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

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Message-ID: <20180808142852.GD24736@iMac.local>
Date:   Wed, 8 Aug 2018 15:28:53 +0100
From:   Catalin Marinas <catalin.marinas@....com>
To:     Mikulas Patocka <mpatocka@...hat.com>
Cc:     Thomas Petazzoni <thomas.petazzoni@...e-electrons.com>,
        Joao Pinto <Joao.Pinto@...opsys.com>,
        libc-alpha@...rceware.org,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Jingoo Han <jingoohan1@...il.com>,
        Will Deacon <will.deacon@....com>,
        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 Wed, Aug 08, 2018 at 10:12:27AM -0400, 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.

It could get it wrong between subsequent writes to the same 64-bit range
(e.g. the address & ~63 is the same but the data strobes for which bytes
to write are different). If it somehow thinks that it's a
write-after-write hazard even though the strobes are different, it could
cancel one of the writes.

It may be worth trying with a byte-only memcpy() function while keeping
the default memcmp().

-- 
Catalin