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

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Message-ID: <CAK8P3a1N8gsXFV=19gWAu033_kCUcf8+DoV4PXyLnaDEitSTXw@mail.gmail.com>
Date:   Thu, 18 Nov 2021 17:27:01 +0100
From:   Arnd Bergmann <arnd@...nel.org>
To:     Nathan Chancellor <nathan@...nel.org>
Cc:     Luca Coelho <luciano.coelho@...el.com>,
        Kalle Valo <kvalo@...eaurora.org>,
        Arnd Bergmann <arnd@...db.de>,
        kernel test robot <lkp@...el.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Johannes Berg <johannes.berg@...el.com>,
        Yaara Baruch <yaara.baruch@...el.com>,
        Matti Gottlieb <matti.gottlieb@...el.com>,
        Emmanuel Grumbach <emmanuel.grumbach@...el.com>,
        linux-wireless <linux-wireless@...r.kernel.org>,
        Networking <netdev@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        llvm@...ts.linux.dev
Subject: Re: [PATCH] [v3] iwlwifi: pcie: fix constant-conversion warning

On Thu, Nov 18, 2021 at 5:03 PM Nathan Chancellor <nathan@...nel.org> wrote:
>
> On Thu, Nov 18, 2021 at 03:21:02PM +0100, Arnd Bergmann wrote:
> > From: Arnd Bergmann <arnd@...db.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

Ok, got it: it turns out this warning /also/ happens with gcc-11 and
the initial changelog was the one for matching the clang warning.
This is the gcc output, which is very similar but has a different
warning option.

drivers/net/wireless/intel/iwlwifi/pcie/drv.c: In function
'iwl_pci_find_dev_info':
include/linux/kernel.h:46:25: error: overflow in conversion from 'long
unsigned int' to 'int' changes value from '18446744073709551615' to
'-1' [-Werror=overflow]
   46 | #define ARRAY_SIZE(arr) (sizeof(arr) / sizeof((arr)[0]) +
__must_be_array(arr))
      |                         ^
drivers/net/wireless/intel/iwlwifi/pcie/drv.c:1344:18: note: in
expansion of macro 'ARRAY_SIZE'
 1344 |         for (i = ARRAY_SIZE(iwl_dev_info_table) - 1; i >= 0; i--) {

My v2 patch only addressed the clang warning, while v3 works with both
gcc and clang. I can send a v4 if I should update the changelog again
to explain that, but I suppose it's still close enough.

      Arnd