Re: [PATCH] HID: ft260: fix format type warning in ft260_word_show()

[Michael Zaidman <michael.zaidman@gmail.com>]

On Sun, May 09, 2021 at 01:39:29PM -0700, Joe Perches wrote:
> On Sun, 2021-05-09 at 22:32 +0300, Michael Zaidman wrote:
> > Fixes: 6a82582d9fa4 ("HID: ft260: add usb hid to i2c host bridge driver")
> > 
> > Fix warning reported by static analysis when built with W=1 for arm64 by
> > clang version 13.0.0
> > 
> > > > drivers/hid/hid-ft260.c:794:44: warning: format specifies type 'short' but
> >    the argument has type 'int' [-Wformat]
> >            return scnprintf(buf, PAGE_SIZE, "%hi\n", le16_to_cpu(*field));
> >                                              ~~~     ^~~~~~~~~~~~~~~~~~~
> >                                              %i
> >    include/linux/byteorder/generic.h:91:21: note: expanded from
> >                                             macro 'le16_to_cpu'
> >    #define le16_to_cpu __le16_to_cpu
> >                        ^
> >    include/uapi/linux/byteorder/big_endian.h:36:26: note: expanded from
> >                                                     macro '__le16_to_cpu'
> >    #define __le16_to_cpu(x) __swab16((__force __u16)(__le16)(x))
> >                             ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> >    include/uapi/linux/swab.h:105:2: note: expanded from macro '__swab16'
> >            (__builtin_constant_p((__u16)(x)) ?     \
> >            ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> > 
> > Signed-off-by: Michael Zaidman <michael.zaidman@gmail.com>
> > Reported-by: kernel test robot <lkp@intel.com>
> > ---
> >  drivers/hid/hid-ft260.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/drivers/hid/hid-ft260.c b/drivers/hid/hid-ft260.c
> > index 047aa85a7c83..38794a29599c 100644
> > --- a/drivers/hid/hid-ft260.c
> > +++ b/drivers/hid/hid-ft260.c
> > @@ -791,7 +791,7 @@ static int ft260_word_show(struct hid_device *hdev, int id, u8 *cfg, int len,
> >  	if (ret != len && ret >= 0)
> >  		return -EIO;
> >  
> > 
> > -	return scnprintf(buf, PAGE_SIZE, "%hi\n", le16_to_cpu(*field));
> > +	return scnprintf(buf, PAGE_SIZE, "%d\n", le16_to_cpu(*field));
> >  }
> 
> There are 2 of these so I wonder about the static analysis.

There is nothing wrong with the static analysis. The first scnprintf format
type is perfectly valid as far as its size is greater than the size of the
data pointed by the *field pointer, which is a one byte size in our case.
The static analysis warned about the second scnprintf case, where the format
type was shorter than the integer returned by the __builtin_constant_p.
This warning can be considered as a false positive since the le16_to_cpu is
all about the 16 bits numbers, but to silence it, I submitted the above fix.

> It's probably better to use sysfs_emit as well.

The sysfs_emit was introduced in the 5.10 kernel:
2efc459d06f16 (Joe Perches 2020-09-16 13:40:38 -0700 335) int sysfs_emit(...)

But, the hid-ft260 driver will be used mostly with older kernels, at least,
for the next couple of years. Since older kernel versions do not have this API,
it will require patching the driver or kernel that I would like to avoid.
Nevertheless, we can reconsider the sysfs_emit usage in this driver in the
future, upon wider 5.10+ kernels' adoption.

> ---
>  drivers/hid/hid-ft260.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/hid/hid-ft260.c b/drivers/hid/hid-ft260.c
> index 7a9ba984a75a..475641682fff 100644
> --- a/drivers/hid/hid-ft260.c
> +++ b/drivers/hid/hid-ft260.c
> @@ -783,7 +783,7 @@ static int ft260_byte_show(struct hid_device *hdev, int id, u8 *cfg, int len,
>  	if (ret != len && ret >= 0)
>  		return -EIO;
>  
> -	return scnprintf(buf, PAGE_SIZE, "%hi\n", *field);
> +	return sysfs_emit(buf, "%d\n", *field);
>  }
>  
>  static int ft260_word_show(struct hid_device *hdev, int id, u8 *cfg, int len,
> @@ -795,7 +795,7 @@ static int ft260_word_show(struct hid_device *hdev, int id, u8 *cfg, int len,
>  	if (ret != len && ret >= 0)
>  		return -EIO;
>  
> -	return scnprintf(buf, PAGE_SIZE, "%hi\n", le16_to_cpu(*field));
> +	return sysfs_emit(buf, "%d\n", le16_to_cpu(*field));
>  }
>  
>  #define FT260_ATTR_SHOW(name, reptype, id, type, func)			       \
>