linux-kernel - Re: [PATCH v2] include/uapi/linux/swab.h: move explicit cast outside ternary

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Date:   Wed, 8 Jun 2022 13:09:40 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Justin Stitt <jstitt007@...il.com>
Cc:     ndesaulniers@...gle.com, linux-kernel@...r.kernel.org,
        llvm@...ts.linux.dev, nathan@...nel.org, trix@...hat.com
Subject: Re: [PATCH v2] include/uapi/linux/swab.h: move explicit cast
 outside ternary

On Tue,  7 Jun 2022 17:14:22 -0700 Justin Stitt <jstitt007@...il.com> wrote:

> A cast inside __builtin_constant_p doesn't do anything since it should evaluate
> as constant at compile time irrespective of this cast. Instead, I moved this
> cast outside the ternary to ensure the return type is as expected.
> 
> For instance, if __HAVE_BUILTIN_BSWAP16__ was not defined then __swab16 is
> actually returning an `int` not a `u16` due to integer promotion as described
> by Nick in this thread. This has repercussions when building with clang
> -Wformat. This fix should solve many of these warnings.
> 

ARM allmodconfig:

In file included from ./include/linux/swab.h:5,
                 from ./arch/arm/include/asm/opcodes.h:86,
                 from ./arch/arm/include/asm/bug.h:7,
                 from ./include/linux/bug.h:5,
                 from ./include/linux/mmdebug.h:5,
                 from ./include/linux/gfp.h:5,
                 from ./include/linux/slab.h:15,
                 from ./fs/xfs/kmem.h:9,
                 from ./fs/xfs/xfs_linux.h:24,
                 from ./fs/xfs/xfs.h:22,
                 from fs/xfs/scrub/agheader.c:6:
fs/xfs/scrub/agheader.c: In function 'xchk_superblock':
./include/uapi/linux/byteorder/little_endian.h:42:52: error: unsigned conversion from 'int' to 'short unsigned int' changes value from '-49265' to '16271' [-Werror=overflow]
   42 | #define __cpu_to_be16(x) ((__force __be16)__swab16((x)))
      |                                                    ^~~
./include/uapi/linux/swab.h:102:46: note: in definition of macro '__swab16'
  102 | #define __swab16(x) (__u16)__builtin_bswap16(x)
      |                                              ^
./include/linux/byteorder/generic.h:96:21: note: in expansion of macro '__cpu_to_be16'
   96 | #define cpu_to_be16 __cpu_to_be16
      |                     ^~~~~~~~~~~~~
fs/xfs/scrub/agheader.c:158:23: note: in expansion of macro 'cpu_to_be16'
  158 |         vernum_mask = cpu_to_be16(~XFS_SB_VERSION_OKBITS |
      |                       ^~~~~~~~~~~
cc1: all warnings being treated as errors
make[2]: *** [scripts/Makefile.build:249: fs/xfs/scrub/agheader.o] Error 1
make[1]: *** [scripts/Makefile.build:466: fs/xfs] Error 2
make: *** [Makefile:1839: fs] Error 2