linux-kernel - RE: [PATCH] include/uapi/linux/swab.h: add __u16 cast to _

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Message-ID: <ef8069e60f7a4e23a26dcbedd0bc77d7@AcuMS.aculab.com>
Date:   Fri, 10 Jun 2022 07:19:38 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Nick Desaulniers' <ndesaulniers@...gle.com>,
        Al Viro <viro@...iv.linux.org.uk>
CC:     Andrew Morton <akpm@...ux-foundation.org>,
        Justin Stitt <jstitt007@...il.com>,
        Nathan Chancellor <nathan@...nel.org>,
        Tom Rix <trix@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "llvm@...ts.linux.dev" <llvm@...ts.linux.dev>,
        Richard Smith <richardsmith@...gle.com>
Subject: RE: [PATCH] include/uapi/linux/swab.h: add __u16 cast to __swab16
 conditional

From: Nick Desaulniers
> Sent: 08 June 2022 20:35
....
> The issue we're facing is more so that `ntohs` is being used in
> printf-like expressions; clang's -Wformat warns about default argument
> promotion so we need to clean up cases where smaller-than-int format
> flags are being used for promoted-to-int params.  While looking at
> that, Nathan noticed that __swab16 will return either a __u16 or an
> int based on whether __HAVE_BUILTIN_BSWAP16__ is defined, which
> depends on BOTH the compiler being used and target architecture.  This
> patch from Justin just cleans that up.

The 'problem' is that the (__u16) cast is likely to add an
extra '& 0xffff' instruction that is almost certainly not
required.

OTOH the lack of this masking has always been a difference between
htons() on BE and LE systems.

But clang is also being over-enthusiastic with its warnings.
IIRC varargs parameters always get integer promotion.
So if %hd ever makes sense for printf then it implies a
mask inside printf.

	David

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