linux-kernel - Re: [PATCH][next] pnpbios: pnpbios.h: Replace zero-length array with flexible-array member

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Message-ID: <bd76497e-7d37-a38b-b12c-d6069714f2f0@intel.com>
Date:   Tue, 24 Mar 2020 17:13:50 +0100
From:   "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>
To:     "Gustavo A. R. Silva" <gustavo@...eddedor.com>
Cc:     linux-kernel@...r.kernel.org, linux-acpi@...r.kernel.org
Subject: Re: [PATCH][next] pnpbios: pnpbios.h: Replace zero-length array with
 flexible-array member

On 3/21/2020 12:18 AM, 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>
> ---
>   drivers/pnp/pnpbios/pnpbios.h | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/drivers/pnp/pnpbios/pnpbios.h b/drivers/pnp/pnpbios/pnpbios.h
> index 37acb8378f39..2ce739ff9c1a 100644
> --- a/drivers/pnp/pnpbios/pnpbios.h
> +++ b/drivers/pnp/pnpbios/pnpbios.h
> @@ -107,7 +107,7 @@ struct pnp_bios_node {
>   	__u32 eisa_id;
>   	__u8 type_code[3];
>   	__u16 flags;
> -	__u8 data[0];
> +	__u8 data[];
>   };
>   #pragma pack()
>   

Please resend this with a CC to linux-acpi, thanks!