Re: [PATCH] rust: page: Simplify overflow check using checked_add()

["Alexandre Courbot" <acourbot@nvidia.com>]

On Sat Dec 20, 2025 at 6:29 AM JST, Kari Argillander wrote:
> Replace the explicit bounds comparisons with a single checked_add()-based
> range check. This avoids redundant comparisons, makes the overflow case
> explicit, and results in simpler generated code (checked with godbolt
> for x86).
>
> Option::is_none_or() would be nicer, but it requires Rust 1.82; the
> kernel currently targets 1.78.
>
> No functional change intended.
>
> Signed-off-by: Kari Argillander <kari.argillander@gmail.com>
> ---
>  rust/kernel/page.rs | 18 ++++++++----------
>  1 file changed, 8 insertions(+), 10 deletions(-)
>
> diff --git a/rust/kernel/page.rs b/rust/kernel/page.rs
> index 432fc0297d4a..a07e6d256860 100644
> --- a/rust/kernel/page.rs
> +++ b/rust/kernel/page.rs
> @@ -239,17 +239,15 @@ fn with_pointer_into_page<T>(
>          len: usize,
>          f: impl FnOnce(*mut u8) -> Result<T>,
>      ) -> Result<T> {
> -        let bounds_ok = off <= PAGE_SIZE && len <= PAGE_SIZE && (off + len) <= PAGE_SIZE;
> -
> -        if bounds_ok {
> -            self.with_page_mapped(move |page_addr| {
> -                // SAFETY: The `off` integer is at most `PAGE_SIZE`, so this pointer offset will
> -                // result in a pointer that is in bounds or one off the end of the page.
> -                f(unsafe { page_addr.add(off) })
> -            })
> -        } else {
> -            Err(EINVAL)
> +        if off.checked_add(len).map_or(true, |end| end > PAGE_SIZE) {

I find this line a bit heavy with the two-steps check, how about
something like:

    if !matches!(off.checked_add(len), Some(e) if e <= PAGE_SIZE) {

Just for your consideration - the original is still valid and an
improvement over the original code, so regardless:

Reviewed-by: Alexandre Courbot <acourbot@nvidia.com>