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 13:01:46 +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 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/

> 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.

Best Regards,
Petr