linux-kernel - Re: [BUG] Use of probe_kernel_address() in task_rcu_dereference() without checking return value

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Date:   Mon, 2 Sep 2019 10:34:46 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>
Cc:     Oleg Nesterov <oleg@...hat.com>,
        Russell King - ARM Linux admin <linux@...linux.org.uk>,
        Peter Zijlstra <peterz@...radead.org>,
        Chris Metcalf <cmetcalf@...hip.com>,
        Christoph Lameter <cl@...ux.com>,
        Kirill Tkhai <tkhai@...dex.ru>, Mike Galbraith <efault@....de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>,
        Linux List Kernel Mailing <linux-kernel@...r.kernel.org>
Subject: Re: [BUG] Use of probe_kernel_address() in task_rcu_dereference()
 without checking return value

On Mon, Sep 2, 2019 at 10:04 AM Eric W. Biederman <ebiederm@...ssion.com> wrote:
>
> I like using the storage we will later use for the rcu_head.
>
> Is the intention your new variable xxx start as 0, and the only
> on the second write it becomes 1 and we take action?
>
> That should work but it is a funny way to encode a decrement.  I think
> it would be more straight forward to use refcount_dec_and_test.
>
> So something like this:

I like how this patch looks. It makes more sense to me than some of
the ad-hoc cases, and I wonder if this might be a pattern in general.

We have a very different "some users don't need RCU" in the dentry
code, and recently in the credential handling code. So I wonder if
this is a larger pattern, but I think your patch looks good
independently on its own.

But this is all based on "that patch _feels_ conceptually right",
rather than any deep thinking or (God forbid) any actual testing.

                Linus