Re: [PATCH] thermal: qcom: tsens.h: Replace zero-length array with flexible-array member

[Amit Kucheria <amit.kucheria@linaro.org>]

On Fri, Mar 20, 2020 at 12:18 AM Gustavo A. R. Silva
<gustavo@embeddedor.com> wrote:
>
> The current codebase makes use of the zero-length array language
> extension to the C90 standard, but the preferred mechanism to declare
> variable-length types such as these ones is a flexible array member[1][2],
> introduced in C99:
>
> struct foo {
>         int stuff;
>         struct boo array[];
> };
>
> By making use of the mechanism above, we will get a compiler warning
> in case the flexible array does not occur last in the structure, which
> will help us prevent some kind of undefined behavior bugs from being
> inadvertently introduced[3] to the codebase from now on.
>
> Also, notice that, dynamic memory allocations won't be affected by
> this change:
>
> "Flexible array members have incomplete type, and so the sizeof operator
> may not be applied. As a quirk of the original implementation of
> zero-length arrays, sizeof evaluates to zero."[1]
>
> This issue was found with the help of Coccinelle.
>
> [1] https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html
> [2] https://github.com/KSPP/linux/issues/21
> [3] commit 76497732932f ("cxgb3/l2t: Fix undefined behaviour")
>
> Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>

Reviewed-by: Amit Kucheria <amit.kucheria@linaro.org>

> ---
>  drivers/thermal/qcom/tsens.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/drivers/thermal/qcom/tsens.h b/drivers/thermal/qcom/tsens.h
> index e24a865fbc34..92503712596a 100644
> --- a/drivers/thermal/qcom/tsens.h
> +++ b/drivers/thermal/qcom/tsens.h
> @@ -488,7 +488,7 @@ struct tsens_priv {
>         struct dentry                   *debug_root;
>         struct dentry                   *debug;
>
> -       struct tsens_sensor             sensor[0];
> +       struct tsens_sensor             sensor[];
>  };
>
>  char *qfprom_read(struct device *dev, const char *cname);
> --
> 2.23.0
>