linux-kernel - Re: [PATCH] pcdp: Replace zero-length array with flexible-array

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Message-ID: <15a0454b-7913-fe76-c39a-fb0ab315360d@gonehiking.org>
Date:   Fri, 8 May 2020 17:29:01 -0600
From:   Khalid Aziz <khalid@...ehiking.org>
To:     "Gustavo A. R. Silva" <gustavoars@...nel.org>
Cc:     linux-kernel@...r.kernel.org
Subject: Re: [PATCH] pcdp: Replace zero-length array with flexible-array

On 5/7/20 1:05 PM, 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@...nel.org>
> ---
>  drivers/firmware/pcdp.h |    2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/firmware/pcdp.h b/drivers/firmware/pcdp.h
> index ce75d1da9e84..e02540571c52 100644
> --- a/drivers/firmware/pcdp.h
> +++ b/drivers/firmware/pcdp.h
> @@ -103,6 +103,6 @@ struct pcdp {
>  	u8			creator_id[4];
>  	u32			creator_rev;
>  	u32			num_uarts;
> -	struct pcdp_uart	uart[0];	/* actual size is num_uarts */
> +	struct pcdp_uart	uart[];	/* actual size is num_uarts */
>  	/* remainder of table is pcdp_device structures */
>  } __attribute__((packed));
> 

Loks good to me.

Acked-by: Khalid Aziz <khalid@...ehiking.org>