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Date:   Mon, 15 Jun 2020 09:03:20 -0700
From:   Sultan Alsawaf <sultan@kerneltoast.com>
To:     Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Cc:     Aaron Ma <aaron.ma@canonical.com>,
        Benjamin Tissoires <benjamin.tissoires@redhat.com>,
        Hans de Goede <hdegoede@redhat.com>,
        HungNien Chen <hn.chen@weidahitech.com>,
        Jarkko Nikula <jarkko.nikula@linux.intel.com>,
        Jiri Kosina <jikos@kernel.org>,
        Kai-Heng Feng <kai.heng.feng@canonical.com>,
        linux-i2c@vger.kernel.org, linux-input@vger.kernel.org,
        linux-kernel@vger.kernel.org,
        Mika Westerberg <mika.westerberg@linux.intel.com>,
        Pavel Balan <admin@kryma.net>, Tin Huynh <tnhuynh@apm.com>,
        Wolfram Sang <wsa@kernel.org>,
        You-Sheng Yang <vicamo.yang@canonical.com>
Subject: Re: [PATCH 1/2] i2c: designware: Only check the first byte for SMBus
 block read length
Message-ID: <20200615160320.GA1949@sultan-book.localdomain>
References: <20200614210255.4641-1-sultan@kerneltoast.com>
 <20200614210255.4641-2-sultan@kerneltoast.com>
 <20200615094019.GP2428291@smile.fi.intel.com>
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On Mon, Jun 15, 2020 at 12:40:19PM +0300, Andy Shevchenko wrote:
> On Sun, Jun 14, 2020 at 02:02:54PM -0700, Sultan Alsawaf wrote:
> > From: Sultan Alsawaf <sultan@kerneltoast.com>
> > 
> > SMBus block reads can be broken because the read function will just skip
> > over bytes it doesn't like until reaching a byte that conforms to the
> > length restrictions for block reads. This is problematic when it isn't
> > known if the incoming payload is indeed a conforming block read.
> > 
> > According to the SMBus specification, block reads will only send the
> > payload length in the first byte, so we can fix this by only considering
> > the first byte in a sequence for block read length purposes.
> 
> I'm wondering if this overlaps with [1]. AFAIU that one is also makes sure that
> the length is not a garbage.
> 
> [1]: https://lore.kernel.org/linux-i2c/20200613104109.2989-1-mans@mansr.com/T/#u

No overlap. That looks like a similar bug for a different driver. In my case,
the adapter provides native SMBus support, so emulation is never used. This is
clear to see by looking at i2c_transfer_buffer_flags(), which only uses the
master_xfer functions provided by the adapter; it doesn't call the emulation
path at all.

Sultan