linux-kernel - [PATCH v2 2/2] kernel/watch_queue: NULL the dangling *pipe, and use it for clear check

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Message-ID: <ef1791b7f1b5207d9ffae053d8641c9a634a25ea.1659771577.git.code@siddh.me>
Date:   Sat,  6 Aug 2022 13:13:42 +0530
From:   Siddh Raman Pant <code@...dh.me>
To:     Eric Biggers <ebiggers@...nel.org>,
        Jonathan Corbet <corbet@....net>,
        David Howells <dhowells@...hat.com>,
        Randy Dunlap <rdunlap@...radead.org>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Christophe JAILLET <christophe.jaillet@...adoo.fr>,
        Eric Dumazet <edumazet@...gle.com>
Cc:     linux-kernel <linux-kernel@...r.kernel.org>,
        linux-kernel-mentees 
        <linux-kernel-mentees@...ts.linuxfoundation.org>
Subject: [PATCH v2 2/2] kernel/watch_queue: NULL the dangling *pipe, and use it for clear check

NULL the dangling pipe reference while clearing watch_queue.

If not done, a reference to a freed pipe remains in the watch_queue,
as this function is called before freeing a pipe in free_pipe_info()
(see line 834 of fs/pipe.c).

The sole use of wqueue->defunct is for checking if the watch queue has
been cleared, but wqueue->pipe is also NULLed while clearing.

Thus, wqueue->defunct is superfluous, as wqueue->pipe can be checked
for NULL. Hence, the former can be removed.

Signed-off-by: Siddh Raman Pant <code@...dh.me>
---
 include/linux/watch_queue.h |  4 +---
 kernel/watch_queue.c        | 12 ++++++------
 2 files changed, 7 insertions(+), 9 deletions(-)

diff --git a/include/linux/watch_queue.h b/include/linux/watch_queue.h
index 7f8b1f15634b..d72ad82a4435 100644
--- a/include/linux/watch_queue.h
+++ b/include/linux/watch_queue.h
@@ -55,7 +55,7 @@ struct watch_filter {
  *
  * @rcu: RCU head
  * @filter: Filter to use on watches
- * @pipe: The pipe we're using as a buffer
+ * @pipe: The pipe we're using as a buffer, NULL when queue is cleared/closed
  * @watches: Contributory watches
  * @notes: Preallocated notifications
  * @notes_bitmap: Allocation bitmap for notes
@@ -63,7 +63,6 @@ struct watch_filter {
  * @lock: To serialize accesses and removes
  * @nr_notes: Number of notes
  * @nr_pages: Number of pages in notes[]
- * @defunct: True when queues closed
  */
 struct watch_queue {
 	struct rcu_head		rcu;
@@ -76,7 +75,6 @@ struct watch_queue {
 	spinlock_t		lock;
 	unsigned int		nr_notes;
 	unsigned int		nr_pages;
-	bool			defunct;
 };
 
 /**
diff --git a/kernel/watch_queue.c b/kernel/watch_queue.c
index a6f9bdd956c3..a70ddfd622ee 100644
--- a/kernel/watch_queue.c
+++ b/kernel/watch_queue.c
@@ -43,7 +43,7 @@ MODULE_LICENSE("GPL");
 static inline bool lock_wqueue(struct watch_queue *wqueue)
 {
 	spin_lock_bh(&wqueue->lock);
-	if (unlikely(wqueue->defunct)) {
+	if (unlikely(!wqueue->pipe)) {
 		spin_unlock_bh(&wqueue->lock);
 		return false;
 	}
@@ -105,9 +105,6 @@ static bool post_one_notification(struct watch_queue *wqueue,
 	unsigned int head, tail, mask, note, offset, len;
 	bool done = false;
 
-	if (!pipe)
-		return false;
-
 	spin_lock_irq(&pipe->rd_wait.lock);
 
 	mask = pipe->ring_size - 1;
@@ -603,8 +600,11 @@ void watch_queue_clear(struct watch_queue *wqueue)
 	rcu_read_lock();
 	spin_lock_bh(&wqueue->lock);
 
-	/* Prevent new notifications from being stored. */
-	wqueue->defunct = true;
+	/*
+	 * This pipe will get freed by the caller free_pipe_info().
+	 * Removing this reference also prevents new notifications.
+	 */
+	wqueue->pipe = NULL;
 
 	while (!hlist_empty(&wqueue->watches)) {
 		watch = hlist_entry(wqueue->watches.first, struct watch, queue_node);
-- 
2.35.1