Re: [bug report] mtd: rawnand: add NVIDIA Tegra NAND Flash controller driver

[Boris Brezillon <boris.brezillon@bootlin.com>]

On Tue, 3 Jul 2018 17:19:57 +0300
Dan Carpenter <dan.carpenter@oracle.com> wrote:

> Hello Stefan Agner,
> 
> The patch d7d9f8ec77fe: "mtd: rawnand: add NVIDIA Tegra NAND Flash
> controller driver" from Jun 24, 2018, leads to the following static
> checker warning:
> 
> 	drivers/mtd/nand/raw/tegra_nand.c:476 tegra_nand_select_chip()
> 	warn: array off by one? 'nand->cs[die_nr]'
> 
> drivers/mtd/nand/raw/tegra_nand.c
>    465  static void tegra_nand_select_chip(struct mtd_info *mtd, int die_nr)
>    466  {
>    467          struct nand_chip *chip = mtd_to_nand(mtd);
>    468          struct tegra_nand_chip *nand = to_tegra_chip(chip);
>    469          struct tegra_nand_controller *ctrl = to_tegra_ctrl(chip->controller);
>    470  
>    471          if (die_nr < 0 || die_nr > 1) {
>    472                  ctrl->cur_cs = -1;
>    473                  return;
>    474          }
>    475  
>    476          ctrl->cur_cs = nand->cs[die_nr];
>    477  }
> 
> The story is that nand->cs[] is a one element array.  Some people use
> one element arrays like this as variable size arrays.  It's better to
> use a zero size array, but I think that might be a GCC feature and not
> everyone knows you can do that.  Smatch treats this one as unknown size
> because apparently it can't tie it back to the kmalloc().
> 
> But it really is a one element array and the condition is off by one.

I don't see where it's off by one? With the above test, die_nr is
guaranteed to be 0 when you reach the
"ctrl->cur_cs = nand->cs[die_nr];" statement, right? Am I missing
something?

> 
> But really one element arrays are super weird.  Why not just use a
> pointer?

The controller supports more than 1 CS, and I guess the plan was to
extend the array when the driver is ready to support this use case. I
guess we could make ->cs a single integer instead of an array of size
1 if that helps.