rust_binder: Add dynamic debug logging mask

Implement a dynamic debug logging mask (`debug_mask`) for the
`rust_binder` module to allow dynamic runtime configuration of log
levels. This enables parity with the legacy C driver's debug mask.

Since the Rust `module!` macro in the current kernel build does not
yet support declaring module parameters directly in Rust, we define
the `debug_mask` variable in Rust as an `Atomic<u32>` exported via
FFI using `#[no_mangle]`, and link to it as `extern` in a C companion
file to expose it to the kernel runtime.

To verify the setup, instrument process lifecycle events (open, flush,
and release) in `process.rs` under the new `BINDER_DEBUG_OPEN_CLOSE`
logging mask. These entry-point events are chosen for initial validation
because they represent the start of the Binder lifecycle and occur
at low frequency, allowing simple runtime verification of the dynamic
toggle without log noise.

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-v4-1-3d7436c2d2f2@google.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/android/binder/debug.rs

@@ -0,0 +1,76 @@
+// SPDX-License-Identifier: GPL-2.0
+// Copyright (C) 2026 Google LLC.
+
+//! Binder debugging helpers.
+
+#![allow(dead_code)]
+
+use kernel::bits::bit_u32;
+use kernel::sync::atomic::Atomic;
+
+kernel::impl_flags!(
+    /// Represents multiple debug mask flags.
+    #[derive(Debug, Clone, Default, Copy, PartialEq, Eq)]
+    pub struct DebugMasks(u32);
+
+    /// Represents a single debug mask category.
+    #[derive(Debug, Clone, Copy, PartialEq, Eq)]
+    pub enum DebugMask {
+        UserError = bit_u32(0),
+        FailedTransaction = bit_u32(1),
+        DeadTransaction = bit_u32(2),
+        OpenClose = bit_u32(3),
+        DeadBinder = bit_u32(4),
+        DeathNotification = bit_u32(5),
+        ReadWrite = bit_u32(6),
+        UserRefs = bit_u32(7),
+        Threads = bit_u32(8),
+        Transaction = bit_u32(9),
+        TransactionComplete = bit_u32(10),
+        FreeBuffer = bit_u32(11),
+        InternalRefs = bit_u32(12),
+        PriorityCap = bit_u32(13),
+        Spinlocks = bit_u32(14),
+    }
+);
+
+#[no_mangle]
+pub(crate) static rust_binder_debug_mask: Atomic<u32> = Atomic::new(
+    (DebugMask::UserError as u32)
+        | (DebugMask::FailedTransaction as u32)
+        | (DebugMask::DeadTransaction as u32),
+);
+
+/// Checks if the given debug logging category is enabled in the mask.
+pub(crate) fn debug_mask_enabled(mask: DebugMask) -> bool {
+    let current_mask = rust_binder_debug_mask.load(kernel::sync::atomic::Relaxed);
+    DebugMasks(current_mask).contains(mask)
+}
+
+/// Prints a debug log if the specified mask category is enabled.
+#[macro_export]
+macro_rules! binder_debug {
+    // Rule to explicitly specify a PID (used in kworkers).
+    (pid=$pid:expr, $mask:ident, $($arg:tt)*) => {
+        if $crate::debug::debug_mask_enabled($crate::debug::DebugMask::$mask) {
+            kernel::pr_info!(
+                "{}: {}\n",
+                $pid,
+                kernel::prelude::fmt!($($arg)*)
+            );
+        }
+    };
+
+    // Default rule (automatically prepends "PID:TID" of the current calling thread).
+    ($mask:ident, $($arg:tt)*) => {
+        if $crate::debug::debug_mask_enabled($crate::debug::DebugMask::$mask) {
+            let thread = kernel::current!();
+            kernel::pr_info!(
+                "{}:{} {}\n",
+                thread.tgid(),
+                thread.pid(),
+                kernel::prelude::fmt!($($arg)*)
+            );
+        }
+    };
+}

drivers/android/binder/process.rs

@@ -1341,6 +1341,7 @@ pub(crate) fn lock_with_nodes(&self) -> WithNodes<'_> {
     }
 
     fn deferred_flush(&self) {
+        binder_debug!(pid = self.task.pid(), OpenClose, "flushing process");
         let inner = self.inner.lock();
         for thread in inner.threads.values() {
             thread.exit_looper();
@@ -1348,6 +1349,8 @@ fn deferred_flush(&self) {
     }
 
     fn deferred_release(self: Arc<Self>) {
+        binder_debug!(pid = self.task.pid(), OpenClose, "releasing process");
+
         let is_manager = {
             let mut inner = self.inner.lock();
             inner.is_dead = true;
@@ -1641,7 +1644,9 @@ fn ioctl_write_read(
 /// The file operations supported by `Process`.
 impl Process {
     pub(crate) fn open(ctx: ArcBorrow<'_, Context>, file: &File) -> Result<Arc<Process>> {
-        Self::new(ctx.into(), ARef::from(file.cred()))
+        let proc = Self::new(ctx.into(), ARef::from(file.cred()))?;
+        binder_debug!(OpenClose, "opened process");
+        Ok(proc)
     }
 
     pub(crate) fn release(this: Arc<Process>, _file: &File) {

drivers/android/binder/rust_binder_main.rs

@@ -31,6 +31,8 @@
 mod context;
 mod deferred_close;
 mod defs;
+#[macro_use]
+mod debug;
 mod error;
 mod node;
 mod page_range;

drivers/android/binder/rust_binderfs.c

@@ -51,6 +51,9 @@ DEFINE_SHOW_ATTRIBUTE(rust_binder_proc);
 char *rust_binder_devices_param = CONFIG_ANDROID_BINDER_DEVICES;
 module_param_named(rust_devices, rust_binder_devices_param, charp, 0444);
 
+extern u32 rust_binder_debug_mask;
+module_param_named(debug_mask, rust_binder_debug_mask, uint, 0644);
+
 static dev_t binderfs_dev;
 static DEFINE_MUTEX(binderfs_minors_mutex);
 static DEFINE_IDA(binderfs_minors);

rust/kernel/task.rs

@@ -210,6 +210,13 @@ pub fn pid(&self) -> Pid {
         unsafe { *ptr::addr_of!((*self.as_ptr()).pid) }
     }
 
+    /// Returns the TGID (Thread Group ID / Process ID) of the given task.
+    pub fn tgid(&self) -> Pid {
+        // SAFETY: The tgid of a task never changes after initialization, so reading this field is
+        // not a data race.
+        unsafe { *ptr::addr_of!((*self.as_ptr()).tgid) }
+    }
+
     /// Returns the UID of the given task.
     #[inline]
     pub fn uid(&self) -> Kuid {