Re: [PATCH] rust: Return Option from page_align and ensure no usize overflow

[Alice Ryhl <aliceryhl@google.com>]

On Fri, Nov 28, 2025 at 7:51 AM Alexandre Courbot <acourbot@nvidia.com> wrote:
>
> On Fri Nov 28, 2025 at 2:29 PM JST, Brendan Shephard wrote:
> > On Fri, Nov 28, 2025 at 03:12:13AM +0100, Miguel Ojeda wrote:
> >> On Fri, Nov 28, 2025 at 1:29 AM Alexandre Courbot <acourbot@nvidia.com> wrote:
> >> >
> >> > Ah, indeed. In that case, can we make the if arms symmetric:
> >> >
> >> >     if let Some(sum) = addr.checked_add(PAGE_SIZE - 1) {
> >> >         Some(sum & PAGE_MASK);
> >> >     } else {
> >> >         None
> >> >     }
> >>
> >> Early return style makes the happy path less indented and is more
> >> consistent with the C side. However, here the happy case would be the
> >> indented case, so `let else` would be better:
> >>
> >>     let Some(sum) = addr.checked_add(PAGE_SIZE - 1) else {
> >>         return None;
> >>     };
> >>
> >>     Some(sum & PAGE_MASK)
> >>
> >> Cheers,
> >> Miguel
> >>
> >
> > A wildcard entrant to this conversation. We could do a match instead if
> > you all might prefer that?
> >
> > match addr.checked_add(PAGE_SIZE - 1) {
> >   Some(v) => Some(v & PAGE_MASK),
> >   None => None,
> > }
>
> Clippy will likely suggest to turn this into an `if { } else { }`, which
> puts us back to where we come from. Miguel's suggestion of returning
> early in the failure path looks good to me as well.

How about using the question mark operator?

Some(addr.checked_add(PAGE_SIZE - 1)? & PAGE_MASK)