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

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Date:   Mon, 25 Sep 2017 02:37:21 -0500
From:   Segher Boessenkool <segher@...nel.crashing.org>
To:     Kees Cook <keescook@...omium.org>
Cc:     Christophe Leroy <christophe.leroy@....fr>,
        Michael Ellerman <mpe@...erman.id.au>,
        Jinbum Park <jinb.park7@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>
Subject: Re: [PATCH] mm: fix RODATA_TEST failure "rodata_test: test data was not read only"

On Sun, Sep 24, 2017 at 12:17:51PM -0700, Kees Cook wrote:
> On Thu, Sep 21, 2017 at 2:37 AM, Christophe Leroy
> <christophe.leroy@....fr> wrote:
> > On powerpc, RODATA_TEST fails with message the following messages:
> >
> > [    6.199505] Freeing unused kernel memory: 528K
> > [    6.203935] rodata_test: test data was not read only
> >
> > This is because GCC allocates it to .data section:
> >
> > c0695034 g     O .data  00000004 rodata_test_data
> 
> Uuuh... that seems like a compiler bug. It's marked "const" -- it
> should never end up in .data. I would argue that this has done exactly
> what it was supposed to do, and shows that something has gone wrong.
> It should always be const. Adding "static" should just change
> visibility. (I'm not opposed to the static change, but it seems to
> paper over a problem with the compiler...)

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.


Segher