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

Liam R. Howlett
2025-08-12T17:32:13Z

* Lorenzo Stoakes <lorenzo.stoakes@oracle.com> [250812 11:47]:
> 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.
>
> Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>

Reviewed-by: Liam R. Howlett <Liam.Howlett@oracle.com>

> ---
> include/linux/mm_types.h | 19 +++++++++----------
> 1 file changed, 9 insertions(+), 10 deletions(-)
>
> diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
> index 46d3fb8935c7..38b3fa927997 100644
> --- a/include/linux/mm_types.h
> +++ b/include/linux/mm_types.h
> @@ -1756,7 +1756,7 @@ enum {
> * the modes are SUID_DUMP_* defined in linux/sched/coredump.h
> */
> #define MMF_DUMPABLE_BITS 2
> -#define MMF_DUMPABLE_MASK ((1 << MMF_DUMPABLE_BITS) - 1)
> +#define MMF_DUMPABLE_MASK (_BITUL(MMF_DUMPABLE_BITS) - 1)
> /* coredump filter bits */
> #define MMF_DUMP_ANON_PRIVATE 2
> #define MMF_DUMP_ANON_SHARED 3
> @@ -1771,13 +1771,13 @@ enum {
> #define MMF_DUMP_FILTER_SHIFT MMF_DUMPABLE_BITS
> #define MMF_DUMP_FILTER_BITS 9
> #define MMF_DUMP_FILTER_MASK \
> - (((1 << MMF_DUMP_FILTER_BITS) - 1) << MMF_DUMP_FILTER_SHIFT)
> + ((_BITUL(MMF_DUMP_FILTER_BITS) - 1) << MMF_DUMP_FILTER_SHIFT)
> #define MMF_DUMP_FILTER_DEFAULT \
> - ((1 << MMF_DUMP_ANON_PRIVATE) | (1 << MMF_DUMP_ANON_SHARED) |\
> - (1 << MMF_DUMP_HUGETLB_PRIVATE) | MMF_DUMP_MASK_DEFAULT_ELF)
> + (_BITUL(MMF_DUMP_ANON_PRIVATE) | _BITUL(MMF_DUMP_ANON_SHARED) | \
> + _BITUL(MMF_DUMP_HUGETLB_PRIVATE) | MMF_DUMP_MASK_DEFAULT_ELF)
>
> #ifdef CONFIG_CORE_DUMP_DEFAULT_ELF_HEADERS
> -# define MMF_DUMP_MASK_DEFAULT_ELF (1 << MMF_DUMP_ELF_HEADERS)
> +# define MMF_DUMP_MASK_DEFAULT_ELF _BITUL(MMF_DUMP_ELF_HEADERS)
> #else
> # define MMF_DUMP_MASK_DEFAULT_ELF 0
> #endif
> @@ -1797,7 +1797,7 @@ enum {
> #define MMF_UNSTABLE 22 /* mm is unstable for copy_from_user */
> #define MMF_HUGE_ZERO_FOLIO 23 /* mm has ever used the global huge zero folio */
> #define MMF_DISABLE_THP 24 /* disable THP for all VMAs */
> -#define MMF_DISABLE_THP_MASK (1 << MMF_DISABLE_THP)
> +#define MMF_DISABLE_THP_MASK _BITUL(MMF_DISABLE_THP)
> #define MMF_OOM_REAP_QUEUED 25 /* mm was queued for oom_reaper */
> #define MMF_MULTIPROCESS 26 /* mm is shared between processes */
> /*
> @@ -1810,16 +1810,15 @@ enum {
> #define MMF_HAS_PINNED 27 /* FOLL_PIN has run, never cleared */
>
> #define MMF_HAS_MDWE 28
> -#define MMF_HAS_MDWE_MASK (1 << MMF_HAS_MDWE)
> -
> +#define MMF_HAS_MDWE_MASK _BITUL(MMF_HAS_MDWE)
>
> #define MMF_HAS_MDWE_NO_INHERIT 29
>
> #define MMF_VM_MERGE_ANY 30
> -#define MMF_VM_MERGE_ANY_MASK (1 << MMF_VM_MERGE_ANY)
> +#define MMF_VM_MERGE_ANY_MASK _BITUL(MMF_VM_MERGE_ANY)
>
> #define MMF_TOPDOWN 31 /* mm searches top down by default */
> -#define MMF_TOPDOWN_MASK (1 << MMF_TOPDOWN)
> +#define MMF_TOPDOWN_MASK _BITUL(MMF_TOPDOWN)
>
> #define MMF_INIT_MASK (MMF_DUMPABLE_MASK | MMF_DUMP_FILTER_MASK |\
> MMF_DISABLE_THP_MASK | MMF_HAS_MDWE_MASK |\
> --
> 2.50.1
>