linux-kernel - Re: [PATCH] mm: fix RODATA_TEST failure "rodata

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Message-ID: <CAGXu5j+bj1NHmrrEmcwPavKubavLh1b01AyyJEFRvycg9FkLpg@mail.gmail.com>
Date:   Mon, 2 Oct 2017 12:29:45 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Segher Boessenkool <segher@...nel.crashing.org>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Michael Ellerman <mpe@...erman.id.au>
Cc:     David Laight <David.Laight@...lab.com>,
        Linux-MM <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Jinbum Park <jinb.park7@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>
Subject: Re: [PATCH] mm: fix RODATA_TEST failure "rodata_test: test data was
 not read only"

On Mon, Sep 25, 2017 at 12:41 PM, Segher Boessenkool
<segher@...nel.crashing.org> wrote:
> On Mon, Sep 25, 2017 at 04:01:55PM +0000, David Laight wrote:
>> From: Segher Boessenkool
>> > The compiler puts this item in .sdata, for 32-bit.  There is no .srodata,
>> > so if it wants to use a small data section, it must use .sdata .
>> >
>> > Non-external, non-referenced symbols are not put in .sdata, that is the
>> > difference you see with the "static".
>> >
>> > I don't think there is a bug here.  If you think there is, please open
>> > a GCC bug.
>>
>> The .sxxx sections are for 'small' data that can be accessed (typically)
>> using small offsets from a global register.
>> This means that all sections must be adjacent in the image.
>> So you can't really have readonly small data.
>>
>> My guess is that the linker script is putting .srodata in with .sdata.
>
> .srodata does not *exist* (in the ABI).

So, I still think this is a bug. The variable is marked const: this is
not a _suggestion_. :) If the compiler produces output where the
variable is writable, that's a bug.

I can't tell if this bug is related:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=9571

-Kees

-- 
Kees Cook
Pixel Security