linux-kernel - Re: [PATCH v2 2/5] staging: r8188eu: cast to restricted _

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Date:   Fri, 20 Aug 2021 16:44:49 -0500
From:   Larry Finger <Larry.Finger@...inger.net>
To:     Aakash Hemadri <aakashhemadri123@...il.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Phillip Potter <phil@...lpotter.co.uk>
Cc:     linux-staging@...ts.linux.dev, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 2/5] staging: r8188eu: cast to restricted __be32

On 8/19/21 3:17 AM, Aakash Hemadri wrote:
> Fix sparse warning:
>> rtw_br_ext.c:836:54: warning: cast to restricted __be32
> 
> Unnecessary double cast, remove them.
> 
> Signed-off-by: Aakash Hemadri <aakashhemadri123@...il.com>
> ---
>   drivers/staging/r8188eu/core/rtw_br_ext.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/staging/r8188eu/core/rtw_br_ext.c b/drivers/staging/r8188eu/core/rtw_br_ext.c
> index 404fa8904e47..6a0462ce6230 100644
> --- a/drivers/staging/r8188eu/core/rtw_br_ext.c
> +++ b/drivers/staging/r8188eu/core/rtw_br_ext.c
> @@ -671,7 +671,7 @@ void dhcp_flag_bcast(struct adapter *priv, struct sk_buff *skb)
>   				    (udph->dest == __constant_htons(SERVER_PORT))) { /*  DHCP request */
>   					struct dhcpMessage *dhcph =
>   						(struct dhcpMessage *)((size_t)udph + sizeof(struct udphdr));
> -					u32 cookie = be32_to_cpu((__be32)dhcph->cookie);
> +					u32 cookie = dhcph->cookie;
>   
>   					if (cookie == DHCP_MAGIC) { /*  match magic word */
>   						if (!(dhcph->flags & htons(BROADCAST_FLAG))) {
> 

This patch is wrong. All the documentation I could find tells me that the 
multi-byte entries in dhcph are big-endian, thus the new line should read:

					u32 cookie = be32_to_cpu(dhcph->cookie);
combined with:

@@ -649,7 +650,7 @@ struct dhcpMessage {
         u_int8_t chaddr[16];
         u_int8_t sname[64];
         u_int8_t file[128];
-       u_int32_t cookie;
+       __be32 cookie;
         u_int8_t options[308]; /* 312 - cookie */
  };

The old code was, in fact, correct, but not in a way that satisfied Sparse.

Larry