The clang warning -Wshorten-64-to-32 can be useful to catch
inadvertent truncation. In some instances this truncation can lead to
changing the sign of a result, for example, truncation to return an
int to fit a sort routine. Silence the warning by making the implicit
truncation explicit. This isn't to say the code is currently incorrect
but without silencing the warning it is hard to spot the erroneous
cases.
Signed-off-by: Ian Rogers <irogers@google.com>
---
include/linux/bitfield.h | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/include/linux/bitfield.h b/include/linux/bitfield.h
index 63928f173223..cc5cfed041bb 100644
--- a/include/linux/bitfield.h
+++ b/include/linux/bitfield.h
@@ -176,7 +176,7 @@ static __always_inline __##type type##_encode_bits(base v, base field) \
{ \
if (__builtin_constant_p(v) && (v & ~field_mask(field))) \
__field_overflow(); \
- return to((v & field_mask(field)) * field_multiplier(field)); \
+ return to((__##type)((v & field_mask(field)) * field_multiplier(field))); \
} \
static __always_inline __##type type##_replace_bits(__##type old, \
base val, base field) \
--
2.49.0.906.g1f30a19c02-googHi Ian,
kernel test robot noticed the following build warnings:
[auto build test WARNING on arnd-asm-generic/master]
[also build test WARNING on soc/for-next linus/master v6.15-rc5 next-20250506]
[If your patch is applied to the wrong git tree, kindly drop us a note.
And when submitting patch, we suggest to use '--base' as documented in
https://git-scm.com/docs/git-format-patch#_base_tree_information]
url: https://github.com/intel-lab-lkp/linux/commits/Ian-Rogers/bitfield-Silence-a-clang-Wshorten-64-to-32-warning/20250501-011830
base: https://git.kernel.org/pub/scm/linux/kernel/git/arnd/asm-generic.git master
patch link: https://lore.kernel.org/r/20250430171534.132774-2-irogers%40google.com
patch subject: [PATCH v2 1/5] bitfield: Silence a clang -Wshorten-64-to-32 warning
config: mips-randconfig-r111-20250501 (https://download.01.org/0day-ci/archive/20250507/202505071638.ZLhUrspy-lkp@intel.com/config)
compiler: clang version 21.0.0git (https://github.com/llvm/llvm-project f819f46284f2a79790038e1f6649172789734ae8)
reproduce: (https://download.01.org/0day-ci/archive/20250507/202505071638.ZLhUrspy-lkp@intel.com/reproduce)
If you fix the issue in a separate patch/commit (i.e. not just a new version of
the same patch/commit), kindly add following tags
| Reported-by: kernel test robot <lkp@intel.com>
| Closes: https://lore.kernel.org/oe-kbuild-all/202505071638.ZLhUrspy-lkp@intel.com/
sparse warnings: (new ones prefixed by >>)
lib/bitfield_kunit.c: note: in included file (through include/linux/fortify-string.h, include/linux/string.h, include/linux/bitmap.h, ...):
>> include/linux/bitfield.h:200:1: sparse: sparse: cast to restricted __le16
>> include/linux/bitfield.h:200:1: sparse: sparse: cast from restricted __le16
>> include/linux/bitfield.h:200:1: sparse: sparse: cast to restricted __be16
>> include/linux/bitfield.h:200:1: sparse: sparse: cast from restricted __be16
>> include/linux/bitfield.h:200:1: sparse: sparse: cast to restricted __le16
>> include/linux/bitfield.h:200:1: sparse: sparse: cast from restricted __le16
>> include/linux/bitfield.h:200:1: sparse: sparse: cast to restricted __be16
>> include/linux/bitfield.h:200:1: sparse: sparse: cast from restricted __be16
>> include/linux/bitfield.h:200:1: sparse: sparse: cast to restricted __le16
>> include/linux/bitfield.h:200:1: sparse: sparse: cast from restricted __le16
>> include/linux/bitfield.h:200:1: sparse: sparse: cast to restricted __be16
>> include/linux/bitfield.h:200:1: sparse: sparse: cast from restricted __be16
>> include/linux/bitfield.h:200:1: sparse: sparse: cast to restricted __le16
>> include/linux/bitfield.h:200:1: sparse: sparse: cast from restricted __le16
>> include/linux/bitfield.h:200:1: sparse: sparse: cast to restricted __be16
>> include/linux/bitfield.h:200:1: sparse: sparse: cast from restricted __be16
>> include/linux/bitfield.h:200:1: sparse: sparse: cast to restricted __le16
>> include/linux/bitfield.h:200:1: sparse: sparse: cast from restricted __le16
>> include/linux/bitfield.h:200:1: sparse: sparse: cast to restricted __be16
>> include/linux/bitfield.h:200:1: sparse: sparse: cast from restricted __be16
>> include/linux/bitfield.h:200:1: sparse: sparse: cast to restricted __le16
>> include/linux/bitfield.h:200:1: sparse: sparse: cast from restricted __le16
>> include/linux/bitfield.h:200:1: sparse: sparse: cast to restricted __be16
>> include/linux/bitfield.h:200:1: sparse: sparse: cast from restricted __be16
>> include/linux/bitfield.h:201:1: sparse: sparse: cast to restricted __le32
>> include/linux/bitfield.h:201:1: sparse: sparse: cast from restricted __le32
>> include/linux/bitfield.h:201:1: sparse: sparse: cast to restricted __be32
>> include/linux/bitfield.h:201:1: sparse: sparse: cast from restricted __be32
>> include/linux/bitfield.h:201:1: sparse: sparse: cast to restricted __le32
>> include/linux/bitfield.h:201:1: sparse: sparse: cast from restricted __le32
>> include/linux/bitfield.h:201:1: sparse: sparse: cast to restricted __be32
>> include/linux/bitfield.h:201:1: sparse: sparse: cast from restricted __be32
>> include/linux/bitfield.h:201:1: sparse: sparse: cast to restricted __le32
>> include/linux/bitfield.h:201:1: sparse: sparse: cast from restricted __le32
>> include/linux/bitfield.h:201:1: sparse: sparse: cast to restricted __be32
>> include/linux/bitfield.h:201:1: sparse: sparse: cast from restricted __be32
>> include/linux/bitfield.h:201:1: sparse: sparse: cast to restricted __le32
>> include/linux/bitfield.h:201:1: sparse: sparse: cast from restricted __le32
>> include/linux/bitfield.h:201:1: sparse: sparse: cast to restricted __be32
>> include/linux/bitfield.h:201:1: sparse: sparse: cast from restricted __be32
>> include/linux/bitfield.h:201:1: sparse: sparse: cast to restricted __le32
>> include/linux/bitfield.h:201:1: sparse: sparse: cast from restricted __le32
>> include/linux/bitfield.h:201:1: sparse: sparse: cast to restricted __be32
>> include/linux/bitfield.h:201:1: sparse: sparse: cast from restricted __be32
>> include/linux/bitfield.h:201:1: sparse: sparse: cast to restricted __le32
>> include/linux/bitfield.h:201:1: sparse: sparse: cast from restricted __le32
>> include/linux/bitfield.h:201:1: sparse: sparse: cast to restricted __be32
>> include/linux/bitfield.h:201:1: sparse: sparse: cast from restricted __be32
>> include/linux/bitfield.h:202:1: sparse: sparse: cast to restricted __le64
>> include/linux/bitfield.h:202:1: sparse: sparse: cast from restricted __le64
>> include/linux/bitfield.h:202:1: sparse: sparse: cast to restricted __be64
>> include/linux/bitfield.h:202:1: sparse: sparse: cast from restricted __be64
>> include/linux/bitfield.h:202:1: sparse: sparse: cast to restricted __le64
>> include/linux/bitfield.h:202:1: sparse: sparse: cast from restricted __le64
>> include/linux/bitfield.h:202:1: sparse: sparse: cast to restricted __be64
>> include/linux/bitfield.h:202:1: sparse: sparse: cast from restricted __be64
>> include/linux/bitfield.h:202:1: sparse: sparse: cast to restricted __le64
>> include/linux/bitfield.h:202:1: sparse: sparse: cast from restricted __le64
>> include/linux/bitfield.h:202:1: sparse: sparse: cast to restricted __be64
>> include/linux/bitfield.h:202:1: sparse: sparse: cast from restricted __be64
>> include/linux/bitfield.h:202:1: sparse: sparse: cast to restricted __le64
>> include/linux/bitfield.h:202:1: sparse: sparse: cast from restricted __le64
>> include/linux/bitfield.h:202:1: sparse: sparse: cast to restricted __be64
>> include/linux/bitfield.h:202:1: sparse: sparse: cast from restricted __be64
>> include/linux/bitfield.h:202:1: sparse: sparse: cast to restricted __le64
>> include/linux/bitfield.h:202:1: sparse: sparse: cast from restricted __le64
>> include/linux/bitfield.h:202:1: sparse: sparse: cast to restricted __be64
>> include/linux/bitfield.h:202:1: sparse: sparse: cast from restricted __be64
>> include/linux/bitfield.h:202:1: sparse: sparse: cast to restricted __le64
>> include/linux/bitfield.h:202:1: sparse: sparse: cast from restricted __le64
>> include/linux/bitfield.h:202:1: sparse: sparse: cast to restricted __be64
>> include/linux/bitfield.h:202:1: sparse: sparse: cast from restricted __be64
lib/bitfield_kunit.c: note: in included file (through include/linux/thread_info.h, include/asm-generic/preempt.h, arch/mips/include/generated/asm/preempt.h, ...):
include/linux/bitops.h:274:19: sparse: sparse: cast truncates bits from constant value (8000000000000000 becomes 0)
include/linux/bitops.h:279:37: sparse: sparse: cast truncates bits from constant value (8000000000000000 becomes 0)
include/linux/bitops.h:274:19: sparse: sparse: cast truncates bits from constant value (7f00000000000000 becomes 0)
include/linux/bitops.h:279:37: sparse: sparse: cast truncates bits from constant value (7f00000000000000 becomes 0)
include/linux/bitops.h:274:19: sparse: sparse: cast truncates bits from constant value (1800000000000 becomes 0)
include/linux/bitops.h:279:37: sparse: sparse: cast truncates bits from constant value (1800000000000 becomes 0)
include/linux/bitops.h:274:19: sparse: sparse: cast truncates bits from constant value (1f8000000 becomes f8000000)
include/linux/bitops.h:279:37: sparse: sparse: cast truncates bits from constant value (1f8000000 becomes f8000000)
vim +200 include/linux/bitfield.h
e2192de59e457a Johannes Berg 2023-01-18 120
e2192de59e457a Johannes Berg 2023-01-18 121 /**
e2192de59e457a Johannes Berg 2023-01-18 122 * FIELD_PREP_CONST() - prepare a constant bitfield element
e2192de59e457a Johannes Berg 2023-01-18 123 * @_mask: shifted mask defining the field's length and position
e2192de59e457a Johannes Berg 2023-01-18 124 * @_val: value to put in the field
e2192de59e457a Johannes Berg 2023-01-18 125 *
e2192de59e457a Johannes Berg 2023-01-18 126 * FIELD_PREP_CONST() masks and shifts up the value. The result should
e2192de59e457a Johannes Berg 2023-01-18 127 * be combined with other fields of the bitfield using logical OR.
e2192de59e457a Johannes Berg 2023-01-18 128 *
e2192de59e457a Johannes Berg 2023-01-18 129 * Unlike FIELD_PREP() this is a constant expression and can therefore
e2192de59e457a Johannes Berg 2023-01-18 130 * be used in initializers. Error checking is less comfortable for this
e2192de59e457a Johannes Berg 2023-01-18 131 * version, and non-constant masks cannot be used.
e2192de59e457a Johannes Berg 2023-01-18 132 */
e2192de59e457a Johannes Berg 2023-01-18 133 #define FIELD_PREP_CONST(_mask, _val) \
e2192de59e457a Johannes Berg 2023-01-18 134 ( \
e2192de59e457a Johannes Berg 2023-01-18 135 /* mask must be non-zero */ \
e2192de59e457a Johannes Berg 2023-01-18 136 BUILD_BUG_ON_ZERO((_mask) == 0) + \
e2192de59e457a Johannes Berg 2023-01-18 137 /* check if value fits */ \
e2192de59e457a Johannes Berg 2023-01-18 138 BUILD_BUG_ON_ZERO(~((_mask) >> __bf_shf(_mask)) & (_val)) + \
e2192de59e457a Johannes Berg 2023-01-18 139 /* check if mask is contiguous */ \
e2192de59e457a Johannes Berg 2023-01-18 140 __BF_CHECK_POW2((_mask) + (1ULL << __bf_shf(_mask))) + \
e2192de59e457a Johannes Berg 2023-01-18 141 /* and create the value */ \
e2192de59e457a Johannes Berg 2023-01-18 142 (((typeof(_mask))(_val) << __bf_shf(_mask)) & (_mask)) \
e2192de59e457a Johannes Berg 2023-01-18 143 )
e2192de59e457a Johannes Berg 2023-01-18 144
3e9b3112ec74f1 Jakub Kicinski 2016-08-31 145 /**
3e9b3112ec74f1 Jakub Kicinski 2016-08-31 146 * FIELD_GET() - extract a bitfield element
3e9b3112ec74f1 Jakub Kicinski 2016-08-31 147 * @_mask: shifted mask defining the field's length and position
7240767450d6d8 Masahiro Yamada 2017-10-03 148 * @_reg: value of entire bitfield
3e9b3112ec74f1 Jakub Kicinski 2016-08-31 149 *
3e9b3112ec74f1 Jakub Kicinski 2016-08-31 150 * FIELD_GET() extracts the field specified by @_mask from the
3e9b3112ec74f1 Jakub Kicinski 2016-08-31 151 * bitfield passed in as @_reg by masking and shifting it down.
3e9b3112ec74f1 Jakub Kicinski 2016-08-31 152 */
3e9b3112ec74f1 Jakub Kicinski 2016-08-31 153 #define FIELD_GET(_mask, _reg) \
3e9b3112ec74f1 Jakub Kicinski 2016-08-31 154 ({ \
3e9b3112ec74f1 Jakub Kicinski 2016-08-31 155 __BF_FIELD_CHECK(_mask, _reg, 0U, "FIELD_GET: "); \
3e9b3112ec74f1 Jakub Kicinski 2016-08-31 156 (typeof(_mask))(((_reg) & (_mask)) >> __bf_shf(_mask)); \
3e9b3112ec74f1 Jakub Kicinski 2016-08-31 157 })
3e9b3112ec74f1 Jakub Kicinski 2016-08-31 158
e7d4a95da86e0b Johannes Berg 2018-06-20 159 extern void __compiletime_error("value doesn't fit into mask")
00b0c9b82663ac Al Viro 2017-12-14 160 __field_overflow(void);
00b0c9b82663ac Al Viro 2017-12-14 161 extern void __compiletime_error("bad bitfield mask")
00b0c9b82663ac Al Viro 2017-12-14 162 __bad_mask(void);
00b0c9b82663ac Al Viro 2017-12-14 163 static __always_inline u64 field_multiplier(u64 field)
00b0c9b82663ac Al Viro 2017-12-14 164 {
00b0c9b82663ac Al Viro 2017-12-14 165 if ((field | (field - 1)) & ((field | (field - 1)) + 1))
00b0c9b82663ac Al Viro 2017-12-14 166 __bad_mask();
00b0c9b82663ac Al Viro 2017-12-14 167 return field & -field;
00b0c9b82663ac Al Viro 2017-12-14 168 }
00b0c9b82663ac Al Viro 2017-12-14 169 static __always_inline u64 field_mask(u64 field)
00b0c9b82663ac Al Viro 2017-12-14 170 {
00b0c9b82663ac Al Viro 2017-12-14 171 return field / field_multiplier(field);
00b0c9b82663ac Al Viro 2017-12-14 172 }
e31a50162feb35 Alex Elder 2020-03-12 173 #define field_max(field) ((typeof(field))field_mask(field))
00b0c9b82663ac Al Viro 2017-12-14 174 #define ____MAKE_OP(type,base,to,from) \
00b0c9b82663ac Al Viro 2017-12-14 175 static __always_inline __##type type##_encode_bits(base v, base field) \
00b0c9b82663ac Al Viro 2017-12-14 176 { \
e7d4a95da86e0b Johannes Berg 2018-06-20 177 if (__builtin_constant_p(v) && (v & ~field_mask(field))) \
00b0c9b82663ac Al Viro 2017-12-14 178 __field_overflow(); \
38e224b77cde3b Ian Rogers 2025-04-30 179 return to((__##type)((v & field_mask(field)) * field_multiplier(field))); \
00b0c9b82663ac Al Viro 2017-12-14 180 } \
00b0c9b82663ac Al Viro 2017-12-14 181 static __always_inline __##type type##_replace_bits(__##type old, \
00b0c9b82663ac Al Viro 2017-12-14 182 base val, base field) \
00b0c9b82663ac Al Viro 2017-12-14 183 { \
00b0c9b82663ac Al Viro 2017-12-14 184 return (old & ~to(field)) | type##_encode_bits(val, field); \
00b0c9b82663ac Al Viro 2017-12-14 185 } \
00b0c9b82663ac Al Viro 2017-12-14 186 static __always_inline void type##p_replace_bits(__##type *p, \
00b0c9b82663ac Al Viro 2017-12-14 187 base val, base field) \
00b0c9b82663ac Al Viro 2017-12-14 188 { \
00b0c9b82663ac Al Viro 2017-12-14 189 *p = (*p & ~to(field)) | type##_encode_bits(val, field); \
00b0c9b82663ac Al Viro 2017-12-14 190 } \
00b0c9b82663ac Al Viro 2017-12-14 191 static __always_inline base type##_get_bits(__##type v, base field) \
00b0c9b82663ac Al Viro 2017-12-14 192 { \
00b0c9b82663ac Al Viro 2017-12-14 193 return (from(v) & field)/field_multiplier(field); \
00b0c9b82663ac Al Viro 2017-12-14 194 }
00b0c9b82663ac Al Viro 2017-12-14 195 #define __MAKE_OP(size) \
00b0c9b82663ac Al Viro 2017-12-14 196 ____MAKE_OP(le##size,u##size,cpu_to_le##size,le##size##_to_cpu) \
00b0c9b82663ac Al Viro 2017-12-14 197 ____MAKE_OP(be##size,u##size,cpu_to_be##size,be##size##_to_cpu) \
00b0c9b82663ac Al Viro 2017-12-14 198 ____MAKE_OP(u##size,u##size,,)
37a3862e123826 Johannes Berg 2018-06-20 199 ____MAKE_OP(u8,u8,,)
00b0c9b82663ac Al Viro 2017-12-14 @200 __MAKE_OP(16)
00b0c9b82663ac Al Viro 2017-12-14 @201 __MAKE_OP(32)
00b0c9b82663ac Al Viro 2017-12-14 @202 __MAKE_OP(64)
00b0c9b82663ac Al Viro 2017-12-14 203 #undef __MAKE_OP
00b0c9b82663ac Al Viro 2017-12-14 204 #undef ____MAKE_OP
00b0c9b82663ac Al Viro 2017-12-14 205
--
0-DAY CI Kernel Test Service
https://github.com/intel/lkp-tests/wiki
On Wed, 30 Apr 2025 10:15:30 -0700
Ian Rogers <irogers@google.com> wrote:
> The clang warning -Wshorten-64-to-32 can be useful to catch
> inadvertent truncation. In some instances this truncation can lead to
> changing the sign of a result, for example, truncation to return an
> int to fit a sort routine. Silence the warning by making the implicit
> truncation explicit. This isn't to say the code is currently incorrect
> but without silencing the warning it is hard to spot the erroneous
> cases.
I suspect that is going to add an explicit mask in many cases.
Probably for u8, u16 and u32 if the return value from the old
code would return a u64 and the value is assigned to a u64.
Now if 'field_mask()' and 'field_multiplier()' are constants
then the compiler might optimise away the extra mask.
But in that case it shouldn't be bleating about the truncation
because it knows it can't happen.
So get the compiler fixed to do proper 'value tracking' and only
report about truncation when the high bits might be non-zero.
It also needs to not warn when the high bits are saved separately
(int h = v64 >> 32, l = v64;).
Then you have to worry about stopping the compiler bleating
for the return values of read (and similar).
Once you've got the compiler fixed to remove most of the false
positives worry about the Linux code.
David
>
> Signed-off-by: Ian Rogers <irogers@google.com>
> ---
> include/linux/bitfield.h | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/include/linux/bitfield.h b/include/linux/bitfield.h
> index 63928f173223..cc5cfed041bb 100644
> --- a/include/linux/bitfield.h
> +++ b/include/linux/bitfield.h
> @@ -176,7 +176,7 @@ static __always_inline __##type type##_encode_bits(base v, base field) \
> { \
> if (__builtin_constant_p(v) && (v & ~field_mask(field))) \
> __field_overflow(); \
> - return to((v & field_mask(field)) * field_multiplier(field)); \
> + return to((__##type)((v & field_mask(field)) * field_multiplier(field))); \
> } \
> static __always_inline __##type type##_replace_bits(__##type old, \
> base val, base field) \
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