linux-kernel - Re: framebuffer corruption due to overlapping stp instructions on arm64

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Message-ID: <b1a0dac5-3cf2-1a7f-ac7f-649126eb7873@arm.com>
Date:   Wed, 8 Aug 2018 16:01:12 +0100
From:   "Richard Earnshaw (lists)" <Richard.Earnshaw@....com>
To:     Mikulas Patocka <mpatocka@...hat.com>,
        Catalin Marinas <catalin.marinas@....com>
Cc:     Will Deacon <will.deacon@....com>,
        Jingoo Han <jingoohan1@...il.com>,
        Joao Pinto <Joao.Pinto@...opsys.com>,
        Thomas Petazzoni <thomas.petazzoni@...e-electrons.com>,
        libc-alpha@...rceware.org,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Russell King <linux@...linux.org.uk>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Matt Sealey <neko@...uhatsu.net>, linux-pci@...r.kernel.org,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>
Subject: Re: framebuffer corruption due to overlapping stp instructions on
 arm64

On 08/08/18 15:12, Mikulas Patocka wrote:
> 
> 
> On Wed, 8 Aug 2018, Catalin Marinas wrote:
> 
>> On Fri, Aug 03, 2018 at 01:09:02PM -0400, Mikulas Patocka wrote:
>>> 	while (1) {
>>> 		start = (unsigned)random() % (LEN + 1);
>>> 		end = (unsigned)random() % (LEN + 1);
>>> 		if (start > end)
>>> 			continue;
>>> 		for (i = start; i < end; i++)
>>> 			data[i] = val++;
>>> 		memcpy(map + start, data + start, end - start);
>>> 		if (memcmp(map, data, LEN)) {
>>
>> It may be worth trying to do a memcmp(map+start, data+start, end-start)
>> here to see whether the hazard logic fails when the writes are unaligned
>> but the reads are not.
>>
>> This problem may as well appear if you do byte writes and read longs
>> back (and I consider this a hardware problem on this specific board).
> 
> I triad to insert usleep(10000) between the memcpy and memcmp, but the 
> same corruption occurs. So, it can't be read-after-write hazard. It is 
> caused by the improper handling of hazard between the overlapping writes 
> inside memcpy.
> 
> Mikulas
> 

I don't think you've told us what form the corruption takes.  Does it
lose some bytes?  Modify values beyond the copy range?  Write completely
arbitrary values?

The overlapping writes in memcpy never write different values to the
same location, so I still feel this must be some sort of HW issue, not a
SW one.

R.