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

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Message-Id: <20230807-test_scanf-wconstant-conversion-v2-1-839ca39083e1@kernel.org>
Date:   Mon, 07 Aug 2023 08:36:28 -0700
From:   Nathan Chancellor <nathan@...nel.org>
To:     pmladek@...e.com, rostedt@...dmis.org
Cc:     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,
        Nathan Chancellor <nathan@...nel.org>
Subject: [PATCH v2] lib: test_scanf: Add explicit type cast to result
 initialization in test_number_prefix()

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
Link: https://github.com/llvm/llvm-project/commit/610ec954e1f81c0e8fcadedcd25afe643f5a094e
Suggested-by: Nick Desaulniers <ndesaulniers@...gle.com>
Signed-off-by: Nathan Chancellor <nathan@...nel.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@...nel.org>