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

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Message-ID: <dc2d025f-0d26-b91c-975a-6c815a7682ef@linaro.org>
Date:   Thu, 19 Mar 2020 21:43:05 +0100
From:   Daniel Lezcano <daniel.lezcano@...aro.org>
To:     "Gustavo A. R. Silva" <gustavo@...eddedor.com>,
        Andy Gross <agross@...nel.org>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        Amit Kucheria <amit.kucheria@...aro.org>,
        Zhang Rui <rui.zhang@...el.com>
Cc:     linux-arm-msm@...r.kernel.org, linux-pm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] thermal: qcom: tsens.h: Replace zero-length array with
 flexible-array member

On 19/03/2020 19:48, Gustavo A. R. Silva 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@...eddedor.com>

Applied, thanks

> ---
>  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);
> 


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