linux-kernel - Re: HAVE_EFFICIENT_UNALIGNED_ACCESS on ARM32 (was: Alignment issues in zImage with Linux 4.12, LZ4 and GCC5.3)

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <CAK8P3a1xX5RfM1kao7WZBmS=0J=MgNZTH2_c-nSOY+GNmjNsUA@mail.gmail.com>
Date:   Thu, 7 Sep 2017 00:38:03 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Ard Biesheuvel <ard.biesheuvel@...aro.org>
Cc:     Romain Izard <romain.izard.pro@...il.com>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Sven Schmidt <4sschmid@...ormatik.uni-hamburg.de>
Subject: Re: HAVE_EFFICIENT_UNALIGNED_ACCESS on ARM32 (was: Alignment issues
 in zImage with Linux 4.12, LZ4 and GCC5.3)

On Thu, Sep 7, 2017 at 12:23 AM, Ard Biesheuvel
<ard.biesheuvel@...aro.org> wrote:
> On 6 September 2017 at 21:57, Arnd Bergmann <arnd@...db.de> wrote:
>> On Mon, Sep 4, 2017 at 6:19 PM, Romain Izard <romain.izard.pro@...il.com> wrote:
>
> HAVE_EFFICIENT_UNALIGNED_ACCESS only affects explicit unaligned
> accesses, and selects between fixups in hardware or in software.
> AFAICT the issue here is implicit unaligned accesses, where char
> pointers are passed as u32 * arguments.

The problem with include/linux/unaligned/access_ok.h is that it
converts pointers
that are known by the caller to be potentially unaligned and accesses them as if
they were aligned. This means we require a software fixup through the
trap handler
on ARM in cases that the compiler already knows how to handle correctly when
using linux/unaligned/le_struct.h. On ARMv7 this means it ends up using normal
load/store instructures but not the ldm/stm or ldrd/stdr instructions
that are not
allowed on unaligned pointers.

Doing that solves the problem that Romain ran into and also makes other
code much more efficient on ARMv7.

       Arnd