linux-kernel - Re: [PATCH 1/2] i2c: designware: Only check the first byte for SMBus block read length

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Date:   Mon, 15 Jun 2020 09:03:20 -0700
From:   Sultan Alsawaf <sultan@...neltoast.com>
To:     Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Cc:     Aaron Ma <aaron.ma@...onical.com>,
        Benjamin Tissoires <benjamin.tissoires@...hat.com>,
        Hans de Goede <hdegoede@...hat.com>,
        HungNien Chen <hn.chen@...dahitech.com>,
        Jarkko Nikula <jarkko.nikula@...ux.intel.com>,
        Jiri Kosina <jikos@...nel.org>,
        Kai-Heng Feng <kai.heng.feng@...onical.com>,
        linux-i2c@...r.kernel.org, linux-input@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Mika Westerberg <mika.westerberg@...ux.intel.com>,
        Pavel Balan <admin@...ma.net>, Tin Huynh <tnhuynh@....com>,
        Wolfram Sang <wsa@...nel.org>,
        You-Sheng Yang <vicamo.yang@...onical.com>
Subject: Re: [PATCH 1/2] i2c: designware: Only check the first byte for SMBus
 block read length

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@...neltoast.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