linux-kernel - Re: [PATCH] x86/boot/compressed/64: Define __force_order only when CONFIG_RANDOMIZE

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Message-ID: <201912301033.84517BA05@keescook>
Date:   Mon, 30 Dec 2019 10:34:07 -0800
From:   Kees Cook <keescook@...omium.org>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     "Kirill A. Shutemov" <kirill@...temov.name>,
        Khem Raj <raj.khem@...il.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        "H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>, x86-ml <x86@...nel.org>
Subject: Re: [PATCH] x86/boot/compressed/64: Define __force_order only when
 CONFIG_RANDOMIZE_BASE is unset

On Mon, Dec 30, 2019 at 01:12:31PM +0100, Arnd Bergmann wrote:
> On Tue, Dec 24, 2019 at 12:08 AM Kirill A. Shutemov
> <kirill@...temov.name> wrote:
> > On Mon, Dec 23, 2019 at 02:25:02PM -0800, Khem Raj wrote:
> > > On Mon, Dec 23, 2019 at 9:10 AM Kirill A. Shutemov <kirill@...temov.name> wrote:
> > > >
> > > > On Sat, Dec 21, 2019 at 07:18:13AM -0800, Khem Raj wrote:
> > > > > Since arch/x86/boot/compressed/Makefile overrides global CFLAGS it loses
> > > > > -fno-common option which would have caught this
> > > >
> > > > If this doesn't cause any visible problems, why bother?
> > > >
> > >
> > > it does break builds with gcc trunk as of now e.g.
> > >
> > > > Hopefully, we will be able to drop it altogether once we ditch GCC 4.X
> > > > support.
> > > >
> > >
> > > gcc10 is switching defaults to -fno-common so we need to solve this one way or
> > > other, I am not sure if gcc 4.x will be dropped before gcc10 release
> > > which would be
> > > in mid of 2020
> >
> > Okay, it makes sense then. Please include this info into the commit
> > message.
> >
> > Also, I wounder if it would be cleaner to define both of them as __weak?
> 
> Or maybe make the #ifdef check for gcc < 5 instead of checking for
> CONFIG_RANDOMIZE_BASE? That way it will be found by whoever
> cleans up the code when we increase the minimum compiler
> version to one that doesn't require the hack.

I agree: that seems a better way to do this.

-- 
Kees Cook