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

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Message-ID: <CAK8P3a0_-YOvPUqZTb+LoU+wCBhy=zCxmk4HProPJY+udbri9Q@mail.gmail.com>
Date:   Mon, 30 Oct 2017 16:09:43 +0100
From:   Arnd Bergmann <arnd@...db.de>
To:     Russell King - ARM Linux <linux@...linux.org.uk>
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 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.

       Arnd