linux-kernel - Re: [PATCH] ipmi_si: fix -Wvoid-pointer-to-enum-cast warning

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Date:   Fri, 11 Aug 2023 15:27:24 -0500
From:   Corey Minyard <minyard@....org>
To:     Justin Stitt <justinstitt@...gle.com>
Cc:     Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Tom Rix <trix@...hat.com>,
        openipmi-developer@...ts.sourceforge.net,
        linux-kernel@...r.kernel.org, llvm@...ts.linux.dev,
        Arnd Bergmann <arnd@...db.de>,
        kernel test robot <lkp@...el.com>
Subject: Re: [PATCH] ipmi_si: fix -Wvoid-pointer-to-enum-cast warning

On Wed, Aug 09, 2023 at 09:05:17PM +0000, Justin Stitt wrote:
> With W=1 we see the following warning:
> 
> |  drivers/char/ipmi/ipmi_si_platform.c:272:15: error: \
> |       cast to smaller integer type 'enum si_type' from \
> |       'const void *' [-Werror,-Wvoid-pointer-to-enum-cast]
> |    272 |         io.si_type      = (enum si_type) match->data;
> |        |                           ^~~~~~~~~~~~~~~~~~~~~~~~~~
> 
> This is due to the fact that the `si_type` enum members are int-width
> and a cast from pointer-width down to int will cause truncation and
> possible data loss. Although in this case `si_type` has only a few
> enumerated fields and thus there is likely no data loss occurring.
> Nonetheless, this patch is necessary to the goal of promoting this
> warning out of W=1.
> 
> Link: https://github.com/ClangBuiltLinux/linux/issues/1902
> Link: https://lore.kernel.org/llvm/202308081000.tTL1ElTr-lkp@intel.com/
> Reported-by: kernel test robot <lkp@...el.com>
> Signed-off-by: Justin Stitt <justinstitt@...gle.com>
> ---
> Note:
> Arnd had mentioned that there perhaps may be some semantic differences
> between GCC and Clang regarding this warning or family of warnings. For
> now, this patch (and others following) will yield less noisy W=1 builds
> and hopefully materialize into this warning getting promoted out of W=1
> to an always-on warning.
> ---
>  drivers/char/ipmi/ipmi_si_platform.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/char/ipmi/ipmi_si_platform.c b/drivers/char/ipmi/ipmi_si_platform.c
> index 505cc978c97a..0d509d683c0f 100644
> --- a/drivers/char/ipmi/ipmi_si_platform.c
> +++ b/drivers/char/ipmi/ipmi_si_platform.c
> @@ -269,7 +269,7 @@ static int of_ipmi_probe(struct platform_device *pdev)
>  	}
>  
>  	memset(&io, 0, sizeof(io));
> -	io.si_type	= (enum si_type) match->data;
> +	io.si_type	= (unsigned long) match->data;

Wouldn't you want to use intptr_t or uintptr_t?

-corey

>  	io.addr_source	= SI_DEVICETREE;
>  	io.irq_setup	= ipmi_std_irq_setup;
>  
> 
> ---
> base-commit: c1a515d3c0270628df8ae5f5118ba859b85464a2
> change-id: 20230809-cbl-1902-7532a747b731
> 
> Best regards,
> --
> Justin Stitt <justinstitt@...gle.com>
>