linux-kernel - [PATCH] binder: fix hang of unregistered readers

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Message-ID: <20240711201452.2017543-1-cmllamas@google.com>
Date: Thu, 11 Jul 2024 20:14:51 +0000
From: Carlos Llamas <cmllamas@...gle.com>
To: Greg Kroah-Hartman <gregkh@...uxfoundation.org>, 
	"Arve Hjønnevåg" <arve@...roid.com>, Todd Kjos <tkjos@...roid.com>, Martijn Coenen <maco@...roid.com>, 
	Joel Fernandes <joel@...lfernandes.org>, Christian Brauner <brauner@...nel.org>, 
	Carlos Llamas <cmllamas@...gle.com>, Suren Baghdasaryan <surenb@...gle.com>
Cc: linux-kernel@...r.kernel.org, kernel-team@...roid.com, 
	stable@...r.kernel.org, Martijn Coenen <maco@...gle.com>, 
	"Arve Hjønnevåg" <arve@...gle.com>
Subject: [PATCH] binder: fix hang of unregistered readers

With the introduction of binder_available_for_proc_work_ilocked() in
commit 1b77e9dcc3da ("ANDROID: binder: remove proc waitqueue") a binder
thread can only "wait_for_proc_work" after its thread->looper has been
marked as BINDER_LOOPER_STATE_{ENTERED|REGISTERED}.

This means an unregistered reader risks waiting indefinitely for work
since it never gets added to the proc->waiting_threads. If there are no
further references to its waitqueue either the task will hang. The same
applies to readers using the (e)poll interface.

I couldn't find the rationale behind this restriction. So this patch
restores the previous behavior of allowing unregistered threads to
"wait_for_proc_work". Note that an error message for this scenario,
which had previously become unreachable, is now re-enabled.

Fixes: 1b77e9dcc3da ("ANDROID: binder: remove proc waitqueue")
Cc: stable@...r.kernel.org
Cc: Martijn Coenen <maco@...gle.com>
Cc: Arve Hjønnevåg <arve@...gle.com>
Signed-off-by: Carlos Llamas <cmllamas@...gle.com>
---
 drivers/android/binder.c | 4 +---
 1 file changed, 1 insertion(+), 3 deletions(-)

diff --git a/drivers/android/binder.c b/drivers/android/binder.c
index b21a7b246a0d..2d0a24a56508 100644
--- a/drivers/android/binder.c
+++ b/drivers/android/binder.c
@@ -570,9 +570,7 @@ static bool binder_has_work(struct binder_thread *thread, bool do_proc_work)
 static bool binder_available_for_proc_work_ilocked(struct binder_thread *thread)
 {
 	return !thread->transaction_stack &&
-		binder_worklist_empty_ilocked(&thread->todo) &&
-		(thread->looper & (BINDER_LOOPER_STATE_ENTERED |
-				   BINDER_LOOPER_STATE_REGISTERED));
+		binder_worklist_empty_ilocked(&thread->todo);
 }

 static void binder_wakeup_poll_threads_ilocked(struct binder_proc *proc,
-- 
2.45.2.993.g49e7a77208-goog