linux-kernel - Re: [PATCH v2] wusb: use correct format characters

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Date:   Thu, 28 Feb 2019 14:23:25 -0800
From:   Jon Flatley <jflat@...omium.org>
To:     Joe Perches <joe@...ches.com>
Cc:     Louis Taylor <louis@...gniz.eu>, gregkh@...uxfoundation.org,
        linux-usb@...r.kernel.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        clang-built-linux@...glegroups.com,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Jon Flatley <jflat@...omium.org>
Subject: Re: [PATCH v2] wusb: use correct format characters

Thanks for the patch and comments.

On Thu, Feb 28, 2019 at 1:53 PM Joe Perches <joe@...ches.com> wrote:
>
> On Thu, 2019-02-28 at 12:24 +0000, Louis Taylor wrote:
> > When compiling with -Wformat, clang warns:
> > ./include/linux/usb/wusb.h:245:5: warning: format specifies type
> > 'unsigned short' but the argument has type 'u8' (aka 'unsigned char')
> > [-Wformat]
> >   ckhdid->data[0],  ckhdid->data[1],
> >   ^~~~~~~~~~~~~~~
>
> I think the message is somewhat misguided as all the
> vararg arguments have implicit integer promotions.

That's a fair point, but Clang checks the arguments against their
format specifier before they're promoted when using -Wformat. When
considering integer promotions it's difficult to say if this is
"wrong", but since 'unsigned char' corresponds to the "hh" length
specifier I don't think this is misguided. Otherwise, why use the "h"
length specifier at all?
>
> > ckhdid->data is unconditionally defined as `u8 data[16]`, so this patch
> > updates the format characters to the correct one for unsigned char types.
> []
> > diff --git a/include/linux/usb/wusb.h b/include/linux/usb/wusb.h
> []
> > @@ -240,8 +240,8 @@ static inline size_t ckhdid_printf(char *pr_ckhdid, size_t size,
> >                                  const struct wusb_ckhdid *ckhdid)
> >  {
> >       return scnprintf(pr_ckhdid, size,
> > -                      "%02hx %02hx %02hx %02hx %02hx %02hx %02hx %02hx "
> > -                      "%02hx %02hx %02hx %02hx %02hx %02hx %02hx %02hx",
> > +                      "%02hhx %02hhx %02hhx %02hhx %02hhx %02hhx %02hhx %02hhx "
> > +                      "%02hhx %02hhx %02hhx %02hhx %02hhx %02hhx %02hhx %02hhx",
> >                        ckhdid->data[0],  ckhdid->data[1],
> >                        ckhdid->data[2],  ckhdid->data[3],
> >                        ckhdid->data[4],  ckhdid->data[5],
>
> Better to use the vsprintf %ph extension insead.

Agreed, so I guess my previous comment is irrelevant in this scenario.
> ---
>  include/linux/usb/wusb.h | 12 +-----------
>  1 file changed, 1 insertion(+), 11 deletions(-)
>
> diff --git a/include/linux/usb/wusb.h b/include/linux/usb/wusb.h
> index 9e4a3213f2c2..8c39ddf62951 100644
> --- a/include/linux/usb/wusb.h
> +++ b/include/linux/usb/wusb.h
> @@ -239,17 +239,7 @@ enum {
>  static inline size_t ckhdid_printf(char *pr_ckhdid, size_t size,
>                                    const struct wusb_ckhdid *ckhdid)
>  {
> -       return scnprintf(pr_ckhdid, size,
> -                        "%02hx %02hx %02hx %02hx %02hx %02hx %02hx %02hx "
> -                        "%02hx %02hx %02hx %02hx %02hx %02hx %02hx %02hx",
> -                        ckhdid->data[0],  ckhdid->data[1],
> -                        ckhdid->data[2],  ckhdid->data[3],
> -                        ckhdid->data[4],  ckhdid->data[5],
> -                        ckhdid->data[6],  ckhdid->data[7],
> -                        ckhdid->data[8],  ckhdid->data[9],
> -                        ckhdid->data[10], ckhdid->data[11],
> -                        ckhdid->data[12], ckhdid->data[13],
> -                        ckhdid->data[14], ckhdid->data[15]);
> +       return scnprintf(pr_ckhdid, size, "%16ph", ckhdid->data);
>  }
>
>  /*
>
>