Re: [PATCH] mdio-mux-gpio: Remove VLA usage

[Andrew Lunn]

On Tue, May 29, 2018 at 03:59:01PM -0700, Kees Cook wrote:
> In the quest to remove all stack VLA usage from the kernel[1], this
> allocates the values buffer during the callback instead of putting it
> on the stack.
> 
> [1] 
> https://lkml.kernel.org/r/CA+55aFzCG-zNmZwX4A2FQpadafLfEzK6CC=qpxydaacu1rq...@mail.gmail.com
> 
> Signed-off-by: Kees Cook <keesc...@chromium.org>
> ---
>  drivers/net/phy/mdio-mux-gpio.c | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/net/phy/mdio-mux-gpio.c b/drivers/net/phy/mdio-mux-gpio.c
> index 082ffef0dec4..e99994f4ed50 100644
> --- a/drivers/net/phy/mdio-mux-gpio.c
> +++ b/drivers/net/phy/mdio-mux-gpio.c
> @@ -26,18 +26,23 @@ static int mdio_mux_gpio_switch_fn(int current_child, int 
> desired_child,
>                                  void *data)
>  {
>       struct mdio_mux_gpio_state *s = data;
> -     int values[s->gpios->ndescs];
> +     int *values;
>       unsigned int n;
>  
>       if (current_child == desired_child)
>               return 0;
>  
> +     values = kmalloc_array(s->gpios->ndescs, sizeof(*values), GFP_KERNEL);
> +     if (!values)
> +             return -ENOMEM;
> +
>       for (n = 0; n < s->gpios->ndescs; n++)
>               values[n] = (desired_child >> n) & 1;
>  
>       gpiod_set_array_value_cansleep(s->gpios->ndescs, s->gpios->desc,
>                                      values);
>  
> +     kfree(values);
>       return 0;
>  }

Hi Kees

This is the 'hot path' for this driver, and you are making it more
expensive. 

Please allocate the memory once in mdio_mux_gpio_probe and make it
part of mdio_mux_gpio_state.

        Thanks
                Andrew