[PATCH] locking/mutex: Test for initialized mutex

[Sebastian Andrzej Siewior]

An uninitialized/ zeroed mutex will go unnoticed because there is no
check for it. There is a magic check in the unlock's slowpath path which
might go unnoticed if the unlock happens in the fastpath.

Add a ->magic check early in the mutex_lock() and mutex_trylock() path.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>
---
Nothing screamed during uninitialized usage of init_mm's context->lock
  https://git.kernel.org/tip/32232b350d7cd93cdc65fe5a453e6a40b539e9f9

 kernel/locking/mutex.c | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/kernel/locking/mutex.c b/kernel/locking/mutex.c
index 0c601ae072b3f..fb1f6f1e1cc61 100644
--- a/kernel/locking/mutex.c
+++ b/kernel/locking/mutex.c
@@ -908,6 +908,10 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
 
 	might_sleep();
 
+#ifdef CONFIG_DEBUG_MUTEXES
+	DEBUG_LOCKS_WARN_ON(lock->magic != lock);
+#endif
+
 	ww = container_of(lock, struct ww_mutex, base);
 	if (use_ww_ctx && ww_ctx) {
 		if (unlikely(ww_ctx == READ_ONCE(ww->ctx)))
@@ -1379,8 +1383,13 @@ __ww_mutex_lock_interruptible_slowpath(struct ww_mutex *lock,
  */
 int __sched mutex_trylock(struct mutex *lock)
 {
-	bool locked = __mutex_trylock(lock);
+	bool locked;
 
+#ifdef CONFIG_DEBUG_MUTEXES
+	DEBUG_LOCKS_WARN_ON(lock->magic != lock);
+#endif
+
+	locked = __mutex_trylock(lock);
 	if (locked)
 		mutex_acquire(&lock->dep_map, 0, 1, _RET_IP_);
 
-- 
2.20.1