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

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Date:   Sun, 25 Feb 2018 02:34:36 +0300
From:   Dan Carpenter <dan.carpenter@...cle.com>
To:     "Luis R. Rodriguez" <mcgrof@...nel.org>
Cc:     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 10:06:01PM +0000, Luis R. Rodriguez wrote:
> 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?

If it's INT_MAX we never do "num_test_devs + 1", we return a NULL.

regards,
dan carpenter