Subject: Re: [PATCH v4] net: mac802154: comparision issue of type cast, finding by EXTRA_CFLAGS=-W

From: Daniel Borkmann <[email-masked]>
Date: Thu, 25 Apr 2013 11:59:06 +0200

On 04/25/2013 11:40 AM, Chen Gang wrote:
> Need change ~(u8)0 to 0xff, or in mac802154_wpan_xmit(), "u8 chan"
> will never == MAC802154_CHAN_NONE.
>
> The related warning:
>    net/mac802154/wpan.c:325:2: warning: comparison is always false
>    due to limited range of data type [-Wtype-limits]

Hm, do you really need to include this huge pile of code below into the
commit message?

Couldn't you make the commit message simpler resp. more compact, e.g.:

Change MAC802154_CHAN_NONE from ~(u8)0 to 0xff, otherwise the comparison
in mac802154_wpan_xmit() for ``chan == MAC802154_CHAN_NONE'' will not succeed.
This bug can be boiled down to ``u8 foo = 0xff; if (foo == ~(u8)0) [...]
else [...]'' where the condition will always take the else branch.

.... something like that maybe?

> The related unit test:
>    cc -Wall -O2 -o test test.c
>    ./test
>    will output 'different'
>
>    (after use '0xff' instead of '~(u8)0', it will output 'equal')
>
> -----------------------test code begin--------------------------------
>
> #include <stdio.h>
>
> typedef unsigned char u8;
>
> int main()
> {
> 	u8 uc = 0xff;
>
> 	if (uc == ~(u8)0)
> 		printf("\nequal\n");
> 	else
> 		printf("\ndifferent\n");
>
> 	return 0;
> }
>
> -----------------------test code end----------------------------------
>
>
> Signed-off-by: Chen Gang <[email-masked]>
> ---
>   net/mac802154/mac802154.h | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/net/mac802154/mac802154.h b/net/mac802154/mac802154.h
> index a4dcaf1..703c121 100644
> --- a/net/mac802154/mac802154.h
> +++ b/net/mac802154/mac802154.h
> @@ -90,7 +90,7 @@ struct mac802154_sub_if_data {
>
>   #define MAC802154_MAX_XMIT_ATTEMPTS	3
>
> -#define MAC802154_CHAN_NONE		(~(u8)0) /* No channel is assigned */
> +#define MAC802154_CHAN_NONE		0xff /* No channel is assigned */
>
>   extern struct ieee802154_reduced_mlme_ops mac802154_mlme_reduced;
>   extern struct ieee802154_mlme_ops mac802154_mlme_wpan;
>