask_test_cpu(0, (const struct cpumask *)local);
+	bpf_rcu_read_unlock();
+
+	return 0;
+}