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Date: Mon, 30 Oct 2017 17:33:22 -0400
From: Steven Rostedt <rostedt@goodmis.org>
To: "Tobin C. Harding" <me@tobin.cc>
Cc: kernel-hardening@lists.openwall.com,
        "Jason A. Donenfeld" <Jason@zx2c4.com>,
        "Theodore Ts'o" <tytso@mit.edu>,
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        Tycho Andersen <tycho@docker.com>,
        "Roberts, William C" <william.c.roberts@intel.com>,
        Tejun Heo <tj@kernel.org>,
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        Dave Weinstein <olorin@google.com>,
        Daniel Micay <danielmicay@gmail.com>,
        Djalal Harouni <tixxdz@gmail.com>, linux-kernel@vger.kernel.org
Subject: Re: [PATCH V8 2/2] printk: hash addresses printed with %p
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On Thu, 26 Oct 2017 13:58:38 +1100
"Tobin C. Harding" <me@tobin.cc> wrote:

> > +static bool have_filled_random_ptr_key;
> > +static siphash_key_t ptr_key __read_mostly;
> > +
> > +static void fill_random_ptr_key(struct random_ready_callback *unused)
> > +{
> > +	get_random_bytes(&ptr_key, sizeof(ptr_key));
> > +	WRITE_ONCE(have_filled_random_ptr_key, true);  
> 
> This usage of WRITE_ONCE was suggested by Jason A. Donenfeld. I read
> include/linux/compiler.h but was not able to grok it. Is this enough to
> stop the compiler re-ordering these two statements? 
> 
> Or do I need to read Documentation/memory-barriers.txt [again]?

No, the WRITE_ONCE does not stop the compiler from reordering those
statements. If you need that, then you need to do:

	get_random_bytes(&ptr_key, sizeof(ptr_key));
	barrier();
	WRITE_ONCE(have_filled_random_ptr_key, true);

and that only works against interrupts. If you need synchronization
across CPUs, then you need smp_mb().

-- Steve