linux-kernel - RE: [PATCH][next] mei: Avoid the use of one-element arrays

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Date:   Tue, 14 Jul 2020 17:54:32 +0000
From:   "Winkler, Tomas" <tomas.winkler@...el.com>
To:     "Gustavo A. R. Silva" <gustavoars@...nel.org>,
        Arnd Bergmann <arnd@...db.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH][next] mei: Avoid the use of one-element arrays

> 
> There is a regular need in the kernel to provide a way to declare having a
> dynamically sized set of trailing elements in a structure.
> Kernel code should always use “flexible array members”[1] for these cases
> or, as in this particular case, replace the one-element array with a simple
> value type u8 reserved once this is just a placeholder for alignment. The older
> style of one-element or zero-length arrays should no longer be used[2].
> 
> Also, while there, use the preferred form for passing a size of a struct.
> The alternative form where struct name is spelled out hurts readability and
> introduces an opportunity for a bug when the variable type is changed but
> the corresponding sizeof that is passed as argument is not.
> 
> [1] https://en.wikipedia.org/wiki/Flexible_array_member
> [2] https://github.com/KSPP/linux/issues/79
> 
> Signed-off-by: Gustavo A. R. Silva <gustavoars@...nel.org>
I'm okay with the patch but in this case the description is a bit off. 
In this case there was no intention for a flexible arrays its just a reserved field. 

> ---
>  drivers/misc/mei/hbm.c | 4 ++--
>  drivers/misc/mei/hw.h  | 6 +++---
>  2 files changed, 5 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/misc/mei/hbm.c b/drivers/misc/mei/hbm.c index
> a44094cdbc36..f020d5594154 100644
> --- a/drivers/misc/mei/hbm.c
> +++ b/drivers/misc/mei/hbm.c
> @@ -408,14 +408,14 @@ static int mei_hbm_add_cl_resp(struct mei_device
> *dev, u8 addr, u8 status)  {
>  	struct mei_msg_hdr mei_hdr;
>  	struct hbm_add_client_response resp;
> -	const size_t len = sizeof(struct hbm_add_client_response);
> +	const size_t len = sizeof(resp);
>  	int ret;
> 
>  	dev_dbg(dev->dev, "adding client response\n");
> 
>  	mei_hbm_hdr(&mei_hdr, len);
> 
> -	memset(&resp, 0, sizeof(struct hbm_add_client_response));
> +	memset(&resp, 0, len);
>  	resp.hbm_cmd = MEI_HBM_ADD_CLIENT_RES_CMD;
>  	resp.me_addr = addr;
>  	resp.status  = status;
> diff --git a/drivers/misc/mei/hw.h b/drivers/misc/mei/hw.h index
> b1a8d5ec88b3..8c0297f0e7f3 100644
> --- a/drivers/misc/mei/hw.h
> +++ b/drivers/misc/mei/hw.h
> @@ -346,13 +346,13 @@ struct hbm_add_client_request {
>   * @hbm_cmd: bus message command header
>   * @me_addr: address of the client in ME
>   * @status: if HBMS_SUCCESS then the client can now accept connections.
> - * @reserved: reserved
> + * @reserved: reserved for alignment.
>   */
>  struct hbm_add_client_response {
>  	u8 hbm_cmd;
>  	u8 me_addr;
>  	u8 status;
> -	u8 reserved[1];
> +	u8 reserved;
>  } __packed;
> 
>  /**
> @@ -461,7 +461,7 @@ struct hbm_notification {
>  	u8 hbm_cmd;
>  	u8 me_addr;
>  	u8 host_addr;
> -	u8 reserved[1];
> +	u8 reserved;
>  } __packed;
> 
>  /**
> --
> 2.27.0