From: Dan Carpenter <dan.carpenter@oracle.com>
To: 周多明 <duoming@zju.edu.cn>
Cc: linux-hams@vger.kernel.org, netdev@vger.kernel.org,
linux-kernel@vger.kernel.org, kuba@kernel.org,
davem@davemloft.net, ralf@linux-mips.org, jreuter@yaina.de,
thomas@osterried.de
Subject: Re: Re: [PATCH net V4 1/2] ax25: Fix refcount leaks caused by ax25_cb_del()
Date: Tue, 15 Mar 2022 17:19:05 +0300 [thread overview]
Message-ID: <20220315141905.GB1841@kadam> (raw)
In-Reply-To: <15e4111b.5339.17f8deb1f24.Coremail.duoming@zju.edu.cn>
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
next prev parent reply other threads:[~2022-03-15 14:19 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-03-15 1:54 [PATCH net V4 1/2] ax25: Fix refcount leaks caused by ax25_cb_del() Duoming Zhou
2022-03-15 10:26 ` Dan Carpenter
2022-03-15 14:11 ` 周多明
2022-03-15 14:19 ` Dan Carpenter [this message]
2022-03-15 14:33 ` 周多明
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=20220315141905.GB1841@kadam \
--to=dan.carpenter@oracle.com \
--cc=davem@davemloft.net \
--cc=duoming@zju.edu.cn \
--cc=jreuter@yaina.de \
--cc=kuba@kernel.org \
--cc=linux-hams@vger.kernel.org \
--cc=linux-kernel@vger.kernel.org \
--cc=netdev@vger.kernel.org \
--cc=ralf@linux-mips.org \
--cc=thomas@osterried.de \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).