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

Guillaume Nault
2022-07-07T07:51:00Z

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@redhat.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@google.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
>