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Date: Wed, 25 Nov 2015 09:31:05 -0800
From: "H. Peter Anvin" <>
To: Mathias Krause <>,
Cc: "" <>,
        Kees Cook <>,
        Andy Lutomirski <>, Ingo Molnar <>,
        Thomas Gleixner <>, x86-ml <>,
        Arnd Bergmann <>, Michael Ellerman <>,, PaX Team <>,
        Emese Revfy <>
Subject: Re: [PATCH 0/2] introduce post-init read-only

On 11/25/15 01:13, Mathias Krause wrote:
> While having that annotation makes perfect sense, not only from a
> security perspective but also from a micro-optimization point of view
> (much like the already existing __read_mostly annotation), it has its
> drawbacks. Violating the "r/o after init" rule by writing to such
> annotated variables from non-init code goes unnoticed as far as it
> concerns the toolchain. Neither the compiler nor the linker will flag
> that incorrect use. It'll just trap at runtime and that's bad.
> I myself had some educating experience seeing my machine triple fault
> when resuming from a S3 sleep. The root cause was a variable that was
> annotated __read_only but that was (unnecessarily) modified during CPU
> bring-up phase. Debugging that kind of problems is sort of a PITA, you
> could imagine.
> So, prior extending the usage of the __read_only annotation some
> toolchain support is needed. Maybe a gcc plugin that'll warn/error on
> code that writes to such a variable but is not __init itself. The
> initify and checker plugins from the PaX patch might be worth to look
> at for that purpose, as they're doing similar things already. Adding
> such a check to sparse might be worth it, too.
> A modpost check probably won't work as it's unable to tell if it's a
> legitimate access (r/o) or a violation (/w access). So the gcc plugin
> is the way to go, IMHO.

We should not wait for compile-time support, that doesn't make any
sense.  What would be useful would be a way to override this on the
command line -- that way, if disabling RO or RO-after-init memory makes
something work, we have an instant diagnosis.


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