[PATCH v2] lib: test_scanf: Add explicit type cast to result initialization in test_number_prefix()

[PATCH v2] lib: test_scanf: Add explicit type cast to result initialization in test_number_prefix()

[Nathan Chancellor]

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@vger.kernel.org
Closes: https://github.com/ClangBuiltLinux/linux/issues/1899
Link: https://github.com/llvm/llvm-project/commit/610ec954e1f81c0e8fcadedcd25afe643f5a094e
Suggested-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
---
Changes in v2:
- Add spaces in initializer to match result (Andy)
- Add 'Cc: stable', as builds with CONFIG_WERROR will be broken, such as
  allmodconfig
- Link to v1: https://lore.kernel.org/r/20230803-test_scanf-wconstant-conversion-v1-1-74da994dedbc@kernel.org
---
 lib/test_scanf.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/lib/test_scanf.c b/lib/test_scanf.c
index b620cf7de503..a2707af2951a 100644
--- a/lib/test_scanf.c
+++ b/lib/test_scanf.c
@@ -606,7 +606,7 @@ static void __init numbers_slice(void)
 #define test_number_prefix(T, str, scan_fmt, expect0, expect1, n_args, fn)	\
 do {										\
 	const T expect[2] = { expect0, expect1 };				\
-	T result[2] = {~expect[0], ~expect[1]};					\
+	T result[2] = { (T)~expect[0], (T)~expect[1] };				\
 										\
 	_test(fn, &expect, str, scan_fmt, n_args, &result[0], &result[1]);	\
 } while (0)

---
base-commit: 5d0c230f1de8c7515b6567d9afba1f196fb4e2f4
change-id: 20230803-test_scanf-wconstant-conversion-d97efbf3bb5d

Best regards,
-- 
Nathan Chancellor <nathan@kernel.org>

Re: [PATCH v2] lib: test_scanf: Add explicit type cast to result initialization in test_number_prefix()

[Petr Mladek]

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@vger.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@google.com>
> Signed-off-by: Nathan Chancellor <nathan@kernel.org>

Reviewed-by: Petr Mladek <pmladek@suse.com>

The patch has been pushed into printk/linux.git, branch for-6.6.

Best Regards,
Petr

Re: [PATCH v2] lib: test_scanf: Add explicit type cast to result initialization in test_number_prefix()

[Nathan Chancellor]

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@vger.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@google.com>
> > Signed-off-by: Nathan Chancellor <nathan@kernel.org>
> 
> Reviewed-by: Petr Mladek <pmladek@suse.com>
> 
> The patch has been pushed into printk/linux.git, branch for-6.6.

Thanks a lot for the review and acceptance!

Cheers,
Nathan

Re: [PATCH v2] lib: test_scanf: Add explicit type cast to result initialization in test_number_prefix()

[Petr Mladek]

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@vger.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