bpf: Remove now-defunct task kfuncs

In commit 22df776a9a ("tasks: Extract rcu_users out of union"), the
'refcount_t rcu_users' field was extracted out of a union with the
'struct rcu_head rcu' field. This allows us to safely perform a
refcount_inc_not_zero() on task->rcu_users when acquiring a reference on
a task struct. A prior patch leveraged this by making struct task_struct
an RCU-protected object in the verifier, and by bpf_task_acquire() to
use the task->rcu_users field for synchronization.

Now that we can use RCU to protect tasks, we no longer need
bpf_task_kptr_get(), or bpf_task_acquire_not_zero(). bpf_task_kptr_get()
is truly completely unnecessary, as we can just use RCU to get the
object. bpf_task_acquire_not_zero() is now equivalent to
bpf_task_acquire().

In addition to these changes, this patch also updates the associated
selftests to no longer use these kfuncs.

Signed-off-by: David Vernet <void@manifault.com>
Link: https://lore.kernel.org/r/20230331195733.699708-3-void@manifault.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

kernel/bpf/helpers.c

@@ -2019,73 +2019,6 @@ __bpf_kfunc struct task_struct *bpf_task_acquire(struct task_struct *p)
 	return NULL;
 }
 
-/**
- * bpf_task_acquire_not_zero - Acquire a reference to a rcu task object. A task
- * acquired by this kfunc which is not stored in a map as a kptr, must be
- * released by calling bpf_task_release().
- * @p: The task on which a reference is being acquired.
- */
-__bpf_kfunc struct task_struct *bpf_task_acquire_not_zero(struct task_struct *p)
-{
-	/* For the time being this function returns NULL, as it's not currently
-	 * possible to safely acquire a reference to a task with RCU protection
-	 * using get_task_struct() and put_task_struct(). This is due to the
-	 * slightly odd mechanics of p->rcu_users, and how task RCU protection
-	 * works.
-	 *
-	 * A struct task_struct is refcounted by two different refcount_t
-	 * fields:
-	 *
-	 * 1. p->usage:     The "true" refcount field which tracks a task's
-	 *		    lifetime. The task is freed as soon as this
-	 *		    refcount drops to 0.
-	 *
-	 * 2. p->rcu_users: An "RCU users" refcount field which is statically
-	 *		    initialized to 2, and is co-located in a union with
-	 *		    a struct rcu_head field (p->rcu). p->rcu_users
-	 *		    essentially encapsulates a single p->usage
-	 *		    refcount, and when p->rcu_users goes to 0, an RCU
-	 *		    callback is scheduled on the struct rcu_head which
-	 *		    decrements the p->usage refcount.
-	 *
-	 * There are two important implications to this task refcounting logic
-	 * described above. The first is that
-	 * refcount_inc_not_zero(&p->rcu_users) cannot be used anywhere, as
-	 * after the refcount goes to 0, the RCU callback being scheduled will
-	 * cause the memory backing the refcount to again be nonzero due to the
-	 * fields sharing a union. The other is that we can't rely on RCU to
-	 * guarantee that a task is valid in a BPF program. This is because a
-	 * task could have already transitioned to being in the TASK_DEAD
-	 * state, had its rcu_users refcount go to 0, and its rcu callback
-	 * invoked in which it drops its single p->usage reference. At this
-	 * point the task will be freed as soon as the last p->usage reference
-	 * goes to 0, without waiting for another RCU gp to elapse. The only
-	 * way that a BPF program can guarantee that a task is valid is in this
-	 * scenario is to hold a p->usage refcount itself.
-	 *
-	 * Until we're able to resolve this issue, either by pulling
-	 * p->rcu_users and p->rcu out of the union, or by getting rid of
-	 * p->usage and just using p->rcu_users for refcounting, we'll just
-	 * return NULL here.
-	 */
-	return NULL;
-}
-
-/**
- * bpf_task_kptr_get - Acquire a reference on a struct task_struct kptr. A task
- * kptr acquired by this kfunc which is not subsequently stored in a map, must
- * be released by calling bpf_task_release().
- * @pp: A pointer to a task kptr on which a reference is being acquired.
- */
-__bpf_kfunc struct task_struct *bpf_task_kptr_get(struct task_struct **pp)
-{
-	/* We must return NULL here until we have clarity on how to properly
-	 * leverage RCU for ensuring a task's lifetime. See the comment above
-	 * in bpf_task_acquire_not_zero() for more details.
-	 */
-	return NULL;
-}
-
 /**
  * bpf_task_release - Release the reference acquired on a task.
  * @p: The task on which a reference is being released.
@@ -2375,8 +2308,6 @@ BTF_ID_FLAGS(func, bpf_list_push_back)
 BTF_ID_FLAGS(func, bpf_list_pop_front, KF_ACQUIRE | KF_RET_NULL)
 BTF_ID_FLAGS(func, bpf_list_pop_back, KF_ACQUIRE | KF_RET_NULL)
 BTF_ID_FLAGS(func, bpf_task_acquire, KF_ACQUIRE | KF_RCU | KF_RET_NULL)
-BTF_ID_FLAGS(func, bpf_task_acquire_not_zero, KF_ACQUIRE | KF_RCU | KF_RET_NULL)
-BTF_ID_FLAGS(func, bpf_task_kptr_get, KF_ACQUIRE | KF_KPTR_GET | KF_RET_NULL)
 BTF_ID_FLAGS(func, bpf_task_release, KF_RELEASE)
 BTF_ID_FLAGS(func, bpf_rbtree_remove, KF_ACQUIRE)
 BTF_ID_FLAGS(func, bpf_rbtree_add)

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

@@ -73,7 +73,7 @@ static const char * const success_tests[] = {
 	"test_task_acquire_release_current",
 	"test_task_acquire_leave_in_map",
 	"test_task_xchg_release",
-	"test_task_get_release",
+	"test_task_map_acquire_release",
 	"test_task_current_acquire_release",
 	"test_task_from_pid_arg",
 	"test_task_from_pid_current",

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

@@ -23,7 +23,7 @@ struct bpf_key *bpf_lookup_user_key(__u32 serial, __u64 flags) __ksym;
 void bpf_key_put(struct bpf_key *key) __ksym;
 void bpf_rcu_read_lock(void) __ksym;
 void bpf_rcu_read_unlock(void) __ksym;
-struct task_struct *bpf_task_acquire_not_zero(struct task_struct *p) __ksym;
+struct task_struct *bpf_task_acquire(struct task_struct *p) __ksym;
 void bpf_task_release(struct task_struct *p) __ksym;
 
 SEC("?fentry.s/" SYS_PREFIX "sys_getpgid")
@@ -159,13 +159,8 @@ int task_acquire(void *ctx)
 		goto out;
 
 	/* acquire a reference which can be used outside rcu read lock region */
-	gparent = bpf_task_acquire_not_zero(gparent);
+	gparent = bpf_task_acquire(gparent);
 	if (!gparent)
-		/* Until we resolve the issues with using task->rcu_users, we
-		 * expect bpf_task_acquire_not_zero() to return a NULL task.
-		 * See the comment at the definition of
-		 * bpf_task_acquire_not_zero() for more details.
-		 */
 		goto out;
 
 	(void)bpf_task_storage_get(&map_a, gparent, 0, 0);

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

@@ -21,7 +21,6 @@ struct hash_map {
 } __tasks_kfunc_map SEC(".maps");
 
 struct task_struct *bpf_task_acquire(struct task_struct *p) __ksym;
-struct task_struct *bpf_task_kptr_get(struct task_struct **pp) __ksym;
 void bpf_task_release(struct task_struct *p) __ksym;
 struct task_struct *bpf_task_from_pid(s32 pid) __ksym;
 void bpf_rcu_read_lock(void) __ksym;

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

@@ -128,59 +128,6 @@ int BPF_PROG(task_kfunc_acquire_unreleased, struct task_struct *task, u64 clone_
 	return 0;
 }
 
-SEC("tp_btf/task_newtask")
-__failure __msg("arg#0 expected pointer to map value")
-int BPF_PROG(task_kfunc_get_non_kptr_param, struct task_struct *task, u64 clone_flags)
-{
-	struct task_struct *kptr;
-
-	/* Cannot use bpf_task_kptr_get() on a non-kptr, even on a valid task. */
-	kptr = bpf_task_kptr_get(&task);
-	if (!kptr)
-		return 0;
-
-	bpf_task_release(kptr);
-
-	return 0;
-}
-
-SEC("tp_btf/task_newtask")
-__failure __msg("arg#0 expected pointer to map value")
-int BPF_PROG(task_kfunc_get_non_kptr_acquired, struct task_struct *task, u64 clone_flags)
-{
-	struct task_struct *kptr, *acquired;
-
-	acquired = bpf_task_acquire(task);
-	if (!acquired)
-		return 0;
-
-	/* Cannot use bpf_task_kptr_get() on a non-kptr, even if it was acquired. */
-	kptr = bpf_task_kptr_get(&acquired);
-	bpf_task_release(acquired);
-	if (!kptr)
-		return 0;
-
-	bpf_task_release(kptr);
-
-	return 0;
-}
-
-SEC("tp_btf/task_newtask")
-__failure __msg("arg#0 expected pointer to map value")
-int BPF_PROG(task_kfunc_get_null, struct task_struct *task, u64 clone_flags)
-{
-	struct task_struct *kptr;
-
-	/* Cannot use bpf_task_kptr_get() on a NULL pointer. */
-	kptr = bpf_task_kptr_get(NULL);
-	if (!kptr)
-		return 0;
-
-	bpf_task_release(kptr);
-
-	return 0;
-}
-
 SEC("tp_btf/task_newtask")
 __failure __msg("Unreleased reference")
 int BPF_PROG(task_kfunc_xchg_unreleased, struct task_struct *task, u64 clone_flags)
@@ -214,26 +161,6 @@ int BPF_PROG(task_kfunc_acquire_release_no_null_check, struct task_struct *task,
 	return 0;
 }
 
-SEC("tp_btf/task_newtask")
-__failure __msg("Unreleased reference")
-int BPF_PROG(task_kfunc_get_unreleased, struct task_struct *task, u64 clone_flags)
-{
-	struct task_struct *kptr;
-	struct __tasks_kfunc_map_value *v;
-
-	v = insert_lookup_task(task);
-	if (!v)
-		return 0;
-
-	kptr = bpf_task_kptr_get(&v->task);
-	if (!kptr)
-		return 0;
-
-	/* Kptr acquired above is never released. */
-
-	return 0;
-}
-
 SEC("tp_btf/task_newtask")
 __failure __msg("Possibly NULL pointer passed to trusted arg0")
 int BPF_PROG(task_kfunc_release_untrusted, struct task_struct *task, u64 clone_flags)

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

@@ -122,7 +122,7 @@ int BPF_PROG(test_task_xchg_release, struct task_struct *task, u64 clone_flags)
 }
 
 SEC("tp_btf/task_newtask")
-int BPF_PROG(test_task_get_release, struct task_struct *task, u64 clone_flags)
+int BPF_PROG(test_task_map_acquire_release, struct task_struct *task, u64 clone_flags)
 {
 	struct task_struct *kptr;
 	struct __tasks_kfunc_map_value *v;
@@ -143,18 +143,18 @@ int BPF_PROG(test_task_get_release, struct task_struct *task, u64 clone_flags)
 		return 0;
 	}
 
-	kptr = bpf_task_kptr_get(&v->task);
-	if (kptr) {
-		/* Until we resolve the issues with using task->rcu_users, we
-		 * expect bpf_task_kptr_get() to return a NULL task. See the
-		 * comment at the definition of bpf_task_acquire_not_zero() for
-		 * more details.
-		 */
-		bpf_task_release(kptr);
+	bpf_rcu_read_lock();
+	kptr = v->task;
+	if (!kptr) {
 		err = 3;
-		return 0;
+	} else {
+		kptr = bpf_task_acquire(kptr);
+		if (!kptr)
+			err = 4;
+		else
+			bpf_task_release(kptr);
 	}
-
+	bpf_rcu_read_unlock();
 
 	return 0;
 }