linux-hardening - Re: [PATCH] MIPS: BCM47XX: Cast memcmp() of function to (void *)

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Message-ID: <YxneeP/9zOHNnt1E@work>
Date:   Thu, 8 Sep 2022 13:22:16 +0100
From:   "Gustavo A. R. Silva" <gustavoars@...nel.org>
To:     Kees Cook <keescook@...omium.org>
Cc:     Hauke Mehrtens <hauke@...ke-m.de>,
        Rafał Miłecki <zajec5@...il.com>,
        Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
        linux-mips@...r.kernel.org, Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        llvm@...ts.linux.dev, kernel test robot <lkp@...el.com>,
        Tom Rix <trix@...hat.com>, linux-kernel@...r.kernel.org,
        linux-hardening@...r.kernel.org
Subject: Re: [PATCH] MIPS: BCM47XX: Cast memcmp() of function to (void *)

On Wed, Sep 07, 2022 at 04:05:56PM -0700, Kees Cook wrote:
> Clang is especially sensitive about argument type matching when using
> __overloaded functions (like memcmp(), etc). Help it see that function
> pointers are just "void *". Avoids this error:
> 
> arch/mips/bcm47xx/prom.c:89:8: error: no matching function for call to 'memcmp'
>                    if (!memcmp(prom_init, prom_init + mem, 32))
>                         ^~~~~~
> include/linux/string.h:156:12: note: candidate function not viable: no known conversion from 'void (void)' to 'const void *' for 1st argument extern int memcmp(const void *,const void *,__kernel_size_t);
> 

Don't we have the same problem at line 162?

(next-20220908)arch/mips/bcm47xx/prom.c:
161         for (extmem = 128 << 20; extmem < 512 << 20; extmem <<= 1) {
162                 if (!memcmp(prom_init, (void *)(off + extmem), 16))
163                         break;
164         }

--
Gustavo

> Cc: Hauke Mehrtens <hauke@...ke-m.de>
> Cc: "Rafał Miłecki" <zajec5@...il.com>
> Cc: Thomas Bogendoerfer <tsbogend@...ha.franken.de>
> Cc: linux-mips@...r.kernel.org
> Cc: Nathan Chancellor <nathan@...nel.org>
> Cc: Nick Desaulniers <ndesaulniers@...gle.com>
> Cc: llvm@...ts.linux.dev
> Reported-by: kernel test robot <lkp@...el.com>
> Link: https://lore.kernel.org/lkml/202209080652.sz2d68e5-lkp@intel.com
> Signed-off-by: Kees Cook <keescook@...omium.org>
> ---
>  arch/mips/bcm47xx/prom.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/mips/bcm47xx/prom.c b/arch/mips/bcm47xx/prom.c
> index ab203e66ba0d..eb5c04a24531 100644
> --- a/arch/mips/bcm47xx/prom.c
> +++ b/arch/mips/bcm47xx/prom.c
> @@ -86,7 +86,7 @@ static __init void prom_init_mem(void)
>  			pr_debug("Assume 128MB RAM\n");
>  			break;
>  		}
> -		if (!memcmp(prom_init, prom_init + mem, 32))
> +		if (!memcmp((void *)prom_init, (void *)prom_init + mem, 32))
>  			break;
>  	}
>  	lowmem = mem;
> -- 
> 2.34.1
>