linux-kernel - Re: [PATCH net-next v6] net: ioctl: Use kernel memory on protocol ioctl callbacks

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Date:   Thu, 8 Jun 2023 01:43:13 -0700
From:   Breno Leitao <leitao@...ian.org>
To:     Kuniyuki Iwashima <kuniyu@...zon.com>
Cc:     alex.aring@...il.com, andrea.righi@...onical.com,
        asml.silence@...il.com, ast@...nel.org, axboe@...nel.dk,
        courmisch@...il.com, davem@...emloft.net, dccp@...r.kernel.org,
        dsahern@...nel.org, edumazet@...gle.com, gnault@...hat.com,
        hbh25y@...il.com, joannelkoong@...il.com, kernelxing@...cent.com,
        kuba@...nel.org, leit@...com, linux-kernel@...r.kernel.org,
        linux-sctp@...r.kernel.org, linux-wpan@...r.kernel.org,
        lucien.xin@...il.com, marcelo.leitner@...il.com,
        martin.lau@...nel.org, martineau@...nel.org,
        matthieu.baerts@...sares.net, miquel.raynal@...tlin.com,
        mptcp@...ts.linux.dev, netdev@...r.kernel.org, pabeni@...hat.com,
        stefan@...enfreihafen.org, willemdebruijn.kernel@...il.com,
        wojciech.drewek@...el.com
Subject: Re: [PATCH net-next v6] net: ioctl: Use kernel memory on protocol
 ioctl callbacks

Hello Kuniyuki,
On Wed, Jun 07, 2023 at 10:31:42AM -0700, Kuniyuki Iwashima wrote:
> > +/* This is the most common ioctl prep function, where the result (4 bytes) is
> > + * copied back to userspace if the ioctl() returns successfully. No input is
> > + * copied from userspace as input argument.
> > + */
> > +static int sock_ioctl_out(struct sock *sk, unsigned int cmd, void __user *arg)
> > +{
> > +	int ret, karg = 0;
> > +
> > +	ret = sk->sk_prot->ioctl(sk, cmd, &karg);
> 
> We need READ_ONCE(sk->sk_prot) as IPv4 conversion or ULP chnage could
> occur at the same time.

Thanks for the heads-up. I would like to pick you brain and understand
a bit more about READ_ONCE() and what is the situation that READ_ONCE()
will solve.

Is the situation related to when sock_ioctl_out() start to execute, and
"sk->sk_prot" changes in a different thread? If that is the case, the
arguments (cmd and arg) will be from the "previous" instance.

Also, grepping for "sk->sk_prot->", I see more than a bunch of calls
that do not use READ_ONCE() barrier. Why is this case different?

Thank you