[PATCH 6/9] bitfield: Update sanity checks

david.laight.linux
2025-12-09T10:03:51Z

From: David Laight <david.laight.linux@gmail.com>

Simplify the check for 'reg' being large enough to hold 'mask' using
sizeof (reg) rather than a convoluted scheme to generate an unsigned
type the same size as 'reg'.

There are three places where the mask is checked for being non-zero
and contiguous. Add a simple expression that checks it and use in
all three places.

Three of the five calls to __BF_FIELD_CHECK_MASK() don't have a 'value'
to check, separate out as was done to __BF_FIELD_CHECK_REG().

There is no point checking a 'val' of zero or a 'reg' of 0ULL (both
are placeholders) - remove/change the calls.

There should be a check of __BF_FIELD_CHECK_REG() when __BF_FIELD_GET()
is called from field_get().
Move the check from FIELD_GET() into __BF_FIELD_GET().

Delete the now-unused __BF_FIELD_CHECK().

Signed-off-by: David Laight <david.laight.linux@gmail.com>
---
include/linux/bitfield.h | 74 ++++++++++++++--------------------------
1 file changed, 26 insertions(+), 48 deletions(-)

diff --git a/include/linux/bitfield.h b/include/linux/bitfield.h
index 3e0e8533bb66..7e8d436b6571 100644
--- a/include/linux/bitfield.h
+++ b/include/linux/bitfield.h
@@ -45,55 +45,38 @@

#define __bf_shf(x) (__builtin_ffsll(x) - 1)

-#define __scalar_type_to_unsigned_cases(type) \
- unsigned type: (unsigned type)0, \
- signed type: (unsigned type)0
+#define __BF_VALIDATE_MASK(mask) \
+ (!(mask) || ((mask) & ((mask) + ((mask) & -(mask)))))

-#define __unsigned_scalar_typeof(x) typeof( \
- _Generic((x), \
- char: (unsigned char)0, \
- __scalar_type_to_unsigned_cases(char), \
- __scalar_type_to_unsigned_cases(short), \
- __scalar_type_to_unsigned_cases(int), \
- __scalar_type_to_unsigned_cases(long), \
- __scalar_type_to_unsigned_cases(long long), \
- default: (x)))
-
-#define __bf_cast_unsigned(type, x) ((__unsigned_scalar_typeof(type))(x))
-
-#define __BF_FIELD_CHECK_MASK(mask, val, pfx) \
- ({ \
+#define __BF_FIELD_CHECK_MASK(mask, pfx) \
+ do { \
BUILD_BUG_ON_MSG(!__builtin_constant_p(mask), \
pfx "mask is not constant"); \
- BUILD_BUG_ON_MSG((mask) == 0, _pfx "mask is zero"); \
- BUILD_BUG_ON_MSG(__builtin_constant_p(val) ? \
- ~((mask) >> __bf_shf(mask)) & \
- (0 + (val)) : 0, \
- pfx "value too large for the field"); \
- __BUILD_BUG_ON_NOT_POWER_OF_2((mask) + \
- (1ULL << __bf_shf(mask))); \
- })
+ BUILD_BUG_ON_MSG(__BF_VALIDATE_MASK(mask), \
+ pfx "mask is zero or not contiguous"); \
+ } while (0)
+
+#define __BF_FIELD_CHECK_VAL(mask, val, pfx) \
+ BUILD_BUG_ON_MSG(__builtin_constant_p(val) && \
+ ~((mask) >> __bf_shf(mask)) & (val), \
+ pfx "value too large for the field")

#define __BF_FIELD_CHECK_REG(mask, reg, pfx) \
- BUILD_BUG_ON_MSG(__bf_cast_unsigned(mask, mask) > \
- __bf_cast_unsigned(reg, ~0ull), \
+ BUILD_BUG_ON_MSG(mask + 0U + 0UL + 0ULL > \
+ ~0ULL >> (64 - 8 * sizeof (reg)), \
pfx "type of reg too small for mask")

-#define __BF_FIELD_CHECK(mask, reg, val, pfx) \
- ({ \
- __BF_FIELD_CHECK_MASK(mask, val, pfx); \
- __BF_FIELD_CHECK_REG(mask, reg, pfx); \
- })
-
#define __BF_FIELD_PREP(mask, val, pfx) \
({ \
- __BF_FIELD_CHECK_MASK(mask, val, pfx); \
+ __BF_FIELD_CHECK_MASK(mask, pfx); \
+ __BF_FIELD_CHECK_VAL(mask, val, pfx); \
((val) << __bf_shf(mask)) & (mask); \
})

#define __BF_FIELD_GET(mask, reg, pfx) \
({ \
- __BF_FIELD_CHECK_MASK(mask, 0U, pfx); \
+ __BF_FIELD_CHECK_MASK(mask, pfx); \
+ __BF_FIELD_CHECK_REG(mask, reg, pfx); \
((reg) & (mask)) >> __bf_shf(mask); \
})

@@ -107,7 +90,7 @@
#define FIELD_MAX(mask) \
({ \
__auto_type _mask = mask; \
- __BF_FIELD_CHECK(_mask, 0ULL, 0ULL, "FIELD_MAX: "); \
+ __BF_FIELD_CHECK_MASK(_mask, "FIELD_MAX: "); \
(_mask >> __bf_shf(_mask)); \
})

@@ -122,7 +105,7 @@
({ \
__auto_type _mask = mask; \
__auto_type _val = 1 ? (val) : _mask; \
- __BF_FIELD_CHECK(_mask, 0ULL, 0ULL, "FIELD_FIT: "); \
+ __BF_FIELD_CHECK_MASK(_mask, "FIELD_FIT: "); \
!((_val << __bf_shf(_mask)) & ~_mask); \
})

@@ -138,12 +121,9 @@
({ \
__auto_type _mask = mask; \
__auto_type _val = 1 ? (val) : _mask; \
- __BF_FIELD_CHECK_REG(_mask, 0ULL, "FIELD_PREP: "); \
__BF_FIELD_PREP(_mask, _val, "FIELD_PREP: "); \
})

-#define __BF_CHECK_POW2(n) BUILD_BUG_ON_ZERO(((n) & ((n) - 1)) != 0)
-
/**
* FIELD_PREP_CONST() - prepare a constant bitfield element
* @mask: shifted mask defining the field's length and position
@@ -158,12 +138,10 @@
*/
#define FIELD_PREP_CONST(mask, val) \
( \
- /* mask must be non-zero */ \
- BUILD_BUG_ON_ZERO((mask) == 0) + \
+ /* mask must be non-zero and contiguous */ \
+ BUILD_BUG_ON_ZERO(__BF_VALIDATE_MASK(mask)) + \
/* check if value fits */ \
BUILD_BUG_ON_ZERO(~((mask) >> __bf_shf(mask)) & (val)) + \
- /* check if mask is contiguous */ \
- __BF_CHECK_POW2((mask) + (1ULL << __bf_shf(mask))) + \
/* and create the value */ \
(((typeof(mask))(val) << __bf_shf(mask)) & (mask)) \
)
@@ -180,7 +158,6 @@
({ \
__auto_type _mask = mask; \
__auto_type _reg = reg; \
- __BF_FIELD_CHECK_REG(_mask, _reg, "FIELD_GET: "); \
__BF_FIELD_GET(_mask, _reg, "FIELD_GET: "); \
})

@@ -198,8 +175,9 @@
__auto_type _mask = mask; \
__auto_type _reg_p = reg_p; \
__auto_type _val = 1 ? (val) : _mask; \
- typecheck_pointer(_reg_p); \
- __BF_FIELD_CHECK(_mask, *_reg_p, _val, "FIELD_MODIFY: "); \
+ __BF_FIELD_CHECK_MASK(_mask, "FIELD_MODIFY: "); \
+ __BF_FIELD_CHECK_VAL(_mask, _val, "FIELD_MODIFY: "); \
+ __BF_FIELD_CHECK_REG(_mask, *_reg_p, "FIELD_MODIFY: "); \
*_reg_p = (*_reg_p & ~_mask) | ((_val << __bf_shf(_mask)) & _mask); \
})

@@ -209,7 +187,7 @@ extern void __compiletime_error("bad bitfield mask")
__bad_mask(void);
static __always_inline u64 field_multiplier(u64 field)
{
- if ((field | (field - 1)) & ((field | (field - 1)) + 1))
+ if (__BF_VALIDATE_MASK(field))
__bad_mask();
return field & -field;
}
--
2.39.5