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

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Message-ID: <20171002200805.GF8421@gate.crashing.org>
Date:   Mon, 2 Oct 2017 15:08:05 -0500
From:   Segher Boessenkool <segher@...nel.crashing.org>
To:     Kees Cook <keescook@...omium.org>
Cc:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        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, Oct 02, 2017 at 12:29:45PM -0700, Kees Cook wrote:
> 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.

C11 6.7.3/6: "If an attempt is made to modify an object defined with a
const-qualified type through use of an lvalue with non-const-qualified
type, the behavior is undefined."

And that is all that "const" means.

The compiler is free to put this var in *no* data section, or to copy
it to the stack before using it, or anything else it thinks is a good
idea.

If you think it would be a good idea for the compiler to change its
behaviour here, please file a PR (or send a patch).  Please bring
arguments why we would want to change this.

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

I don't think so: the only remaining bug there is that a copy of the
constant is put in .rodata.cst8 (although there is a copy in .sdata2
already).

Thanks,


Segher