Messages in this thread | | | Date | Tue, 15 Mar 2022 17:19:05 +0300 | From | Dan Carpenter <> | Subject | Re: Re: [PATCH net V4 1/2] ax25: Fix refcount leaks caused by ax25_cb_del() |
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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: > > 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. > > > 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
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