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From:   Ramana Radhakrishnan <ramana.gcc@googlemail.com>
Date:   Fri, 3 Aug 2018 10:15:52 +0100
Message-ID: <CAJA7tRZbmnZq7RfvQeYEy_a1ZObWqpFpVdvgsXgsioQ3RyPMuA@mail.gmail.com>
Subject: Re: framebuffer corruption due to overlapping stp instructions on arm64
To:     Florian Weimer <fweimer@redhat.com>
Cc:     Andrew Pinski <pinskia@gmail.com>, mpatocka@redhat.com,
        Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will.deacon@arm.com>, linux@armlinux.org.uk,
        thomas.petazzoni@free-electrons.com,
        linux-arm-kernel@lists.infradead.org,
        LKML <linux-kernel@vger.kernel.org>,
        GNU C Library <libc-alpha@sourceware.org>
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On Fri, Aug 3, 2018 at 8:53 AM, Florian Weimer <fweimer@redhat.com> wrote:
> On 08/03/2018 09:11 AM, Andrew Pinski wrote:
>>
>> Yes fix Links not to use memcpy on the framebuffer.
>> It is undefined behavior to use device memory with memcpy.
>
>
> Some (de facto) ABIs require that it is supported, though.  For example, the
> POWER string functions avoid unaligned loads and stores for this reason
> because the platform has the same issue with device memory.  And yes, GCC
> will expand memcpy on POWER to something that is incompatible with device
> memory. 8-(

GCC for AArch64 - use -mstrict-align
GCC for AArch32 - use -mno-unaligned-access.

If you see unaligned accesses coming out of the compiler for well
defined programs then that's a bug. Frequently we see undefined
programs that get the compiler to produce traps - atleast one or 2
bugs a year  in  GCC .


>
> If we don't want people to use memcpy, we probably need to provide a
> credible alternative.

I believe a number of packages have rolled their own to take these
constraints into account
for AArch32, perhaps it needs to be expanded for AArch64 as well.

regards
Ramana

>
> Thanks,
> Florian