bpf: Remove bpf_cpumask_kptr_get() kfunc

Now that struct bpf_cpumask is RCU safe, there's no need for this kfunc.
Rather than doing the following:

private(MASK) static struct bpf_cpumask __kptr *global;

int BPF_PROG(prog, s32 cpu, ...)
{
	struct bpf_cpumask *cpumask;

	bpf_rcu_read_lock();
	cpumask = bpf_cpumask_kptr_get(&global);
	if (!cpumask) {
		bpf_rcu_read_unlock();
		return -1;
	}
	bpf_cpumask_setall(cpumask);
	...
	bpf_cpumask_release(cpumask);
	bpf_rcu_read_unlock();
}

Programs can instead simply do (assume same global cpumask):

int BPF_PROG(prog, ...)
{
	struct bpf_cpumask *cpumask;

	bpf_rcu_read_lock();
	cpumask = global;
	if (!cpumask) {
		bpf_rcu_read_unlock();
		return -1;
	}
	bpf_cpumask_setall(cpumask);
	...
	bpf_rcu_read_unlock();
}

In other words, no extra atomic acquire / release, and less boilerplate
code.

This patch removes both the kfunc, as well as its selftests and
documentation.

Signed-off-by: David Vernet <void@manifault.com>
Link: https://lore.kernel.org/r/20230316054028.88924-5-void@manifault.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

kernel/bpf/cpumask.c

@@ -82,34 +82,6 @@ __bpf_kfunc struct bpf_cpumask *bpf_cpumask_acquire(struct bpf_cpumask *cpumask)
 	return cpumask;
 }
 
-/**
- * bpf_cpumask_kptr_get() - Attempt to acquire a reference to a BPF cpumask
- *			    stored in a map.
- * @cpumaskp: A pointer to a BPF cpumask map value.
- *
- * Attempts to acquire a reference to a BPF cpumask stored in a map value. The
- * cpumask returned by this function must either be embedded in a map as a
- * kptr, or freed with bpf_cpumask_release(). This function may return NULL if
- * no BPF cpumask was found in the specified map value.
- */
-__bpf_kfunc struct bpf_cpumask *bpf_cpumask_kptr_get(struct bpf_cpumask **cpumaskp)
-{
-	struct bpf_cpumask *cpumask;
-
-	/* The BPF memory allocator frees memory backing its caches in an RCU
-	 * callback. Thus, we can safely use RCU to ensure that the cpumask is
-	 * safe to read.
-	 */
-	rcu_read_lock();
-
-	cpumask = READ_ONCE(*cpumaskp);
-	if (cpumask && !refcount_inc_not_zero(&cpumask->usage))
-		cpumask = NULL;
-
-	rcu_read_unlock();
-	return cpumask;
-}
-
 static void cpumask_free_cb(struct rcu_head *head)
 {
 	struct bpf_cpumask *cpumask;
@@ -435,7 +407,6 @@ BTF_SET8_START(cpumask_kfunc_btf_ids)
 BTF_ID_FLAGS(func, bpf_cpumask_create, KF_ACQUIRE | KF_RET_NULL)
 BTF_ID_FLAGS(func, bpf_cpumask_release, KF_RELEASE | KF_TRUSTED_ARGS)
 BTF_ID_FLAGS(func, bpf_cpumask_acquire, KF_ACQUIRE | KF_TRUSTED_ARGS)
-BTF_ID_FLAGS(func, bpf_cpumask_kptr_get, KF_ACQUIRE | KF_KPTR_GET | KF_RET_NULL)
 BTF_ID_FLAGS(func, bpf_cpumask_first, KF_RCU)
 BTF_ID_FLAGS(func, bpf_cpumask_first_zero, KF_RCU)
 BTF_ID_FLAGS(func, bpf_cpumask_set_cpu, KF_RCU)

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

@@ -16,7 +16,6 @@ static const char * const cpumask_success_testcases[] = {
 	"test_copy_any_anyand",
 	"test_insert_leave",
 	"test_insert_remove_release",
-	"test_insert_kptr_get_release",
 	"test_global_mask_rcu",
 };
 

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

@@ -26,7 +26,6 @@ struct array_map {
 struct bpf_cpumask *bpf_cpumask_create(void) __ksym;
 void bpf_cpumask_release(struct bpf_cpumask *cpumask) __ksym;
 struct bpf_cpumask *bpf_cpumask_acquire(struct bpf_cpumask *cpumask) __ksym;
-struct bpf_cpumask *bpf_cpumask_kptr_get(struct bpf_cpumask **cpumask) __ksym;
 u32 bpf_cpumask_first(const struct cpumask *cpumask) __ksym;
 u32 bpf_cpumask_first_zero(const struct cpumask *cpumask) __ksym;
 void bpf_cpumask_set_cpu(u32 cpu, struct bpf_cpumask *cpumask) __ksym;

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

@@ -94,30 +94,6 @@ int BPF_PROG(test_insert_remove_no_release, struct task_struct *task, u64 clone_
 	return 0;
 }
 
-SEC("tp_btf/task_newtask")
-__failure __msg("Unreleased reference")
-int BPF_PROG(test_kptr_get_no_release, struct task_struct *task, u64 clone_flags)
-{
-	struct bpf_cpumask *cpumask;
-	struct __cpumask_map_value *v;
-
-	cpumask = create_cpumask();
-	if (!cpumask)
-		return 0;
-
-	if (cpumask_map_insert(cpumask))
-		return 0;
-
-	v = cpumask_map_value_lookup();
-	if (!v)
-		return 0;
-
-	cpumask = bpf_cpumask_kptr_get(&v->cpumask);
-
-	/* cpumask is never released. */
-	return 0;
-}
-
 SEC("tp_btf/task_newtask")
 __failure __msg("NULL pointer passed to trusted arg0")
 int BPF_PROG(test_cpumask_null, struct task_struct *task, u64 clone_flags)

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

@@ -394,36 +394,6 @@ int BPF_PROG(test_insert_remove_release, struct task_struct *task, u64 clone_fla
 	return 0;
 }
 
-SEC("tp_btf/task_newtask")
-int BPF_PROG(test_insert_kptr_get_release, struct task_struct *task, u64 clone_flags)
-{
-	struct bpf_cpumask *cpumask;
-	struct __cpumask_map_value *v;
-
-	cpumask = create_cpumask();
-	if (!cpumask)
-		return 0;
-
-	if (cpumask_map_insert(cpumask)) {
-		err = 3;
-		return 0;
-	}
-
-	v = cpumask_map_value_lookup();
-	if (!v) {
-		err = 4;
-		return 0;
-	}
-
-	cpumask = bpf_cpumask_kptr_get(&v->cpumask);
-	if (cpumask)
-		bpf_cpumask_release(cpumask);
-	else
-		err = 5;
-
-	return 0;
-}
-
 SEC("tp_btf/task_newtask")
 int BPF_PROG(test_global_mask_rcu, struct task_struct *task, u64 clone_flags)
 {