rust_binder: Implement BINDER_DEBUG_USER_ERROR for refcounting and death notifications

This adds dynamic debug logs for:
- Decrementing handle reference counts that are already zero.
- Mismatched reference states (calling inc_ref_done with no active
  inc_refs, or using a weak reference as a strong reference).
- Requesting or clearing death notifications on invalid references,
  already active notifications, or with mismatched cookies.

Reviewed-by: Carlos Llamas <cmllamas@google.com>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Signed-off-by: Jahnavi MN <jahnavimn@google.com>
Link: https://patch.msgid.link/20260716-rust_binder_debug_mask-v4-3-3d7436c2d2f2@google.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/android/binder/node.rs

@@ -345,7 +345,7 @@ pub(crate) fn inc_ref_done_locked(
     ) -> Option<DLArc<Node>> {
         let inner = self.inner.access_mut(owner_inner);
         if inner.active_inc_refs == 0 {
-            pr_err!("inc_ref_done called when no active inc_refs");
+            binder_debug!(UserError, "inc_ref_done called when no active inc_refs");
             return None;
         }
 
@@ -821,6 +821,7 @@ pub(crate) fn get_count(&self) -> (usize, usize) {
 
     pub(crate) fn clone(&self, strong: bool) -> Result<NodeRef> {
         if strong && self.strong_count == 0 {
+            binder_debug!(UserError, "tried to use weak ref as strong ref");
             return Err(EINVAL);
         }
         Ok(self
@@ -861,9 +862,10 @@ pub(crate) fn update(&mut self, inc: bool, strong: bool) -> bool {
             *count += 1;
         } else {
             if *count == 0 {
-                pr_warn!(
-                    "pid {} performed invalid decrement on ref\n",
-                    kernel::current!().pid()
+                binder_debug!(
+                    UserError,
+                    "performed invalid {} decrement on ref",
+                    if strong { "strong" } else { "weak" }
                 );
                 return false;
             }

drivers/android/binder/process.rs

@@ -912,7 +912,13 @@ pub(crate) fn get_transaction_node(&self, handle: u32) -> BinderResult<NodeRef>
             }
             Ok(node_ref)
         } else {
-            Ok(self.get_node_from_handle(handle, true)?)
+            match self.get_node_from_handle(handle, true) {
+                Ok(node_ref) => Ok(node_ref),
+                Err(err) => {
+                    binder_debug!(UserError, "got transaction to invalid handle {handle}");
+                    Err(err.into())
+                }
+            }
         }
     }
 
@@ -997,7 +1003,7 @@ pub(crate) fn update_ref(
         } else {
             // All refs are cleared in process exit, so this warning is expected in that case.
             if !self.inner.lock().is_dead {
-                pr_warn!("{}: no such ref {handle}\n", self.pid_in_current_ns());
+                binder_debug!(UserError, "no such ref {handle}");
             }
         }
         Ok(())
@@ -1250,13 +1256,19 @@ pub(crate) fn request_death(
         })?;
         let mut refs = self.node_refs.lock();
         let Some(info) = refs.by_handle.get_mut(&handle) else {
-            pr_warn!("BC_REQUEST_DEATH_NOTIFICATION invalid ref {handle}\n");
+            binder_debug!(
+                UserError,
+                "BC_REQUEST_DEATH_NOTIFICATION invalid ref {handle}"
+            );
             return Ok(());
         };
 
         // Nothing to do if there is already a death notification request for this handle.
         if info.death().is_some() {
-            pr_warn!("BC_REQUEST_DEATH_NOTIFICATION death notification already set\n");
+            binder_debug!(
+                UserError,
+                "BC_REQUEST_DEATH_NOTIFICATION death notification already set"
+            );
             return Ok(());
         }
 
@@ -1293,17 +1305,26 @@ pub(crate) fn clear_death(&self, reader: &mut UserSliceReader, thread: &Thread)
 
         let mut refs = self.node_refs.lock();
         let Some(info) = refs.by_handle.get_mut(&handle) else {
-            pr_warn!("BC_CLEAR_DEATH_NOTIFICATION invalid ref {handle}\n");
+            binder_debug!(
+                UserError,
+                "BC_CLEAR_DEATH_NOTIFICATION invalid ref {handle}"
+            );
             return Ok(());
         };
 
         let Some(death) = info.death().take() else {
-            pr_warn!("BC_CLEAR_DEATH_NOTIFICATION death notification not active\n");
+            binder_debug!(
+                UserError,
+                "BC_CLEAR_DEATH_NOTIFICATION death notification not active"
+            );
             return Ok(());
         };
         if death.cookie != cookie {
             *info.death() = Some(death);
-            pr_warn!("BC_CLEAR_DEATH_NOTIFICATION death notification cookie mismatch\n");
+            binder_debug!(
+                UserError,
+                "BC_CLEAR_DEATH_NOTIFICATION death notification cookie mismatch"
+            );
             return Ok(());
         }