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

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Date:   Thu, 07 Jul 2022 11:01:26 -0700
From:   Joe Perches <joe@...ches.com>
To:     Justin Stitt <justinstitt@...gle.com>
Cc:     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>,
        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 10:47 -0700, Justin Stitt wrote:
> On Thu, Jul 7, 2022 at 10:40 AM Joe Perches <joe@...ches.com> wrote:
> > 
> > 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.
> Yeah, that would also work. An integer (even a signed one) fully
> encompasses a u16 so it's really a choice of style. Do you think the
> change to %u warrants a v2 of this patch?

As it's rather odd output to use '%u != %d', probably.

Your patch, up to you.

> > 
> > > 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);
> >