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Date:   Fri, 8 May 2020 11:20:50 -0700 (PDT)
From:   matthew.gerlach@linux.intel.com
To:     "Gustavo A. R. Silva" <gustavoars@kernel.org>
cc:     Wu Hao <hao.wu@intel.com>, linux-fpga@vger.kernel.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH] fpga: dfl: Replace zero-length array with
 flexible-array
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This looks like a a good change to me.

Tested-by: Matthew Gerlach <matthew.gerlach@linux.intel.com>

On Thu, 7 May 2020, 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]
>
> sizeof(flexible-array-member) triggers a warning because flexible array
> members have incomplete type[1]. There are some instances of code in
> which the sizeof operator is being incorrectly/erroneously applied to
> zero-length arrays and the result is zero. Such instances may be hiding
> some bugs. So, this work (flexible-array member conversions) will also
> help to get completely rid of those sorts of issues.
>
> 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 <gustavoars@kernel.org>
> ---
> drivers/fpga/dfl.h |    2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/drivers/fpga/dfl.h b/drivers/fpga/dfl.h
> index 9f0e656de720..1cd51ea52ce1 100644
> --- a/drivers/fpga/dfl.h
> +++ b/drivers/fpga/dfl.h
> @@ -235,7 +235,7 @@ struct dfl_feature_platform_data {
> 	unsigned long dev_status;
> 	void *private;
> 	int num;
> -	struct dfl_feature features[0];
> +	struct dfl_feature features[];
> };
>
> static inline
>
>