Re: [PATCH 07/10] mm: correct sign-extension issue in MMF_* flag masks

David Hildenbrand
2025-08-26T13:05:35Z

On 12.08.25 17:44, Lorenzo Stoakes wrote:
> There is an issue with the mask declarations in linux/mm_types.h, which
> naively do (1 << bit) operations. Unfortunately this results in the 1 being
> defaulted as a signed (32-bit) integer.
>
> When the compiler expands the MMF_INIT_MASK bitmask it comes up with:
>
> (((1 << 2) - 1) | (((1 << 9) - 1) << 2) | (1 << 24) | (1 << 28) | (1 << 30)
> | (1 << 31))
>
> Which overflows the signed integer to -788,527,105. Implicitly casting this
> to an unsigned integer results in sign-expansion, and thus this value
> becomes 0xffffffffd10007ff, rather than the intended 0xd10007ff.
>
> While we're limited to a maximum of 32 bits in mm->flags, this isn't an
> issue as the remaining bits being masked will always be zero.
>
> However, now we are moving towards having more bits in this flag, this
> becomes an issue.
>
> Simply resolve this by using the _BITUL() helper to cast the shifted value
> to an unsigned long.

Hmm, I thought BIT() should be used and would just fine?

include/linux/bits.h includes <vdso/bits.h> where we have

#define BIT(nr) (UL(1) << (nr))

In contrast, _BITUL is a uapi thingy from include/uapi/linux/const.h ...
as it seems.

$ git grep "_BITUL" -- include/linux/
include/linux/mm_types.h:#define MMF_DUMPABLE_MASK (_BITUL(MMF_DUMPABLE_BITS) - 1)
include/linux/mm_types.h: ((_BITUL(MMF_DUMP_FILTER_BITS) - 1) << MMF_DUMP_FILTER_SHIFT)
include/linux/mm_types.h: (_BITUL(MMF_DUMP_ANON_PRIVATE) | _BITUL(MMF_DUMP_ANON_SHARED) | \
include/linux/mm_types.h: _BITUL(MMF_DUMP_HUGETLB_PRIVATE) | MMF_DUMP_MASK_DEFAULT_ELF)
include/linux/mm_types.h:# define MMF_DUMP_MASK_DEFAULT_ELF _BITUL(MMF_DUMP_ELF_HEADERS)
include/linux/mm_types.h:#define MMF_DISABLE_THP_MASK (_BITUL(MMF_DISABLE_THP_COMPLETELY) | \
include/linux/mm_types.h: _BITUL(MMF_DISABLE_THP_EXCEPT_ADVISED))
include/linux/mm_types.h:#define MMF_HAS_MDWE_MASK _BITUL(MMF_HAS_MDWE)
include/linux/mm_types.h:#define MMF_VM_MERGE_ANY_MASK _BITUL(MMF_VM_MERGE_ANY)
include/linux/mm_types.h:#define MMF_TOPDOWN_MASK _BITUL(MMF_TOPDOWN)

Oh, hey, it's only your changes :P

We should better just be using BIT().

--
Cheers

David / dhildenb