linux-kernel - Re: [PATCH] iio: potentiometer: ds1803: Remove VLA usage

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Date:   Fri, 9 Mar 2018 16:35:10 +0530
From:   Himanshu Jha <himanshujha199640@...il.com>
To:     Kees Cook <keescook@...omium.org>
Cc:     jic23@...nel.org, Hartmut Knaack <knaack.h@....de>,
        Lars-Peter Clausen <lars@...afoo.de>,
        Peter Meerwald-Stadler <pmeerw@...erw.net>,
        linux-iio@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>,
        "Gustavo A. R. Silva" <garsilva@...eddedor.com>
Subject: Re: [PATCH] iio: potentiometer: ds1803: Remove VLA usage

On Thu, Mar 08, 2018 at 11:39:15AM -0800, Kees Cook wrote:
> On Thu, Mar 8, 2018 at 10:45 AM, Himanshu Jha
> <himanshujha199640@...il.com> wrote:
> > In preparation to enabling -Wvla, remove VLA usage and replace it
> > with fixed a fixed length array and therefore, prevent potential
> > stack overflow attacks.
> >
> > Fixed as a part of the discussion to remove all VLAs from the kernel:
> > https://lkml.org/lkml/2018/3/7/621
> >
> > Cc: keescook@...omium.org
> > Signed-off-by: Himanshu Jha <himanshujha199640@...il.com>
> > ---
> >  drivers/iio/potentiometer/ds1803.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/drivers/iio/potentiometer/ds1803.c b/drivers/iio/potentiometer/ds1803.c
> > index 9b0ff4a..6bf12c9 100644
> > --- a/drivers/iio/potentiometer/ds1803.c
> > +++ b/drivers/iio/potentiometer/ds1803.c
> > @@ -64,7 +64,7 @@ static int ds1803_read_raw(struct iio_dev *indio_dev,
> >         struct ds1803_data *data = iio_priv(indio_dev);
> >         int pot = chan->channel;
> >         int ret;
> > -       u8 result[indio_dev->num_channels];
> > +       u8 result[ARRAY_SIZE(ds1803_channels)];
> 
> It seems like num_channels is always ARRAY_SIZE(ds1803_channels).
> Could the entire field be dropped?

If you're asking to remove num_channels then certainly it is not
possible
since it is a member of industrial I/O device struct and it is not just
a member of regular struct local to this file.

We certainly know that there are only two channels BTW and it can be
tranformed to simply:

        u8 result[2];

But then I might have to add an additional comment explaining the magic
number 2.

-- 
Thanks
Himanshu Jha