diff mbox series

[1/5] seccomp: don't use semaphore and wait_queue together

Message ID 20221020011048.156415-2-avagin@gmail.com
State New
Headers
Series seccomp: add the synchronous mode for seccomp_unotify |

Commit Message

Andrei Vagin Oct. 20, 2022, 1:10 a.m. UTC 
  Here is no reason to use two different primitives that do similar things.

Signed-off-by: Andrei Vagin <avagin@gmail.com>
---
 kernel/seccomp.c | 41 ++++++++++++++++++++++++++++++++++++-----
 1 file changed, 36 insertions(+), 5 deletions(-)

Comments

Kees Cook Oct. 20, 2022, 5:10 a.m. UTC | #1 
On October 19, 2022 6:10:44 PM PDT, Andrei Vagin <avagin@gmail.com> wrote:
>Here is no reason to use two different primitives that do similar things.
>
>Signed-off-by: Andrei Vagin <avagin@gmail.com>
>---
> kernel/seccomp.c | 41 ++++++++++++++++++++++++++++++++++++-----
> 1 file changed, 36 insertions(+), 5 deletions(-)

The commit log does not justify adding 29 lines to "do similar things". :) Can you describe the rationale and benefits here a bit more? I assume this to use the the future new wake_up helper?
Andrei Vagin Oct. 21, 2022, 4:30 a.m. UTC | #2 
On Wed, Oct 19, 2022 at 10:10:44PM -0700, Kees Cook wrote:
> On October 19, 2022 6:10:44 PM PDT, Andrei Vagin <avagin@gmail.com> wrote:
> >Here is no reason to use two different primitives that do similar things.
> >
> >Signed-off-by: Andrei Vagin <avagin@gmail.com>
> >---
> > kernel/seccomp.c | 41 ++++++++++++++++++++++++++++++++++++-----
> > 1 file changed, 36 insertions(+), 5 deletions(-)
> 
> The commit log does not justify adding 29 lines to "do similar
> things". :) Can you describe the rationale and benefits here a bit
> more? I assume this to use the the future new wake_up helper?

The main reason is to use new wake_up helpers, but there are a few
other reasons:

* if we use two different ways, we always need to call them both. This
  patch fixes seccomp_notify_recv where we forgot to call wake_up_poll
  in the error path.

* If we use one primitive, we can control how many waiters are woken up
  for each request. Our goal is to wake up just one that will handle a
  request. Right now, wake_up_poll may wake up one waiter and
  up(&match->notif->request) may wake up one more.

I will update the commit message. Thanks!

> 
> 
> -- 
> Kees Cook
diff mbox series

Patch

diff --git a/kernel/seccomp.c b/kernel/seccomp.c
index e9852d1b4a5e..876022e9c88c 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);
 
 	/*
@@ -1450,6 +1450,37 @@  find_notification(struct seccomp_filter *filter, u64 id)
 	return NULL;
 }
 
+static int recv_wake_function(wait_queue_entry_t *wait, unsigned int mode, int sync,
+				  void *key)
+{
+	/* Avoid a wakeup if event not interesting for us. */
+	if (key && !(key_to_poll(key) & (EPOLLIN | EPOLLERR)))
+		return 0;
+	return autoremove_wake_function(wait, mode, sync, key);
+}
+
+static int recv_wait_event(struct seccomp_filter *filter)
+{
+	DEFINE_WAIT_FUNC(wait, recv_wake_function);
+	int ret;
+
+	if (atomic_add_unless(&filter->notif->requests, -1, 0) != 0)
+		return 0;
+
+	for (;;) {
+		ret = prepare_to_wait_event(&filter->wqh, &wait, TASK_INTERRUPTIBLE);
+
+		if (atomic_add_unless(&filter->notif->requests, -1, 0) != 0)
+			break;
+
+		if (ret)
+			return ret;
+
+		schedule();
+	}
+	finish_wait(&filter->wqh, &wait);
+	return 0;
+}
 
 static long seccomp_notify_recv(struct seccomp_filter *filter,
 				void __user *buf)
@@ -1467,7 +1498,7 @@  static long seccomp_notify_recv(struct seccomp_filter *filter,
 
 	memset(&unotif, 0, sizeof(unotif));
 
-	ret = down_interruptible(&filter->notif->request);
+	ret = recv_wait_event(filter);
 	if (ret < 0)
 		return ret;
 
@@ -1515,7 +1546,8 @@  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);
 	}
@@ -1777,7 +1809,6 @@  static struct file *init_listener(struct seccomp_filter *filter)
 	if (!filter->notif)
 		goto out;
 
-	sema_init(&filter->notif->request, 0);
 	filter->notif->next_id = get_random_u64();
 	INIT_LIST_HEAD(&filter->notif->notifications);