netdev - Re: [PATCH] net: l2tp: fix clang -Wformat warning

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Message-ID: <20220707075052.GA3912@debian.home>
Date:   Thu, 7 Jul 2022 09:50:52 +0200
From:   Guillaume Nault <gnault@...hat.com>
To:     Justin Stitt <justinstitt@...gle.com>
Cc:     James Chapman <jchapman@...alix.com>,
        "David S . Miller" <davem@...emloft.net>,
        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: l2tp: fix clang -Wformat warning

On Wed, Jul 06, 2022 at 04:08:33PM -0700, Justin Stitt wrote:
> When building with clang we encounter this warning:
> | net/l2tp/l2tp_ppp.c:1557:6: error: format specifies type 'unsigned
> | short' but the argument has type 'u32' (aka 'unsigned int')
> | [-Werror,-Wformat] session->nr, session->ns,
> 
> Both session->nr and session->ns are of type u32. The format specifier
> previously used is `%hu` which would truncate our unsigned integer from
> 32 to 16 bits. This doesn't seem like intended behavior, if it is then
> perhaps we need to consider suppressing the warning with pragma clauses.

pppol2tp_seq_session_show() is only called for L2TPv2 sessions, where
ns and nr are 2 bytes long (L2TPv3 uses 3 bytes, hence the u32 type in
the generic l2tp_session structure). So %hu shouldn't truncate anything
here.

However %u doesn't harm and is cleaner than silencing the warning with
pragma.

Acked-by: Guillaume Nault <gnault@...hat.com>


> This patch should get us closer to the goal of enabling the -Wformat
> flag for Clang builds.
> 
> Link: https://github.com/ClangBuiltLinux/linux/issues/378
> Signed-off-by: Justin Stitt <justinstitt@...gle.com>
> ---
>  net/l2tp/l2tp_ppp.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/net/l2tp/l2tp_ppp.c b/net/l2tp/l2tp_ppp.c
> index 8be1fdc68a0b..db2e584c625e 100644
> --- a/net/l2tp/l2tp_ppp.c
> +++ b/net/l2tp/l2tp_ppp.c
> @@ -1553,7 +1553,7 @@ static void pppol2tp_seq_session_show(struct seq_file *m, void *v)
>  		   session->lns_mode ? "LNS" : "LAC",
>  		   0,
>  		   jiffies_to_msecs(session->reorder_timeout));
> -	seq_printf(m, "   %hu/%hu %ld/%ld/%ld %ld/%ld/%ld\n",
> +	seq_printf(m, "   %u/%u %ld/%ld/%ld %ld/%ld/%ld\n",
>  		   session->nr, session->ns,
>  		   atomic_long_read(&session->stats.tx_packets),
>  		   atomic_long_read(&session->stats.tx_bytes),
> -- 
> 2.37.0.rc0.161.g10f37bed90-goog
>