linux-kernel - [PATCH 1/4] seccomp: don't use semaphore and wait

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Date:   Mon, 29 Aug 2022 18:43:53 -0700
From:   Andrei Vagin <avagin@...il.com>
To:     linux-kernel@...r.kernel.org
Cc:     Andrei Vagin <avagin@...il.com>,
        Andy Lutomirski <luto@...capital.net>,
        Christian Brauner <brauner@...nel.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Kees Cook <keescook@...omium.org>,
        Ingo Molnar <mingo@...hat.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Peter Oskolkov <posk@...gle.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Tycho Andersen <tycho@...ho.pizza>,
        Will Drewry <wad@...omium.org>,
        Vincent Guittot <vincent.guittot@...aro.org>
Subject: [PATCH 1/4] seccomp: don't use semaphore and wait_queue together

Here is no reason to use two different primitives that do similar things.

Signed-off-by: Andrei Vagin <avagin@...il.com>
---
 kernel/seccomp.c | 38 ++++++++++++++++++++++++++------------
 1 file changed, 26 insertions(+), 12 deletions(-)

diff --git a/kernel/seccomp.c b/kernel/seccomp.c
index e9852d1b4a5e..667fd2d89464 100644
--- a/kernel/seccomp.c
+++ b/kernel/seccomp.c
@@ -145,7 +145,7 @@ struct seccomp_kaddfd {
  * @notifications: A list of struct seccomp_knotif elements.
  */
 struct notification {
-	struct semaphore request;
+	atomic_t requests;
 	u64 next_id;
 	struct list_head notifications;
 };
@@ -1116,7 +1116,7 @@ static int seccomp_do_user_notification(int this_syscall,
 	list_add_tail(&n.list, &match->notif->notifications);
 	INIT_LIST_HEAD(&n.addfd);
 
-	up(&match->notif->request);
+	atomic_add(1, &match->notif->requests);
 	wake_up_poll(&match->wqh, EPOLLIN | EPOLLRDNORM);
 
 	/*
@@ -1388,8 +1388,10 @@ static long seccomp_set_mode_strict(void)
 #ifdef CONFIG_SECCOMP_FILTER
 static void seccomp_notify_free(struct seccomp_filter *filter)
 {
-	kfree(filter->notif);
-	filter->notif = NULL;
+	struct notification *notif = filter->notif;
+
+	WRITE_ONCE(filter->notif, NULL);
+	kfree_rcu(notif);
 }
 
 static void seccomp_notify_detach(struct seccomp_filter *filter)
@@ -1450,6 +1452,16 @@ find_notification(struct seccomp_filter *filter, u64 id)
 	return NULL;
 }
 
+static bool notify_wakeup(struct seccomp_filter *filter)
+{
+	bool ret;
+
+	rcu_read_lock();
+	ret = atomic_add_unless(&filter->notif->requests, -1, 0);
+	rcu_read_unlock();
+
+	return ret;
+}
 
 static long seccomp_notify_recv(struct seccomp_filter *filter,
 				void __user *buf)
@@ -1467,7 +1479,7 @@ static long seccomp_notify_recv(struct seccomp_filter *filter,
 
 	memset(&unotif, 0, sizeof(unotif));
 
-	ret = down_interruptible(&filter->notif->request);
+	ret = wait_event_interruptible(filter->wqh, notify_wakeup(filter));
 	if (ret < 0)
 		return ret;
 
@@ -1515,7 +1527,8 @@ static long seccomp_notify_recv(struct seccomp_filter *filter,
 			if (should_sleep_killable(filter, knotif))
 				complete(&knotif->ready);
 			knotif->state = SECCOMP_NOTIFY_INIT;
-			up(&filter->notif->request);
+			atomic_add(1, &filter->notif->requests);
+			wake_up_poll(&filter->wqh, EPOLLIN | EPOLLRDNORM);
 		}
 		mutex_unlock(&filter->notify_lock);
 	}
@@ -1771,15 +1784,15 @@ static const struct file_operations seccomp_notify_ops = {
 static struct file *init_listener(struct seccomp_filter *filter)
 {
 	struct file *ret;
+	struct notification *notif;
 
 	ret = ERR_PTR(-ENOMEM);
-	filter->notif = kzalloc(sizeof(*(filter->notif)), GFP_KERNEL);
-	if (!filter->notif)
+	notif = kzalloc(sizeof(*notif), GFP_KERNEL);
+	if (!notif)
 		goto out;
 
-	sema_init(&filter->notif->request, 0);
-	filter->notif->next_id = get_random_u64();
-	INIT_LIST_HEAD(&filter->notif->notifications);
+	notif->next_id = get_random_u64();
+	INIT_LIST_HEAD(&notif->notifications);
 
 	ret = anon_inode_getfile("seccomp notify", &seccomp_notify_ops,
 				 filter, O_RDWR);
@@ -1788,10 +1801,11 @@ static struct file *init_listener(struct seccomp_filter *filter)
 
 	/* The file has a reference to it now */
 	__get_seccomp_filter(filter);
+	WRITE_ONCE(filter->notif, notif);
 
 out_notif:
 	if (IS_ERR(ret))
-		seccomp_notify_free(filter);
+		kfree(notif);
 out:
 	return ret;
 }
-- 
2.37.2