[PATCH] rust_binder: Avoid holding lock when dropping delivered_death

[PATCH] rust_binder: Avoid holding lock when dropping delivered_death

[Matthew Maurer]

In 6c37bebd8c926, we switched to looping over the list and dropping each
individual node, ostensibly without the lock held in the loop body.

If the kernel were using Rust Edition 2024, the comment would be
accurate, and the lock would not be held across the drop. However, the
kernel is currently using 2021, so tail expression lifetime extension
results in the lock being held across the drop. Explicitly binding the
expression result to a variable makes the lockguard no longer part of a
tail expression, causing the lock to be dropped before entering the loop
body.

This was detected via `CONFIG_PROVE_LOCKING` identifying an invalid wait
context at the drop site.

Reported-by: David Stevens <stevensd@google.com>
Signed-off-by: Matthew Maurer <mmaurer@google.com>
Cc: stable@vger.kernel.org
Fixes: eafedbc7c050 ("rust_binder: add Rust Binder driver")
---
 drivers/android/binder/process.rs | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/drivers/android/binder/process.rs b/drivers/android/binder/process.rs
index f06498129aa9765ecbe7a34af36074f15b7490f1..9812c52dc16ee52044dbaf86e30a0a3861effefb 100644
--- a/drivers/android/binder/process.rs
+++ b/drivers/android/binder/process.rs
@@ -1402,7 +1402,12 @@ fn deferred_release(self: Arc<Self>) {
         // Clear delivered_deaths list.
         //
         // Scope ensures that MutexGuard is dropped while executing the body.
-        while let Some(delivered_death) = { self.inner.lock().delivered_deaths.pop_front() } {
+        while let Some(delivered_death) = {
+            // Explicitly bind to avoid tail expression lifetime extension of the lockguard
+            // Can be removed when the kernel moves to edition 2024
+            let maybe_death = self.inner.lock().delivered_deaths.pop_front();
+            maybe_death
+        } {
             drop(delivered_death);
         }
 

---
base-commit: d8a9a4b11a137909e306e50346148fc5c3b63f9d
change-id: 20260403-lockhold-bbc2398794c0

Best regards,
-- 
Matthew Maurer <mmaurer@google.com>

Re: [PATCH] rust_binder: Avoid holding lock when dropping delivered_death

[Alice Ryhl]

On Fri, Apr 3, 2026 at 8:19 PM Matthew Maurer <mmaurer@google.com> wrote:
>
> In 6c37bebd8c926, we switched to looping over the list and dropping each
> individual node, ostensibly without the lock held in the loop body.
>
> If the kernel were using Rust Edition 2024, the comment would be
> accurate, and the lock would not be held across the drop. However, the
> kernel is currently using 2021, so tail expression lifetime extension
> results in the lock being held across the drop. Explicitly binding the
> expression result to a variable makes the lockguard no longer part of a
> tail expression, causing the lock to be dropped before entering the loop
> body.
>
> This was detected via `CONFIG_PROVE_LOCKING` identifying an invalid wait
> context at the drop site.
>
> Reported-by: David Stevens <stevensd@google.com>
> Signed-off-by: Matthew Maurer <mmaurer@google.com>
> Cc: stable@vger.kernel.org
> Fixes: eafedbc7c050 ("rust_binder: add Rust Binder driver")

Thanks
Reviewed-by: Alice Ryhl <aliceryhl@google.com>