Re: [PATCH] [v3] iwlwifi: pcie: fix constant-conversion warning

[Nathan Chancellor <nathan@kernel.org>]

On Thu, Nov 18, 2021 at 03:21:02PM +0100, Arnd Bergmann wrote:
> From: Arnd Bergmann <arnd@arndb.de>
> 
> gcc-11 points out a potential issue with integer overflow when
> the iwl_dev_info_table[] array is empty:
> 
> drivers/net/wireless/intel/iwlwifi/pcie/drv.c:1344:42: error: implicit conversion from 'unsigned long' to 'int' changes value from 18446744073709551615 to -1 [-Werror,-Wconstant-conversion]
>         for (i = ARRAY_SIZE(iwl_dev_info_table) - 1; i >= 0; i--) {
>                ~ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~

For what it's worth, I do see this warning with Clang when both
CONFIG_IWLDVM and CONFIG_IWLMVM are disabled and looking through the GCC
warning docs [1], I do not see a -Wconstant-conversion option? Maybe
there is another warning that is similar but that warning right there
appears to have come from clang, as it matches mine exactly.

drivers/net/wireless/intel/iwlwifi/pcie/drv.c:1344:42: error: implicit conversion from 'unsigned long' to 'int' changes value from 18446744073709551615 to -1 [-Werror,-Wconstant-conversion]
        for (i = ARRAY_SIZE(iwl_dev_info_table) - 1; i >= 0; i--) {
               ~ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~
1 error generated.

[1]: https://gcc.gnu.org/onlinedocs/gcc/Warning-Options.html

> This is still harmless, as the loop correctly terminates, but adding
> an extra range check makes that obvious to both readers and to the
> compiler.
> 
> Fixes: 3f7320428fa4 ("iwlwifi: pcie: simplify iwl_pci_find_dev_info()")
> Reported-by: kernel test robot <lkp@intel.com>
> Cc: Nick Desaulniers <ndesaulniers@google.com>
> Signed-off-by: Arnd Bergmann <arnd@arndb.de>

Regardless of above, this resolves the warning for clang.

Reviewed-by: Nathan Chancellor <nathan@kernel.org>

> ---
> Changes in v3:
> - make it actually work with gcc-11
> - fix commit message s/clang/gcc-11/
> 
> Changes in v2:
> - replace int cast with a range check
> ---
>  drivers/net/wireless/intel/iwlwifi/pcie/drv.c | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/net/wireless/intel/iwlwifi/pcie/drv.c b/drivers/net/wireless/intel/iwlwifi/pcie/drv.c
> index c574f041f096..395e328c6a07 100644
> --- a/drivers/net/wireless/intel/iwlwifi/pcie/drv.c
> +++ b/drivers/net/wireless/intel/iwlwifi/pcie/drv.c
> @@ -1339,9 +1339,13 @@ iwl_pci_find_dev_info(u16 device, u16 subsystem_device,
>  		      u16 mac_type, u8 mac_step,
>  		      u16 rf_type, u8 cdb, u8 rf_id, u8 no_160, u8 cores)
>  {
> +	int num_devices = ARRAY_SIZE(iwl_dev_info_table);
>  	int i;
>  
> -	for (i = ARRAY_SIZE(iwl_dev_info_table) - 1; i >= 0; i--) {
> +	if (!num_devices)
> +		return NULL;
> +
> +	for (i = num_devices - 1; i >= 0; i--) {
>  		const struct iwl_dev_info *dev_info = &iwl_dev_info_table[i];
>  
>  		if (dev_info->device != (u16)IWL_CFG_ANY &&
> -- 
> 2.29.2
>