netdev - Re: [PATCH] net: ipv4: fix clang -Wformat warning

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Date:   Thu, 07 Jul 2022 10:40:29 -0700
From:   Joe Perches <joe@...ches.com>
To:     Justin Stitt <justinstitt@...gle.com>,
        Steffen Klassert <steffen.klassert@...unet.com>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        "David S . Miller" <davem@...emloft.net>,
        Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
        David Ahern <dsahern@...nel.org>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>
Cc:     Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Tom Rix <trix@...hat.com>, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, llvm@...ts.linux.dev
Subject: Re: [PATCH] net: ipv4: fix clang -Wformat warning

On Thu, 2022-07-07 at 10:30 -0700, Justin Stitt wrote:
> When building with Clang we encounter this warning:
> > net/ipv4/ah4.c:513:4: error: format specifies type 'unsigned short' but
> > the argument has type 'int' [-Werror,-Wformat]
> > aalg_desc->uinfo.auth.icv_fullbits / 8);
> 
> `aalg_desc->uinfo.auth.icv_fullbits` is a u16 but due to default
> argument promotion becomes an int.
> 
> Variadic functions (printf-like) undergo default argument promotion.
> Documentation/core-api/printk-formats.rst specifically recommends using
> the promoted-to-type's format flag.
> 
> As per C11 6.3.1.1:
> (https://www.open-std.org/jtc1/sc22/wg14/www/docs/n1548.pdf) `If an int
> can represent all values of the original type ..., the value is
> converted to an int; otherwise, it is converted to an unsigned int.
> These are called the integer promotions.` Thus it makes sense to change
> %hu to %d not only to follow this standard but to suppress the warning
> as well.

I think it also makes sense to use %u and not %d
as the original type is unsigned.

> diff --git a/net/ipv4/ah4.c b/net/ipv4/ah4.c
[]
> @@ -507,7 +507,7 @@ static int ah_init_state(struct xfrm_state *x)
>  
>  	if (aalg_desc->uinfo.auth.icv_fullbits/8 !=
>  	    crypto_ahash_digestsize(ahash)) {
> -		pr_info("%s: %s digestsize %u != %hu\n",
> +		pr_info("%s: %s digestsize %u != %d\n",
>  			__func__, x->aalg->alg_name,
>  			crypto_ahash_digestsize(ahash),
>  			aalg_desc->uinfo.auth.icv_fullbits / 8);