[PATCH 2/3] rust_binder: track caller location in BinderError

[Alice Ryhl <aliceryhl@google.com>]

Record the caller's location in BinderError using #[track_caller] and a
new ErrorLocation wrapper around &'static Location<'static> across all
BinderError constructors and From implementations. ErrorLocation
implements Display by stripping the directory prefix so locations are
formatted as filename.rs:line (e.g. process.rs:422).

In Context::get_manager_node and Process::buffer_alloc, avoid passing
BinderError::new_dead() or BinderError::from() as function pointers to
Option::ok_or_else() and Result::map_err() so that #[track_caller]
captures the call site in rust_binder correctly.

Include the error location in the FailedTransaction debug print and
prepare for recording it in the binder transaction log. Example output:

  rust_binder: 840:4583 transaction async to 7656:0 failed ESRCH, code 1 size 428-16 line thread.rs:711

Assisted-by: LLM
Signed-off-by: Alice Ryhl <aliceryhl@google.com>
---
 drivers/android/binder/context.rs |  8 ++++----
 drivers/android/binder/error.rs   | 39 +++++++++++++++++++++++++++++++++++++++
 drivers/android/binder/process.rs |  5 ++++-
 drivers/android/binder/thread.rs  |  5 +++--
 4 files changed, 50 insertions(+), 7 deletions(-)

diff --git a/drivers/android/binder/context.rs b/drivers/android/binder/context.rs
index ddddb66b3557..bcc97ddaacd7 100644
--- a/drivers/android/binder/context.rs
+++ b/drivers/android/binder/context.rs
@@ -134,13 +134,13 @@ pub(crate) fn unset_manager_node(&self) {
     }
 
     pub(crate) fn get_manager_node(&self, strong: bool) -> Result<NodeRef, BinderError> {
-        self.manager
+        Ok(self
+            .manager
             .lock()
             .node
             .as_ref()
-            .ok_or_else(BinderError::new_dead)?
-            .clone(strong)
-            .map_err(BinderError::from)
+            .ok_or_else(|| BinderError::new_dead())?
+            .clone(strong)?)
     }
 
     pub(crate) fn for_each_proc<F>(&self, mut func: F)
diff --git a/drivers/android/binder/error.rs b/drivers/android/binder/error.rs
index 1296072c35d9..41b07d687b30 100644
--- a/drivers/android/binder/error.rs
+++ b/drivers/android/binder/error.rs
@@ -2,6 +2,7 @@
 
 // Copyright (C) 2025 Google LLC.
 
+use core::panic::Location;
 use kernel::fmt;
 use kernel::prelude::*;
 
@@ -9,32 +10,65 @@
 
 pub(crate) type BinderResult<T = ()> = core::result::Result<T, BinderError>;
 
+/// Wraps a `&'static Location` to format it as `filename.rs:line` without the directory prefix.
+#[derive(Copy, Clone)]
+#[repr(transparent)]
+pub(crate) struct ErrorLocation(&'static Location<'static>);
+
+// SAFETY: `ErrorLocation` is `repr(transparent)` over a shared reference, which is part of the
+// option layout optimization guarantee, so all zeroes is a valid representation for `None`.
+unsafe impl pin_init::ZeroableOption for ErrorLocation {}
+
+impl ErrorLocation {
+    #[track_caller]
+    pub(crate) const fn caller() -> Self {
+        Self(Location::caller())
+    }
+}
+
+impl fmt::Display for ErrorLocation {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        let file = match self.0.file().rsplit_once('/') {
+            Some((_, file)) => file,
+            None => self.0.file(),
+        };
+        write!(f, "{}:{}", file, self.0.line())
+    }
+}
+
 /// An error that will be returned to userspace via the `BINDER_WRITE_READ` ioctl rather than via
 /// errno.
 pub(crate) struct BinderError {
     pub(crate) reply: u32,
     pub(crate) source: Option<Error>,
+    pub(crate) line: ErrorLocation,
 }
 
 impl BinderError {
+    #[track_caller]
     pub(crate) fn new_dead() -> Self {
         Self {
             reply: BR_DEAD_REPLY,
             source: None,
+            line: ErrorLocation::caller(),
         }
     }
 
+    #[track_caller]
     pub(crate) fn new_frozen() -> Self {
         Self {
             reply: BR_FROZEN_REPLY,
             source: None,
+            line: ErrorLocation::caller(),
         }
     }
 
+    #[track_caller]
     pub(crate) fn new_frozen_oneway() -> Self {
         Self {
             reply: BR_TRANSACTION_PENDING_FROZEN,
             source: None,
+            line: ErrorLocation::caller(),
         }
     }
 
@@ -46,25 +80,30 @@ pub(crate) fn is_dead(&self) -> bool {
 /// Convert an errno into a `BinderError` and store the errno used to construct it. The errno
 /// should be stored as the thread's extended error when given to userspace.
 impl From<Error> for BinderError {
+    #[track_caller]
     fn from(source: Error) -> Self {
         Self {
             reply: BR_FAILED_REPLY,
             source: Some(source),
+            line: ErrorLocation::caller(),
         }
     }
 }
 
 impl From<kernel::fs::file::BadFdError> for BinderError {
+    #[track_caller]
     fn from(source: kernel::fs::file::BadFdError) -> Self {
         BinderError::from(Error::from(source))
     }
 }
 
 impl From<kernel::alloc::AllocError> for BinderError {
+    #[track_caller]
     fn from(_: kernel::alloc::AllocError) -> Self {
         Self {
             reply: BR_FAILED_REPLY,
             source: Some(ENOMEM),
+            line: ErrorLocation::caller(),
         }
     }
 }
diff --git a/drivers/android/binder/process.rs b/drivers/android/binder/process.rs
index 5372bfbd93b3..dcf6ba4bc71b 100644
--- a/drivers/android/binder/process.rs
+++ b/drivers/android/binder/process.rs
@@ -1066,7 +1066,10 @@ pub(crate) fn buffer_alloc(
 
         let (new_alloc, addr) = loop {
             let mut inner = self.inner.lock();
-            let mapping = inner.mapping.as_mut().ok_or_else(BinderError::new_dead)?;
+            let mapping = inner
+                .mapping
+                .as_mut()
+                .ok_or_else(|| BinderError::new_dead())?;
             let alloc_request = match mapping.alloc.reserve_new(reserve_new_args)? {
                 ReserveNew::Success(new_alloc) => break (new_alloc, mapping.address),
                 ReserveNew::NeedAlloc(request) => request,
diff --git a/drivers/android/binder/thread.rs b/drivers/android/binder/thread.rs
index 18a14aa8a835..0891a36e9cbd 100644
--- a/drivers/android/binder/thread.rs
+++ b/drivers/android/binder/thread.rs
@@ -1310,7 +1310,7 @@ fn transaction(self: &Arc<Self>, cmd: u32, reader: &mut UserSliceReader) -> Resu
 
                     binder_debug!(
                         FailedTransaction,
-                        "transaction {} to {}:{} failed {:?}, code {} size {}-{}",
+                        "transaction {} to {}:{} failed {:?}, code {} size {}-{} line {}",
                         if info.is_reply {
                             "reply"
                         } else if info.is_oneway() {
@@ -1323,7 +1323,8 @@ fn transaction(self: &Arc<Self>, cmd: u32, reader: &mut UserSliceReader) -> Resu
                         err,
                         info.code,
                         info.data_size,
-                        info.offsets_size
+                        info.offsets_size,
+                        err.line
                     );
                 }
             }

-- 
2.56.0.rc1.315.gc6ed9934b7-goog