linux-kernel - Re: [PATCH] ptrace: restore smp_rmb() in __ptrace_may

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Message-ID: <20190530120531.GE22536@redhat.com>
Date:   Thu, 30 May 2019 14:05:31 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     Jann Horn <jannh@...gle.com>
Cc:     "Eric W . Biederman" <ebiederm@...ssion.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Kees Cook <keescook@...omium.org>,
        David Howells <dhowells@...hat.com>,
        kernel list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] ptrace: restore smp_rmb() in __ptrace_may_access()

On 05/29, Jann Horn wrote:
>
> > (I am wondering if smp_acquire__after_ctrl_dep() could be used instead, just to
> >  make this code look more confusing)
>
> Uuh, I had no idea that that barrier type exists. The helper isn't
> even explicitly mentioned in Documentation/memory-barriers.rst. I
> don't really want to use dark magic in the middle of ptrace access
> logic...

Yes. and if it was not clear I didn't try to seriously suggest to use this
barrier. I was just curious if it can be used or not in this particular case.

> Anyway, looking at it, I think smp_acquire__after_ctrl_dep() doesn't
> make sense here;

Well I still _think_ it should work, it provides the LOAD-LOAD ordering
and this is what we need.

But I can be easily wrong, and again, I wasn't serious.

Oleg.