Re: [PATCH v7 16/31] rust: ptr: add const_align_up()

[Alice Ryhl]

On Fri, Mar 20, 2026 at 9:38 AM David Rheinsberg <david@xxxxxxxxxxxx> wrote:
>
> Hi
>
> On Tue, Mar 17, 2026, at 11:53 PM, John Hubbard wrote:
> > Add const_align_up() to kernel::ptr as the const-compatible equivalent
> > of Alignable::align_up().
> >
> > Suggested-by: Danilo Krummrich <dakr@xxxxxxxxxx>
> > Suggested-by: Gary Guo <gary@xxxxxxxxxxx>
> > Suggested-by: Miguel Ojeda <ojeda@xxxxxxxxxx>
> > Signed-off-by: John Hubbard <jhubbard@xxxxxxxxxx>
> > ---
> >  rust/kernel/ptr.rs | 24 ++++++++++++++++++++++++
> >  1 file changed, 24 insertions(+)
> >
> > diff --git a/rust/kernel/ptr.rs b/rust/kernel/ptr.rs
> > index bdc2d79ff669..7e99f129543b 100644
> > --- a/rust/kernel/ptr.rs
> > +++ b/rust/kernel/ptr.rs
> > @@ -253,3 +253,27 @@ fn size(p: *const Self) -> usize {
> >          p.len() * size_of::<T>()
> >      }
> >  }
> > +
> > +/// Aligns `value` up to `align`.
> > +///
> > +/// This is the const-compatible equivalent of [`Alignable::align_up`].
> > +///
> > +/// Returns [`None`] on overflow.
> > +///
> > +/// # Examples
> > +///
> > +/// ```
> > +/// use kernel::ptr::{const_align_up, Alignment};
> > +/// use kernel::sizes::SZ_4K;
> > +///
> > +/// assert_eq!(const_align_up(0x4f, Alignment::new::<16>()), Some(0x50));
> > +/// assert_eq!(const_align_up(0x40, Alignment::new::<16>()), Some(0x40));
> > +/// assert_eq!(const_align_up(1, Alignment::new::<SZ_4K>()), Some(SZ_4K));
> > +/// ```
> > +#[inline(always)]
> > +pub const fn const_align_up(value: usize, align: Alignment) -> Option<usize> {
> > +    match value.checked_add(align.as_usize() - 1) {
> > +        Some(v) => Some(v & align.mask()),
> > +        None => None,
> > +    }
>
> This would return `None` if the value is already aligned, but the addition overflows `usize`, right? For instance, this would incorrectly return `None`: `const_align_up(usize::MAX - 1, 2.into())`

No, in that case it computes `usize::MAX-1 + (2-1)` which is just
usize::MAX and does not overflow. After applying the mask, it returns
`usize::MAX-1` as the return value.

> FYI, `core` provides `usize::checked_next_multiple_of()` ((const-)stable since 1.73). So an alternative would be:
>
> pub const fn const_align_up(value: usize, align: Alignment) -> Option<usize> {
>     value.checked_next_multiple_of(align.as_usize())
> }

That would return value+align when value is already aligned, which is wrong.

Alice