linux-kernel - Re: [PATCH] firmware: dmi: Check DMI structure length

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Date:   Sat, 3 Jun 2017 23:14:23 +0200
From:   Jean Delvare <jdelvare@...e.de>
To:     Andy Shevchenko <andy.shevchenko@...il.com>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Dmitry Torokhov <dmitry.torokhov@...il.com>,
        Mika Westerberg <mika.westerberg@...ux.intel.com>,
        Linus Walleij <linus.walleij@...aro.org>
Subject: Re: [PATCH] firmware: dmi: Check DMI structure length

On Fri, 2 Jun 2017 21:45:37 +0300, Andy Shevchenko wrote:
> On Fri, Jun 2, 2017 at 9:40 PM, Jean Delvare <jdelvare@...e.de> wrote:
> > On Thu, 1 Jun 2017 19:06:36 +0300, Andy Shevchenko wrote:  
> >> Your commit message should answer to the question why and what.
> >> You didn't put it there.
> >> Moreover, the change above per se doesn't belong to this — one logical
> >> change per patch.  
> >
> > I'm confused. These changes totally belong to this patch. They belong
> > so much to it, that's the very reason why they are not described
> > separately in the commit message.
> >
> > The purpose of the patch is to check that the records are large enough
> > to contain the fields we need to access. Setting a pointer beyond the
> > end of the record _before_ performing that check makes no sense.
> >
> > I did not include these changes as performance optimizations, I
> > included them because they make the code conceptually correct. It's
> > even clearer for the last instance, as we are dereferencing the pointer
> > immediately, but in my opinion, even setting a pointer to a location
> > which may not exist is equally wrong and confusing for the reader.
> > That's why I moved that code after the length checks.  
> 
> You are talking here explicitly about third case which I agreed on.
> 
> The two first ones are not the same.
> You didn't dereference them before check since your check is not
> against pointer.
>
> So, basically it means you are checking pointer _indirectly_.

Correct.

> I think we already spent too much time on this one.

Agreed.

> If you wish to leave your changes, update commit message accordingly.

No.

-- 
Jean Delvare
SUSE L3 Support