linux-kernel - Re: [PATCH] lib/test_kmod: Fix an integer overflow test

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Message-ID: <20180224220601.GG14069@wotan.suse.de>
Date:   Sat, 24 Feb 2018 22:06:01 +0000
From:   "Luis R. Rodriguez" <mcgrof@...nel.org>
To:     Dan Carpenter <dan.carpenter@...cle.com>
Cc:     "Luis R. Rodriguez" <mcgrof@...nel.org>,
        linux-kernel@...r.kernel.org, kernel-janitors@...r.kernel.org,
        cocci@...teme.lip6.fr
Subject: Re: [PATCH] lib/test_kmod: Fix an integer overflow test

On Sat, Feb 24, 2018 at 11:45:16AM +0300, Dan Carpenter wrote:
> On Sat, Feb 24, 2018 at 02:59:41AM +0000, Luis R. Rodriguez wrote:
> > On Mon, Jan 22, 2018 at 01:27:54PM +0300, Dan Carpenter wrote:
> > > The main problem is that the parentheses are in the wrong place and the
> > > unlikely() call returns either 0 or 1 so it's never less than zero.
> > 
> > Doh, thanks, yes. Seems worth considering a grammar rule for it.
> > 
> > > The other problem is that signed integer overflows like "INT_MAX + 1" are
> > > undefined behavior.
> > 
> > Likewise.
> > 
> > This seems like another possible generic typo issue. But I would not resolve it
> > the way you did, in this particular case below num_test_devs represents the
> > number of already registered devs, before we increment. So the way to resolve
> > this would be:
> > 
> > 	if (num_test_devs + 1 == INT_MAX)
> > 
> > I'll get this upstream, thanks!
> 
> There is no issue if num_test_devs is INT_MAX.  But capping it at
> INT_MAX - 1 is also fine.

If num_test_devs is INT_MAX, then doing num_test_devs + 1 overflows
and as you noted that is undefined?

  Luis