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Date: Wed, 25 Nov 2015 12:05:25 +0100
From: "PaX Team" <>
        Mathias Krause <>
CC: "" <>,
        Kees Cook <>,
        Andy Lutomirski <>, Ingo Molnar <>,
        Thomas Gleixner <>, "H. Peter Anvin" <>,
        x86-ml <>, Arnd Bergmann <>,
        Michael Ellerman <>,,
        Emese Revfy <>
Subject: Re: [PATCH 0/2] introduce post-init read-only memory

On 25 Nov 2015 at 10:13, Mathias Krause wrote:

> 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.

actually the kernel could silently recover from this given how the
page fault handler could easily determine that the fault address fell
into the data..read_only section and just silently undo the read-only
property, log the event to dmesg and retry the faulting access.

> 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.

this is exactly what i suggested earlier in the constify thread ;).
note that this will produce false positives because __init* annotations
are not propagated everywhere they could be.

> 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.

one of our plans for initify is to add the discovery and propagation
of _init* annotations as well, it'd not only fix the false positives
mentioned above but also help reduce the kernel size (code/data/rodata).

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