linux-kernel - Re: [PATCH v2] lib: test_scanf: Add explicit type cast to result initialization in test_number

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Date:   Wed, 16 Aug 2023 17:01:43 +0200
From:   Petr Mladek <pmladek@...e.com>
To:     Nathan Chancellor <nathan@...nel.org>
Cc:     rostedt@...dmis.org, senozhatsky@...omium.org,
        andriy.shevchenko@...ux.intel.com, linux@...musvillemoes.dk,
        ndesaulniers@...gle.com, trix@...hat.com,
        linux-kernel@...r.kernel.org, llvm@...ts.linux.dev,
        patches@...ts.linux.dev, stable@...r.kernel.org
Subject: Re: [PATCH v2] lib: test_scanf: Add explicit type cast to result
 initialization in test_number_prefix()

On Wed 2023-08-16 07:01:12, Nathan Chancellor wrote:
> Hi Petr,
> 
> On Wed, Aug 16, 2023 at 01:01:46PM +0200, Petr Mladek wrote:
> > On Mon 2023-08-07 08:36:28, Nathan Chancellor wrote:
> > > A recent change in clang allows it to consider more expressions as
> > > compile time constants, which causes it to point out an implicit
> > > conversion in the scanf tests:
> > > 
> > >   lib/test_scanf.c:661:2: warning: implicit conversion from 'int' to 'unsigned char' changes value from -168 to 88 [-Wconstant-conversion]
> > >     661 |         test_number_prefix(unsigned char,       "0xA7", "%2hhx%hhx", 0, 0xa7, 2, check_uchar);
> > >         |         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> > >   lib/test_scanf.c:609:29: note: expanded from macro 'test_number_prefix'
> > >     609 |         T result[2] = {~expect[0], ~expect[1]};                                 \
> > >         |                       ~            ^~~~~~~~~~
> > >   1 warning generated.
> > > 
> > > The result of the bitwise negation is the type of the operand after
> > > going through the integer promotion rules, so this truncation is
> > > expected but harmless, as the initial values in the result array get
> > > overwritten by _test() anyways. Add an explicit cast to the expected
> > > type in test_number_prefix() to silence the warning. There is no
> > > functional change, as all the tests still pass with GCC 13.1.0 and clang
> > > 18.0.0.
> > > 
> > > Cc: stable@...r.kernel.org
> > > Closes: https://github.com/ClangBuiltLinux/linux/issues/1899
> > 
> > "Closes:" is not a valid tag. It was proposed and rejected in the end.
> > I replaced it with "Link:" as suggested by ./scripts/checkpatch.pl/
> 
> I don't really care about "Closes:" vs. "Link:", either is fine with me,
> but checkpatch.pl did not warn me about it and I still see commit
> 44c31888098a ("checkpatch: allow Closes tags with links") in mainline
> and -next that explicitly allows this (and even requires Closes: instead
> of Link: when following Reported-by:).

Good to know. It is possible that I mixed this with another tag.
I recall that people wanted to add some new tags recently and
Linus was strongly against it. Unfortunately, I can't find
the discussion now.

It seems that the Closes: tag is acceptable. But Linus is still
going to "put my foot down" when it gets misused, see
https://lore.kernel.org/linux-doc/CAHk-=wh0v1EeDV3v8TzK81nDC40=XuTdY2MCr0xy3m3FiBV3+Q@mail.gmail.com/

I called the checkpatch.pl from printk/linux.git in a branch based on 6.4.
It did not have the commit 44c31888098a ("checkpatch: allow Closes
tags with links").

If you do not mind, I'll keep the "Link:" tag to avoid rebase in
the for-6.6 branch in printk/linug.git.


> > The patch has been pushed into printk/linux.git, branch for-6.6.
> 
> Thanks a lot for the review and acceptance!

You are welcome.

Best Regards,
Petr