Re: [PATCH] selinux: use explicit 64-bit division

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [Low] selinux: unnecessary 64-bit division for 24-bit bounded value
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commit 7622bbc45f42944796ad657eff450b2c4b7eb729
Author: Arnd Bergmann <arnd@arndb.de>

selinux: use explicit 64-bit division

The patch addresses a 32-bit build error where dividing a 64-bit integer
by 33 caused an undefined reference to __aeabi_uldivmod. It fixes this
by replacing standard division and modulo with explicit 64-bit division
helpers.

> diff --git a/security/selinux/selinuxfs.c b/security/selinux/selinuxfs.c
> index f60f92e2ac341..7894a2f480f74 100644
> --- a/security/selinux/selinuxfs.c
> +++ b/security/selinux/selinuxfs.c
> @@ -1573,7 +1573,7 @@ static inline u64 sel_class_to_ino(u16 class)
>  
>  static inline u16 sel_ino_to_class(u64 ino)
>  {
> -	return (ino & SEL_INO_MASK) / (SEL_VEC_MAX + 1);
> +	return div_u64(ino & SEL_INO_MASK, SEL_VEC_MAX + 1);
>  }

[Severity: Low]
Since SEL_INO_MASK restricts the dividend to 24 bits, the masked value fits
comfortably in a standard 32-bit unsigned integer.

Would it be better to downcast the masked value to u32 before division?

This would avoid the 64-bit division overhead entirely and allow the
compiler to use fast 32-bit math or optimize the constant division into a
multiply-and-shift.

>  static inline u64 sel_perm_to_ino(u16 class, u32 perm)
> @@ -1583,7 +1583,11 @@ static inline u64 sel_perm_to_ino(u16 class, u32 perm)
>  
>  static inline u32 sel_ino_to_perm(u64 ino)
>  {
> -	return (ino & SEL_INO_MASK) % (SEL_VEC_MAX + 1);
> +	u32 rem;
> +
> +	div_u64_rem(ino & SEL_INO_MASK, SEL_VEC_MAX + 1, &rem);
> +
> +	return rem;
>  }

[Severity: Low]
Similar to sel_ino_to_class(), does this need a full 64-bit quotient and
remainder calculation?

Given the dividend is also bounded to 24 bits by SEL_INO_MASK here, could
this be replaced with a 32-bit downcast and a standard modulo operation to be
more efficient while remaining mathematically identical?

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260915195728.3519469-1-arnd@kernel.org?part=1