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

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Message-ID: <20230816140112.GA2109327@dev-arch.thelio-3990X>
Date:   Wed, 16 Aug 2023 07:01:12 -0700
From:   Nathan Chancellor <nathan@...nel.org>
To:     Petr Mladek <pmladek@...e.com>
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()

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:).

> > Link: https://github.com/llvm/llvm-project/commit/610ec954e1f81c0e8fcadedcd25afe643f5a094e
> > Suggested-by: Nick Desaulniers <ndesaulniers@...gle.com>
> > Signed-off-by: Nathan Chancellor <nathan@...nel.org>
> 
> Reviewed-by: Petr Mladek <pmladek@...e.com>
> 
> The patch has been pushed into printk/linux.git, branch for-6.6.

Thanks a lot for the review and acceptance!

Cheers,
Nathan