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

[Dan Carpenter]

On Mon, May 10, 2021 at 02:52:14AM -0700, Joe Perches wrote:
> On Mon, 2021-05-10 at 12:17 +0300, Michael Zaidman wrote:
> > 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@xxxxxxxxx>
> > > > Reported-by: kernel test robot <lkp@xxxxxxxxx>
> > > > ---
> > > >  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.
> 
> $ git grep __arch_swab16 arch/arm*/
> arch/arm/include/asm/swab.h:#define __arch_swab16(x) ((__u16)__arch_swahb32(x))
> 
> otherwise:
> 
> static inline __attribute_const__ __u16 __fswab16(__u16 val)
> {
> #if defined (__arch_swab16)
> 	return __arch_swab16(val);
> #else
> 	return ___constant_swab16(val);
> #endif
> }
> 
> #define ___constant_swab16(x) ((__u16)(				\
> 	(((__u16)(x) & (__u16)0x00ffU) << 8) |			\
> 	(((__u16)(x) & (__u16)0xff00U) >> 8)))
> 
> /**
>  * __swab16 - return a byteswapped 16-bit value
>  * @x: value to byteswap
>  */
> #ifdef __HAVE_BUILTIN_BSWAP16__
> #define __swab16(x) (__u16)__builtin_bswap16((__u16)(x))
> #else
> #define __swab16(x)				\
> 	(__builtin_constant_p((__u16)(x)) ?	\
> 	___constant_swab16(x) :			\
> 	__fswab16(x))
> #endif
> 
> Under what condition does the ?: return an int sized value
> rather than a u16 sized value?  I fail to see a path where
> the compiler should promote the returned value to int _before_
> the promotion done for the varargs use.
> 
> If it's for the varargs use, then both instances are promoted.
> 

Ternary type promotion is a horrible thing.

	foo = a ? b : c;

Everything is type promoted which ever of a, b or c has the most
positive bits.  Or if none of them have 31 positive bits it's
type promoted to int.

I sent a series of patches earlier where one the a, b, or c was
a negative error code and another was a unsigned int.  And foo
was a ssize_t.  Because you end up type promoting the -ENOMEM
to something close to UINT_MAX and then it doesn't sign extend
so the ssize_t value is not negative.

regards,
dan carpenter