binder: frozen notification

Frozen processes present a significant challenge in binder transactions.
When a process is frozen, it cannot, by design, accept and/or respond to
binder transactions. As a result, the sender needs to adjust its
behavior, such as postponing transactions until the peer process
unfreezes. However, there is currently no way to subscribe to these
state change events, making it impossible to implement frozen-aware
behaviors efficiently.

Introduce a binder API for subscribing to frozen state change events.
This allows programs to react to changes in peer process state,
mitigating issues related to binder transactions sent to frozen
processes.

Implementation details:
For a given binder_ref, the state of frozen notification can be one of
the followings:
1. Userspace doesn't want a notification. binder_ref->freeze is null.
2. Userspace wants a notification but none is in flight.
   list_empty(&binder_ref->freeze->work.entry) = true
3. A notification is in flight and waiting to be read by userspace.
   binder_ref_freeze.sent is false.
4. A notification was read by userspace and kernel is waiting for an ack.
   binder_ref_freeze.sent is true.

When a notification is in flight, new state change events are coalesced into
the existing binder_ref_freeze struct. If userspace hasn't picked up the
notification yet, the driver simply rewrites the state. Otherwise, the
notification is flagged as requiring a resend, which will be performed
once userspace acks the original notification that's inflight.

See https://r.android.com/3070045 for how userspace is going to use this
feature.

Signed-off-by: Yu-Ting Tseng <yutingtseng@google.com>
Acked-by: Carlos Llamas <cmllamas@google.com>
Link: https://lore.kernel.org/r/20240709070047.4055369-4-yutingtseng@google.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

1 files changed, 19 insertions, 2 deletions

diff --git a/drivers/android/binder_internal.h b/drivers/android/binder_internal.h
index 7d4fc53f7a732..f8d6be682f238 100644
--- a/drivers/android/binder_internal.h
+++ b/drivers/android/binder_internal.h
@@ -130,12 +130,13 @@ enum binder_stat_types {
 	BINDER_STAT_DEATH,
 	BINDER_STAT_TRANSACTION,
 	BINDER_STAT_TRANSACTION_COMPLETE,
+	BINDER_STAT_FREEZE,
 	BINDER_STAT_COUNT
 };
 
 struct binder_stats {
-	atomic_t br[_IOC_NR(BR_TRANSACTION_PENDING_FROZEN) + 1];
-	atomic_t bc[_IOC_NR(BC_REPLY_SG) + 1];
+	atomic_t br[_IOC_NR(BR_CLEAR_FREEZE_NOTIFICATION_DONE) + 1];
+	atomic_t bc[_IOC_NR(BC_FREEZE_NOTIFICATION_DONE) + 1];
 	atomic_t obj_created[BINDER_STAT_COUNT];
 	atomic_t obj_deleted[BINDER_STAT_COUNT];
 };
@@ -160,6 +161,8 @@ struct binder_work {
 		BINDER_WORK_DEAD_BINDER,
 		BINDER_WORK_DEAD_BINDER_AND_CLEAR,
 		BINDER_WORK_CLEAR_DEATH_NOTIFICATION,
+		BINDER_WORK_FROZEN_BINDER,
+		BINDER_WORK_CLEAR_FREEZE_NOTIFICATION,
 	} type;
 };
 
@@ -276,6 +279,14 @@ struct binder_ref_death {
 	binder_uintptr_t cookie;
 };
 
+struct binder_ref_freeze {
+	struct binder_work work;
+	binder_uintptr_t cookie;
+	bool is_frozen:1;
+	bool sent:1;
+	bool resend:1;
+};
+
 /**
  * struct binder_ref_data - binder_ref counts and id
  * @debug_id:        unique ID for the ref
@@ -308,6 +319,8 @@ struct binder_ref_data {
  *               @node indicates the node must be freed
  * @death:       pointer to death notification (ref_death) if requested
  *               (protected by @node->lock)
+ * @freeze:      pointer to freeze notification (ref_freeze) if requested
+ *               (protected by @node->lock)
  *
  * Structure to track references from procA to target node (on procB). This
  * structure is unsafe to access without holding @proc->outer_lock.
@@ -324,6 +337,7 @@ struct binder_ref {
 	struct binder_proc *proc;
 	struct binder_node *node;
 	struct binder_ref_death *death;
+	struct binder_ref_freeze *freeze;
 };
 
 /**
@@ -377,6 +391,8 @@ struct binder_ref {
  *                        (atomics, no lock needed)
  * @delivered_death:      list of delivered death notification
  *                        (protected by @inner_lock)
+ * @delivered_freeze:     list of delivered freeze notification
+ *                        (protected by @inner_lock)
  * @max_threads:          cap on number of binder threads
  *                        (protected by @inner_lock)
  * @requested_threads:    number of binder threads requested but not
@@ -424,6 +440,7 @@ struct binder_proc {
 	struct list_head todo;
 	struct binder_stats stats;
 	struct list_head delivered_death;
+	struct list_head delivered_freeze;
 	u32 max_threads;
 	int requested_threads;
 	int requested_threads_started;