Hello Kuniyuki,
On Wed, Jun 07, 2023 at 10:31:42AM -0700, Kuniyuki Iwashima wrote:

quoted

+/* 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