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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Wed, 22 Dec 2021 17:44:19 -0800
Message-ID: <CAHk-=wgtFAA9SbVYg0gR1tqPMC17-NYcs0GQkaYg1bGhh1uJQQ@mail.gmail.com>
Subject: Re: [PATCH 09/10] kthread: Ensure struct kthread is present for all kthreads
To:     "Eric W. Biederman" <ebiederm@xmission.com>
Cc:     Nathan Chancellor <nathan@kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        linux-arch <linux-arch@vger.kernel.org>,
        Alexey Gladkov <legion@kernel.org>,
        Kyle Huey <me@kylehuey.com>, Oleg Nesterov <oleg@redhat.com>,
        Kees Cook <keescook@chromium.org>,
        Al Viro <viro@zeniv.linux.org.uk>,
        Heiko Carstens <hca@linux.ibm.com>,
        Vasily Gorbik <gor@linux.ibm.com>,
        Christian Borntraeger <borntraeger@de.ibm.com>,
        Alexander Gordeev <agordeev@linux.ibm.com>,
        Martin Schwidefsky <schwidefsky@de.ibm.com>
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On Wed, Dec 22, 2021 at 3:25 PM Eric W. Biederman <ebiederm@xmission.com> wrote:
>
> Solve this by skipping the put_user for all kthreads.

Ugh.

While this fixes the problem, could we please just not mis-use that
'set_child_tid' as that kthread pointer any more?

It was always kind of hacky. I think a new pointer with the proper
'struct kthread *' type would be an improvement.

One of the "arguments" in the comment for re-using that set_child_tid
pointer was that 'fork()' used to not wrongly copy it, but your patch
literally now does that "allocate new kthread struct" at fork-time, so
that argument is actually bogus now.

              Linus