linux-kernel - Re: [PATCH] ARM: add a private asm/unaligned.h

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Message-ID: <20171030155029.GP20805@n2100.armlinux.org.uk>
Date:   Mon, 30 Oct 2017 15:50:30 +0000
From:   Russell King - ARM Linux <linux@...linux.org.uk>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Gregory CLEMENT <gregory.clement@...e-electrons.com>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Romain Izard <romain.izard.pro@...il.com>,
        Sven Schmidt <4sschmid@...ormatik.uni-hamburg.de>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Petr Cvek <petrcvekcz@...il.com>,
        Aaro Koskinen <aaro.koskinen@....fi>,
        Andrea Adami <andrea.adami@...il.com>,
        Robert Jarzmik <robert.jarzmik@...e.fr>
Subject: Re: [PATCH] ARM: add a private asm/unaligned.h

On Mon, Oct 30, 2017 at 04:09:43PM +0100, Arnd Bergmann wrote:
> On Fri, Oct 27, 2017 at 5:27 PM, Russell King - ARM Linux
> <linux@...linux.org.uk> wrote:
> 
> >
> > There's three things wrong, all of which I have patches to address:
> >
> > 1. The decompressor code reading the image data sometimes issues unaligned
> >    reads.  Some compilers get this wrong and cause an abort.  Arnds patch
> >    addresses this.
> >
> > 2. Additional sections can appear in the zImage binary which adds extra
> >    bytes on the end of the image.  Concatenating the zImage with the
> >    extra bytes onto a DTB is the same thing as doing this:
> >
> >         cat zImage extrabytes foo.dtb > image
> >
> >    and the decompressor tolerates no additional bytes between the
> >    _official_ end of the zImage and the DTB.  I've added a patch which
> >    detects this situation and fails the kernel build when it happens.
> >
> > 3. Ard's patch "efi/libstub: arm: omit sorting of the UEFI memory map"
> >    gets rid of the additional sections that (a) change the alignment
> >    of the compressed data, and (b) add additional unexpected bytes on
> >    the end of zImage.
> 
> It's possible that we still need yet another patch to address the gcc bug that
> Alex Graf found, i.e. https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82445
> 
> Without the latest gcc, we might still get into a situation in which we get
> an unaligned strd when compiling for armv5te or armv6 with gcc-7.[012].
> As someone mentioned in the bug report, that problem doesn't seem
> to happen with gcc-6 or -mtune=xscale, or with gcc-7 -march=armv7-a.

There is another solution which we've used in the past - we could detect
these compiler versions and refuse to build with the broken compilers.

-- 
RMK's Patch system: http://www.armlinux.org.uk/developer/patches/
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