[PATCH 0/3] rust_binder: error line numbers and failed_transaction_log

[PATCH 0/3] rust_binder: error line numbers and failed_transaction_log

[Alice Ryhl]

When diagnosing Binder issues, it's useful to know what the most recent
failed transactionas are, so include a transaction log with this content
matching the C Binder transaction log.

While we're at it, improve the line number information emitted by Rust
Binder using #[track_caller].

Signed-off-by: Alice Ryhl <aliceryhl@google.com>
---
Alice Ryhl (3):
      rust_binder: start debug_id at 1
      rust_binder: track caller location in BinderError
      rust_binder: add transaction_log and failed_transaction_log

 drivers/android/binder/context.rs             |   8 +-
 drivers/android/binder/error.rs               |  39 ++++++
 drivers/android/binder/process.rs             |   5 +-
 drivers/android/binder/rust_binder_internal.h |   1 +
 drivers/android/binder/rust_binder_main.rs    |  26 +++-
 drivers/android/binder/rust_binderfs.c        |  19 +++
 drivers/android/binder/thread.rs              |  12 +-
 drivers/android/binder/transaction.rs         | 193 +++++++++++++++++++++++++-
 8 files changed, 293 insertions(+), 10 deletions(-)
---
base-commit: 72d3fcf802c45d00b300f25b848a93c3a2bd7c7e
change-id: 20260930-binder-transaction-log-94989f47b142

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

[PATCH 1/3] rust_binder: start debug_id at 1

[Alice Ryhl]

In the C Binder driver, debug IDs are allocated with
atomic_inc_return(&binder_last_id), so valid debug IDs start at 1 and 0
is never assigned.

Start NEXT_DEBUG_ID at 1 in Rust Binder as well so that 0 can be used as
a sentinel for an absent or uninitialized debug_id (such as in
TransactionInfo::to_node_debug_id and in transaction log entries).

Assisted-by: LLM
Signed-off-by: Alice Ryhl <aliceryhl@google.com>
---
 drivers/android/binder/rust_binder_main.rs | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/android/binder/rust_binder_main.rs b/drivers/android/binder/rust_binder_main.rs
index 955c4c348f73..e360df991d51 100644
--- a/drivers/android/binder/rust_binder_main.rs
+++ b/drivers/android/binder/rust_binder_main.rs
@@ -96,7 +96,7 @@ fn default() -> Self {
 };
 
 fn next_debug_id() -> usize {
-    static NEXT_DEBUG_ID: Atomic<usize> = Atomic::new(0);
+    static NEXT_DEBUG_ID: Atomic<usize> = Atomic::new(1);
 
     NEXT_DEBUG_ID.fetch_add(1, Relaxed)
 }

-- 
2.56.0.rc1.315.gc6ed9934b7-goog

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

[Alice Ryhl]

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

[PATCH 3/3] rust_binder: add transaction_log and failed_transaction_log

[Alice Ryhl]

Implement the binder_logs/transaction_log and
binder_logs/failed_transaction_log binderfs files in Rust Binder,
matching the circular 32-entry transaction logs in C Binder.

Example output from /dev/binderfs/binder_logs/transaction_log:

  261251: call  from 6157:6171 to 391:0 context binder node 170 handle 34 size 256:0 ret 0/0 l=0
  261254: async from 606:783 to 669:0 context binder node 2048 handle 2 size 144:0 ret 0/0 l=0
  261255: reply from 391:552 to 6157:6171 context binder node 0 handle -1 size 2436:8 ret 0/0 l=0

Example output from /dev/binderfs/binder_logs/failed_transaction_log:

  194556: async from 6646:6853 to 7452:0 context binder node 177682 handle 450 size 160:0 ret 29189/0 l=process.rs:1081
  194846: reply from 6646:6775 to 7452:7544 context binder node 0 handle -1 size 8:0 ret 29201/0 l=process.rs:1081
  201573: call  from 7771:7793 to 6646:0 context binder node 198883 handle 28 size 0:0 ret 29189/0 l=process.rs:1081

Unlike C Binder, which writes to log entries without a lock using memory
barriers (smp_wmb/smp_rmb) and a debug_id_done field, each
TransactionLog uses an atomic index cursor with 32 cacheline-aligned
SpinLock<TransactionLogEntry> slots initialized in BinderModule::init
via SetOnce<_>. Using a SpinLock per slot avoids data races (such as
tearing when the ring buffer wraps around while an entry is being
printed) without requiring atomic accesses for every field, and has very
low risk of contention: concurrent transactions are spread across 32
separate cacheline-aligned locks, each lock is only held briefly for a
fixed-size copy, and two writers only contend on a slot if 32
transactions occur before a single copy completes.

Assisted-by: LLM
Signed-off-by: Alice Ryhl <aliceryhl@google.com>
---
 drivers/android/binder/rust_binder_internal.h |   1 +
 drivers/android/binder/rust_binder_main.rs    |  24 ++++
 drivers/android/binder/rust_binderfs.c        |  19 +++
 drivers/android/binder/thread.rs              |   7 +
 drivers/android/binder/transaction.rs         | 193 +++++++++++++++++++++++++-
 5 files changed, 242 insertions(+), 2 deletions(-)

diff --git a/drivers/android/binder/rust_binder_internal.h b/drivers/android/binder/rust_binder_internal.h
index 78288fe7964d..50a50df14c77 100644
--- a/drivers/android/binder/rust_binder_internal.h
+++ b/drivers/android/binder/rust_binder_internal.h
@@ -44,6 +44,7 @@ struct binder_device {
 int rust_binder_stats_show(struct seq_file *m, void *unused);
 int rust_binder_state_show(struct seq_file *m, void *unused);
 int rust_binder_transactions_show(struct seq_file *m, void *unused);
+int rust_binder_transaction_log_show(struct seq_file *m, void *unused);
 int rust_binder_proc_show(struct seq_file *m, void *pid);
 
 extern const struct file_operations rust_binder_fops;
diff --git a/drivers/android/binder/rust_binder_main.rs b/drivers/android/binder/rust_binder_main.rs
index e360df991d51..32f189fed0aa 100644
--- a/drivers/android/binder/rust_binder_main.rs
+++ b/drivers/android/binder/rust_binder_main.rs
@@ -305,6 +305,9 @@ fn init(_module: &'static kernel::ThisModule) -> Result<Self> {
         // SAFETY: The module initializer never runs twice, so we only call this once.
         unsafe { crate::context::CONTEXTS.init() };
 
+        crate::transaction::TRANSACTION_LOG.init()?;
+        crate::transaction::FAILED_TRANSACTION_LOG.init()?;
+
         let netlink = crate::netlink::BINDER_NL_FAMILY.register()?;
         BINDER_SHRINKER.register(c"android-binder")?;
 
@@ -545,6 +548,27 @@ unsafe impl<T> Sync for AssertSync<T> {}
     0
 }
 
+/// # Safety
+/// Only called by binderfs.
+#[no_mangle]
+unsafe extern "C" fn rust_binder_transaction_log_show(
+    ptr: *mut seq_file,
+    _: *mut kernel::ffi::c_void,
+) -> kernel::ffi::c_int {
+    // SAFETY: Accessing the private field of `seq_file` is okay.
+    let is_failed = !unsafe { (*ptr).private }.is_null();
+    // SAFETY: The caller ensures that the pointer is valid and exclusive for the duration in which
+    // this method is called.
+    let m = unsafe { SeqFile::from_raw(ptr) };
+    let log = if is_failed {
+        &transaction::FAILED_TRANSACTION_LOG
+    } else {
+        &transaction::TRANSACTION_LOG
+    };
+    log.debug_print(m);
+    0
+}
+
 fn rust_binder_transactions_show_impl(m: &SeqFile) -> Result<()> {
     seq_print!(m, "binder transactions:\n");
     let contexts = context::get_all_contexts()?;
diff --git a/drivers/android/binder/rust_binderfs.c b/drivers/android/binder/rust_binderfs.c
index 300cc65562d1..ef83d5a502af 100644
--- a/drivers/android/binder/rust_binderfs.c
+++ b/drivers/android/binder/rust_binderfs.c
@@ -46,6 +46,7 @@
 DEFINE_SHOW_ATTRIBUTE(rust_binder_stats);
 DEFINE_SHOW_ATTRIBUTE(rust_binder_state);
 DEFINE_SHOW_ATTRIBUTE(rust_binder_transactions);
+DEFINE_SHOW_ATTRIBUTE(rust_binder_transaction_log);
 DEFINE_SHOW_ATTRIBUTE(rust_binder_proc);
 
 char *rust_binder_devices_param = CONFIG_ANDROID_BINDER_DEVICES;
@@ -603,6 +604,24 @@ static int init_binder_logs(struct super_block *sb)
 		goto out;
 	}
 
+	dentry = rust_binderfs_create_file(binder_logs_root_dir,
+				      "transaction_log",
+				      &rust_binder_transaction_log_fops,
+				      (void *)0);
+	if (IS_ERR(dentry)) {
+		ret = PTR_ERR(dentry);
+		goto out;
+	}
+
+	dentry = rust_binderfs_create_file(binder_logs_root_dir,
+				      "failed_transaction_log",
+				      &rust_binder_transaction_log_fops,
+				      (void *)1);
+	if (IS_ERR(dentry)) {
+		ret = PTR_ERR(dentry);
+		goto out;
+	}
+
 	proc_log_dir = binderfs_create_dir(binder_logs_root_dir, "proc");
 	if (IS_ERR(proc_log_dir)) {
 		ret = PTR_ERR(proc_log_dir);
diff --git a/drivers/android/binder/thread.rs b/drivers/android/binder/thread.rs
index 0891a36e9cbd..875daaef5f53 100644
--- a/drivers/android/binder/thread.rs
+++ b/drivers/android/binder/thread.rs
@@ -1299,6 +1299,7 @@ fn transaction(self: &Arc<Self>, cmd: u32, reader: &mut UserSliceReader) -> Resu
             self.push_return_work(err.reply);
             if err.reply != BR_TRANSACTION_COMPLETE {
                 info.reply = err.reply;
+                info.error_line = Some(err.line);
                 if let Some(source) = &err.source {
                     info.errno = source.to_errno();
 
@@ -1330,6 +1331,8 @@ fn transaction(self: &Arc<Self>, cmd: u32, reader: &mut UserSliceReader) -> Resu
             }
         }
 
+        info.write_log(&self.process.ctx);
+
         if info.oneway_spam_suspect {
             // If this is both a oneway spam suspect and a failure, we report it twice. This is
             // useful in case the transaction failed with BR_TRANSACTION_PENDING_FROZEN.
@@ -1345,6 +1348,7 @@ fn transaction(self: &Arc<Self>, cmd: u32, reader: &mut UserSliceReader) -> Resu
     fn transaction_inner(self: &Arc<Self>, info: &mut TransactionInfo) -> BinderResult {
         let node_ref = self.process.get_transaction_node(info.target_handle)?;
         info.to_pid = node_ref.node.owner.task.pid();
+        info.to_node_debug_id = node_ref.node.debug_id;
         security::binder_transaction(&self.process.cred, &node_ref.node.owner.cred)?;
         // TODO: We need to ensure that there isn't a pending transaction in the work queue. How
         // could this happen?
@@ -1431,6 +1435,8 @@ fn reply_inner(self: &Arc<Self>, info: &mut TransactionInfo) -> BinderResult {
             orig.from
                 .deliver_reply(Err(BR_FAILED_REPLY), &orig, Some(ee));
             info.reply = BR_FAILED_REPLY;
+            info.errno = param;
+            info.error_line = Some(err.line);
             err.reply = BR_TRANSACTION_COMPLETE;
             err
         });
@@ -1441,6 +1447,7 @@ fn reply_inner(self: &Arc<Self>, info: &mut TransactionInfo) -> BinderResult {
     fn oneway_transaction_inner(self: &Arc<Self>, info: &mut TransactionInfo) -> BinderResult {
         let node_ref = self.process.get_transaction_node(info.target_handle)?;
         info.to_pid = node_ref.node.owner.task.pid();
+        info.to_node_debug_id = node_ref.node.debug_id;
         security::binder_transaction(&self.process.cred, &node_ref.node.owner.cred)?;
         let transaction = Transaction::new(node_ref, None, self, info)?;
         let code = if self.process.is_oneway_spam_detection_enabled() && info.oneway_spam_suspect {
diff --git a/drivers/android/binder/transaction.rs b/drivers/android/binder/transaction.rs
index 245f1556b5db..f1608651c636 100644
--- a/drivers/android/binder/transaction.rs
+++ b/drivers/android/binder/transaction.rs
@@ -7,8 +7,9 @@
     prelude::*,
     seq_file::SeqFile,
     seq_print,
+    str::BStr,
     sync::atomic::{ordering::Relaxed, Atomic},
-    sync::{Arc, SpinLock},
+    sync::{Arc, SetOnce, SpinLock},
     task::{Kuid, Pid},
     time::{Instant, Monotonic},
     types::ScopeGuard,
@@ -18,7 +19,7 @@
 use crate::{
     allocation::{Allocation, TranslatedFds},
     defs::*,
-    error::{BinderError, BinderResult},
+    error::{BinderError, BinderResult, ErrorLocation},
     netlink::Report,
     node::{Node, NodeRef},
     process::{Process, ProcessInner},
@@ -54,12 +55,191 @@ pub(crate) fn is_oneway(self) -> bool {
     }
 }
 
+const LOG_SIZE: usize = 32;
+
+pub(crate) static TRANSACTION_LOG: TransactionLog = TransactionLog::new();
+pub(crate) static FAILED_TRANSACTION_LOG: TransactionLog = TransactionLog::new();
+
+#[derive(Copy, Clone)]
+enum CallType {
+    Call,
+    Async,
+    Reply,
+}
+
+#[derive(Copy, Clone)]
+pub(crate) struct TransactionLogEntry {
+    debug_id: usize,
+    call_type: CallType,
+    from_proc: Pid,
+    from_thread: Pid,
+    // The kernel ignores `target.handle` on replies (where `libbinder` passes `-1` / `0xffffffff`),
+    // and C Binder stores and prints this field as a signed `int` (`%d`).
+    target_handle: i32,
+    to_proc: Pid,
+    to_thread: Pid,
+    to_node: usize,
+    data_size: usize,
+    offsets_size: usize,
+    return_error_line: Option<ErrorLocation>,
+    return_error: u32,
+    return_error_param: i32,
+    context_name: [u8; 16],
+}
+
+impl TransactionLogEntry {
+    const fn empty() -> Self {
+        Self {
+            debug_id: 0,
+            call_type: CallType::Call,
+            from_proc: 0,
+            from_thread: 0,
+            target_handle: 0,
+            to_proc: 0,
+            to_thread: 0,
+            to_node: 0,
+            data_size: 0,
+            offsets_size: 0,
+            return_error_line: None,
+            return_error: 0,
+            return_error_param: 0,
+            context_name: [0; 16],
+        }
+    }
+
+    fn new(info: &TransactionInfo, ctx: &crate::Context) -> Self {
+        let call_type = if info.is_reply {
+            CallType::Reply
+        } else if info.is_oneway() {
+            CallType::Async
+        } else {
+            CallType::Call
+        };
+        let name_bytes = ctx.name.to_bytes();
+        let mut context_name = [0u8; 16];
+        let len = usize::min(name_bytes.len(), context_name.len());
+        context_name[..len].copy_from_slice(&name_bytes[..len]);
+
+        let failed = info.reply != 0 && info.reply != BR_TRANSACTION_PENDING_FROZEN;
+        Self {
+            debug_id: info.debug_id,
+            call_type,
+            from_proc: info.from_pid,
+            from_thread: info.from_tid,
+            target_handle: info.target_handle as i32,
+            to_proc: info.to_pid,
+            to_thread: info.to_tid,
+            to_node: info.to_node_debug_id,
+            data_size: info.data_size,
+            offsets_size: info.offsets_size,
+            return_error_line: if failed { info.error_line } else { None },
+            return_error: if failed { info.reply } else { 0 },
+            return_error_param: if failed { info.errno } else { 0 },
+            context_name,
+        }
+    }
+}
+
+#[pin_data]
+#[repr(align(64))]
+struct TransactionLogSlot {
+    #[pin]
+    entry: SpinLock<TransactionLogEntry>,
+}
+
+pub(crate) struct TransactionLog {
+    cur: Atomic<usize>,
+    entries: SetOnce<Pin<KBox<[TransactionLogSlot; LOG_SIZE]>>>,
+}
+
+impl TransactionLog {
+    const fn new() -> Self {
+        Self {
+            cur: Atomic::new(0),
+            entries: SetOnce::new(),
+        }
+    }
+
+    pub(crate) fn init(&self) -> Result<()> {
+        let entries = KBox::pin_init(
+            pin_init::pin_init_array_from_fn(|_| {
+                pin_init!(TransactionLogSlot {
+                    entry <- kernel::new_spinlock!(
+                        TransactionLogEntry::empty(),
+                        "TransactionLog::entries"
+                    ),
+                })
+            }),
+            GFP_KERNEL,
+        )?;
+        self.entries.populate(entries);
+        Ok(())
+    }
+
+    fn add(&self, entry: &TransactionLogEntry) {
+        let Some(entries) = self.entries.as_ref() else {
+            return;
+        };
+        let idx = self.cur.fetch_add(1, Relaxed) % LOG_SIZE;
+        let mut slot = entries[idx].entry.lock();
+        if slot.debug_id < entry.debug_id {
+            *slot = *entry;
+        }
+    }
+
+    pub(crate) fn debug_print(&self, m: &SeqFile) {
+        let Some(entries) = self.entries.as_ref() else {
+            return;
+        };
+        let cur = self.cur.load(Relaxed);
+        for i in 0..LOG_SIZE {
+            let idx = cur.wrapping_add(i) % LOG_SIZE;
+            let entry = *entries[idx].entry.lock();
+            if entry.debug_id == 0 {
+                continue;
+            }
+            let call_type = match entry.call_type {
+                CallType::Call => "call ",
+                CallType::Async => "async",
+                CallType::Reply => "reply",
+            };
+            let ctx_name = match CStr::from_bytes_until_nul(&entry.context_name) {
+                Ok(cstr) => BStr::from_bytes(cstr.to_bytes()),
+                Err(_) => BStr::from_bytes(&entry.context_name),
+            };
+            let return_error_line: &dyn kernel::fmt::Display = match &entry.return_error_line {
+                Some(line) => line,
+                None => &0,
+            };
+            seq_print!(
+                m,
+                "{}: {} from {}:{} to {}:{} context {} node {} handle {} size {}:{} ret {}/{} l={}\n",
+                entry.debug_id,
+                call_type,
+                entry.from_proc,
+                entry.from_thread,
+                entry.to_proc,
+                entry.to_thread,
+                ctx_name,
+                entry.to_node,
+                entry.target_handle,
+                entry.data_size,
+                entry.offsets_size,
+                entry.return_error,
+                entry.return_error_param,
+                return_error_line,
+            );
+        }
+    }
+}
+
 #[derive(Zeroable)]
 pub(crate) struct TransactionInfo {
     pub(crate) from_pid: Pid,
     pub(crate) from_tid: Pid,
     pub(crate) to_pid: Pid,
     pub(crate) to_tid: Pid,
+    pub(crate) to_node_debug_id: usize,
     pub(crate) code: u32,
     pub(crate) flags: TransactionFlags,
     pub(crate) data_ptr: UserPtr,
@@ -70,6 +250,7 @@ pub(crate) struct TransactionInfo {
     pub(crate) target_handle: u32,
     pub(crate) errno: i32,
     pub(crate) reply: u32,
+    pub(crate) error_line: Option<ErrorLocation>,
     pub(crate) oneway_spam_suspect: bool,
     pub(crate) is_reply: bool,
     pub(crate) debug_id: usize,
@@ -81,6 +262,14 @@ pub(crate) fn is_oneway(&self) -> bool {
         self.flags.is_oneway()
     }
 
+    pub(crate) fn write_log(&self, ctx: &crate::Context) {
+        let entry = TransactionLogEntry::new(self, ctx);
+        TRANSACTION_LOG.add(&entry);
+        if self.reply != 0 && self.reply != BR_TRANSACTION_PENDING_FROZEN {
+            FAILED_TRANSACTION_LOG.add(&entry);
+        }
+    }
+
     pub(crate) fn report_netlink(&self, reply: u32, ctx: &crate::Context) {
         if let Err(err) = self.report_netlink_inner(reply, ctx) {
             pr_warn!(

-- 
2.56.0.rc1.315.gc6ed9934b7-goog

Re: [PATCH 0/3] rust_binder: error line numbers and failed_transaction_log

[Alice Ryhl]

On Wed, Sep 30, 2026 at 2:48 PM Alice Ryhl <aliceryhl@google.com> wrote:
>
> When diagnosing Binder issues, it's useful to know what the most recent
> failed transactionas are, so include a transaction log with this content
> matching the C Binder transaction log.
>
> While we're at it, improve the line number information emitted by Rust
> Binder using #[track_caller].
>
> Signed-off-by: Alice Ryhl <aliceryhl@google.com>
> ---
> Alice Ryhl (3):
>       rust_binder: start debug_id at 1
>       rust_binder: track caller location in BinderError
>       rust_binder: add transaction_log and failed_transaction_log
>
>  drivers/android/binder/context.rs             |   8 +-
>  drivers/android/binder/error.rs               |  39 ++++++
>  drivers/android/binder/process.rs             |   5 +-
>  drivers/android/binder/rust_binder_internal.h |   1 +
>  drivers/android/binder/rust_binder_main.rs    |  26 +++-
>  drivers/android/binder/rust_binderfs.c        |  19 +++
>  drivers/android/binder/thread.rs              |  12 +-
>  drivers/android/binder/transaction.rs         | 193 +++++++++++++++++++++++++-
>  8 files changed, 293 insertions(+), 10 deletions(-)

I'm going to send a v2 addressing the sashiko review right away.

Alice