Messages in this thread Patch in this message | | | Date | Sun, 22 Oct 2023 22:39:54 +0300 | From | Alexey Dobriyan <> | Subject | PSA: BITS_TO_LONGS() most likely returns size_t, not what you think |
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I wrote the following code today (don't ask):
for (int i = 0; i <= BITS_TO_LONGS(NR_CPUS) - 2; i += 1) { }
only to get near infinite loop. Note that NR_CPUS is "int".
This is because BITS_TO_LONGS() and __KERNEL_DIV_ROUND_UP() macros work together to promote everything to "size_t". The loop essientially becomes:
for (int i = 0; i <= (size_t)-1; i += 1) { }
This is easily fixable by doing very sketchy and obvious thing.
--- a/include/linux/bitops.h +++ b/include/linux/bitops.h @@ -16,7 +16,7 @@ # define aligned_byte_mask(n) (~0xffUL << (BITS_PER_LONG - 8 - 8*(n))) #endif -#define BITS_PER_TYPE(type) (sizeof(type) * BITS_PER_BYTE) +#define BITS_PER_TYPE(type) ((int)sizeof(type) * BITS_PER_BYTE) #define BITS_TO_LONGS(nr) __KERNEL_DIV_ROUND_UP(nr, BITS_PER_TYPE(long)) #define BITS_TO_U64(nr) __KERNEL_DIV_ROUND_UP(nr, BITS_PER_TYPE(u64)) #define BITS_TO_U32(nr) __KERNEL_DIV_ROUND_UP(nr, BITS_PER_TYPE(u32)) Ideally __KERNEL_DIV_ROUND_UP() should return the type of the first argument but this stuff is UAPI header which complicates things.
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