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Date:   Fri, 31 May 2019 11:55:53 +0200
From:   Oleg Nesterov <oleg@redhat.com>
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Jann Horn <jannh@google.com>,
        "Eric W . Biederman" <ebiederm@xmission.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Kees Cook <keescook@chromium.org>,
        David Howells <dhowells@redhat.com>,
        kernel list <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] ptrace: restore smp_rmb() in __ptrace_may_access()
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On 05/31, Peter Zijlstra wrote:
>
> 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,

Yes,

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

and it is still not clear to me if __ptrace_may_acess() has a control
dependency or not,

		if (uid_eq(caller_uid, tcred->euid) && ...)
			goto ok;
		retuurn;

	ok:
		// provide LOAD->LOAD
		smp_acquire__after_ctrl_dep();


again, again, I didn't suggest to change the patch, I was just curious
if it would be correct or not.

Oleg.