[PATCH] rust_binder: avoid calling pending_oneway_finished() on TF_UPDATE_TXN

[Alice Ryhl <aliceryhl@google.com>]

When an outdated transaction is removed from `oneway_todo` due to
`TF_UPDATE_TXN`, its `Allocation` is dropped. The current implementation
of `Allocation::drop` calls `pending_oneway_finished()`, assuming the
transaction was executed. This leads to premature execution of the next
queued one-way transaction.

Fix this by taking the `oneway_node` from the `Allocation` of the
outdated transaction before it is dropped. This prevents
`Allocation::drop` from signaling completion.

We do not call `take_oneway_node()` from `Transaction::cancel` because
it's actually correct to call `pending_oneway_finished()` on cancel if
the transaction did not come from `oneway_todo`. This ensures that if
`BINDER_THREAD_EXIT` is invoked and cancels a oneway transaction, then
the next transaction is taken from `oneway_todo`.

This bug does not lead to any issues in the kernel, but may lead to
Binder delivering transactions to userspace earlier than userspace
expected to receive them.

Cc: stable@vger.kernel.org
Fixes: eafedbc7c050 ("rust_binder: add Rust Binder driver")
Assisted-by: Antigravity:gemini
Signed-off-by: Alice Ryhl <aliceryhl@google.com>
---
 drivers/android/binder/allocation.rs  |  8 ++++++++
 drivers/android/binder/transaction.rs | 11 ++++++++++-
 2 files changed, 18 insertions(+), 1 deletion(-)

diff --git a/drivers/android/binder/allocation.rs b/drivers/android/binder/allocation.rs
index 0cab959e4b7e..b7b05e72970a 100644
--- a/drivers/android/binder/allocation.rs
+++ b/drivers/android/binder/allocation.rs
@@ -157,6 +157,14 @@ pub(crate) fn set_info_target_node(&mut self, target_node: NodeRef) {
         self.get_or_init_info().target_node = Some(target_node);
     }
 
+    pub(crate) fn take_oneway_node(&mut self) -> Option<DArc<Node>> {
+        if let Some(info) = self.allocation_info.as_mut() {
+            info.oneway_node.take()
+        } else {
+            None
+        }
+    }
+
     /// Reserve enough space to push at least `num_fds` fds.
     pub(crate) fn info_add_fd_reserve(&mut self, num_fds: usize) -> Result {
         self.get_or_init_info()
diff --git a/drivers/android/binder/transaction.rs b/drivers/android/binder/transaction.rs
index 47d5e4d88b07..1d9b66920a21 100644
--- a/drivers/android/binder/transaction.rs
+++ b/drivers/android/binder/transaction.rs
@@ -270,7 +270,8 @@ fn drop_outstanding_txn(&self) {
     /// Not used for replies.
     pub(crate) fn submit(self: DLArc<Self>, info: &mut TransactionInfo) -> BinderResult {
         // Defined before `process_inner` so that the destructor runs after releasing the lock.
-        let mut _t_outdated;
+        let _t_outdated;
+        let _oneway_node;
 
         let oneway = self.flags & TF_ONE_WAY != 0;
         let process = self.to.clone();
@@ -287,6 +288,14 @@ pub(crate) fn submit(self: DLArc<Self>, info: &mut TransactionInfo) -> BinderRes
                         if let Some(t_outdated) =
                             target_node.take_outdated_transaction(&self, &mut process_inner)
                         {
+                            let mut alloc_guard = t_outdated.allocation.lock();
+                            if let Some(alloc) = (*alloc_guard).as_mut() {
+                                // Take the oneway node to prevent `Allocation::drop` from calling
+                                // `pending_oneway_finished()`, which would be incorrect as this
+                                // transaction is not being submitted.
+                                _oneway_node = alloc.take_oneway_node();
+                            }
+                            drop(alloc_guard);
                             // Save the transaction to be dropped after locks are released.
                             _t_outdated = t_outdated;
                         }

---
base-commit: 0990a71f678aa0f045f2c126b39b6b581844d3b0
change-id: 20260414-tf-update-txn-fix-8cef91add11c

Best regards,
-- 
Alice Ryhl <aliceryhl@google.com>