On Tue, Mar 15, 2022 at 10:11:10PM +0800, 周多明 wrote:

Hello,

On Tue, 15 Mar 2022 13:26:57 +0300, Dan Carpenter wrote:

quoted

I'm happy that this is simpler.  I'm not super happy about the
if (sk->sk_wq) check.  That seems like a fragile side-effect condition
instead of something deliberate.  But I don't know networking so maybe
this is something which we can rely on.

The variable sk->sk_wq is the address of waiting queue of sock, it is initialized to the 
address of sock->wq through the following path:
sock_create->__sock_create->ax25_create()->sock_init_data()->RCU_INIT_POINTER(sk->sk_wq, &sock->wq).
Because we have used sock_alloc() to allocate the socket in __sock_create(), sock or the address of
sock->wq is not null.
What`s more, sk->sk_wq is set to null only in sock_orphan().

Another solution:
We could also use sk->sk_socket to check. We set sk->sk_socket to sock in the following path:
sock_create()->__sock_create()->ax25_create()->sock_init_data()->sk_set_socket(sk, sock).
Because we have used sock_alloc() to allocate the socket in __sock_create(), sock or sk->sk_socket
is not null.
What`s more, sk->sk_socket is set to null only in sock_orphan().

I will change the if (sk->sk_wq) check to if(sk->sk_socket) check, because I think it is 
easier to understand.

quoted

When you sent the earlier patch then I asked if the devices in
ax25_kill_by_device() were always bound and if we could just use a local
variable instead of something tied to the ax25_dev struct.  I still
wonder about that.  In other words, could we just do this?

diff --git a/net/ax25/af_ax25.c b/net/ax25/af_ax25.c
index 6bd097180772..4af9d9a939c6 100644
--- a/net/ax25/af_ax25.c
+++ b/net/ax25/af_ax25.c

@@ -78,6 +78,7 @@ static void ax25_kill_by_device(struct net_device *dev)
 	ax25_dev *ax25_dev;
 	ax25_cb *s;
 	struct sock *sk;
+	bool found = false;
 
 	if ((ax25_dev = ax25_dev_ax25dev(dev)) == NULL)
 		return;

@@ -86,6 +87,7 @@ static void ax25_kill_by_device(struct net_device *dev)
 again:
 	ax25_for_each(s, &ax25_list) {
 		if (s->ax25_dev == ax25_dev) {
+			found = true;
 			sk = s->sk;
 			if (!sk) {
 				spin_unlock_bh(&ax25_list_lock);

@@ -115,6 +117,11 @@ static void ax25_kill_by_device(struct net_device *dev)
 		}
 	}
 	spin_unlock_bh(&ax25_list_lock);
+
+	if (!found) {
+		dev_put_track(ax25_dev->dev, &ax25_dev->dev_tracker);
+		ax25_dev_put(ax25_dev);
+	}
 }

If we just use ax25_dev_device_up() to bring device up without using ax25_bind(),
the "found" flag could be false when we enter ax25_kill_by_device() and the refcounts 
underflow will happen. So we should use two additional variables.

That answers my question.  Thank you.

If we use additional variables to fix the bug, I think there is a problem.

So the v3 patch was buggy?

Why was this not explained under the --- cut off line?

regards,
dan carpenter