Re: [PATCH] bitfield: avoid gcc-8 -Wint-in-bool-context warning

[Andrew Morton]

On Tue, 14 Aug 2018 00:09:34 +0200 Arnd Bergmann <arnd@xxxxxxxx> wrote:

> Passing an enum into FIELD_GET() produces a long but harmless warning on
> newer compilers:
> 
>                  from include/linux/linkage.h:7,
>                  from include/linux/kernel.h:7,
>                  from include/linux/skbuff.h:17,
>                  from include/linux/if_ether.h:23,
>                  from include/linux/etherdevice.h:25,
>                  from drivers/net/wireless/intel/iwlwifi/mvm/rxmq.c:63:
> drivers/net/wireless/intel/iwlwifi/mvm/rxmq.c: In function 'iwl_mvm_rx_mpdu_mq':
> include/linux/bitfield.h:56:20: error: enum constant in boolean context [-Werror=int-in-bool-context]
>    BUILD_BUG_ON_MSG(!(_mask), _pfx "mask is zero"); \
>                     ^
> ...
> include/linux/bitfield.h:103:3: note: in expansion of macro '__BF_FIELD_CHECK'
>    __BF_FIELD_CHECK(_mask, _reg, 0U, "FIELD_GET: "); \
>    ^~~~~~~~~~~~~~~~
> drivers/net/wireless/intel/iwlwifi/mvm/rxmq.c:1025:21: note: in expansion of macro 'FIELD_GET'
>     le16_encode_bits(FIELD_GET(IWL_RX_HE_PHY_SIBG_SYM_OR_USER_NUM_MASK,

Newer compilers will previously be used on older kernels, so I'll add a
cc:stable here.

> The problem here is that the caller has no idea how the macro gets
> expanding, leading to a false-positive. It can be trivially avoided
> by doing a comparison against zero.
> 
> This only recently started appearing as the iwlwifi driver was patched
> to use FIELD_GET.
> 
> Fixes: 514c30696fbc ("iwlwifi: add support for IEEE802.11ax")
> Signed-off-by: Arnd Bergmann <arnd@xxxxxxxx>
> ---
>  include/linux/bitfield.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/include/linux/bitfield.h b/include/linux/bitfield.h
> index 65a6981eef7b..3f1ef4450a7c 100644
> --- a/include/linux/bitfield.h
> +++ b/include/linux/bitfield.h
> @@ -53,7 +53,7 @@
>  	({								\
>  		BUILD_BUG_ON_MSG(!__builtin_constant_p(_mask),		\
>  				 _pfx "mask is not constant");		\
> -		BUILD_BUG_ON_MSG(!(_mask), _pfx "mask is zero");	\
> +		BUILD_BUG_ON_MSG((_mask) == 0, _pfx "mask is zero");	\
>  		BUILD_BUG_ON_MSG(__builtin_constant_p(_val) ?		\
>  				 ~((_mask) >> __bf_shf(_mask)) & (_val) : 0, \

I'm not understanding how a switch from !x to x==0 can fix anything. 
Help!