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

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Message-ID: <20190531091245.GN2677@hirez.programming.kicks-ass.net>
Date:   Fri, 31 May 2019 11:12:45 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Oleg Nesterov <oleg@...hat.com>
Cc:     Jann Horn <jannh@...gle.com>,
        "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 Thu, May 30, 2019 at 02:05:31PM +0200, Oleg Nesterov wrote:
> > 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.

So it hard relies on being part of a control dependency, note how the
comment says that architectures that do not do load speculation can
override the smp_rmb() default with barrier() (and we used have an
architecture that made use of that, although it's been since removed).

IOW, it is an error to use smp_acquire__after_ctrl_dep() without an
(immediate) preceding branch.