From mboxrd@z Thu Jan  1 00:00:00 1970
From: David Miller <davem@davemloft.net>
Subject: Re: [PATCH net] ppp: ensure file->private_data can't be overridden
Date: Mon, 14 Mar 2016 14:57:59 -0400 (EDT)
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From: Guillaume Nault <g.nault@alphalink.fr>
Date: Mon, 14 Mar 2016 18:59:40 +0100

> Testing ->private_data without lock in ppp_ioctl() before calling
> ppp_unattached_ioctl() is fine, because either ->private_data is
> not NULL and thus is stable, or it is and ppp_unattached_ioctl()
> takes care of not overriding ->private_data, should its value get
> modified before taking the mutex.

This is exactly the ambiguous behavior I want you to avoid.

The decision should be atomic from ppp_ioctl()'s test all the way
until ppp_unattached_ioctl() does it's work.