Re: [PATCH v2] rust: page: Simplify overflow check using checked_add()

[Gary Guo <gary@garyguo.net>]

On Tue, 23 Dec 2025 13:29:10 +0100
Dirk Behme <dirk.behme@gmail.com> wrote:

> On 23.12.25 12:57, Gary Guo wrote:
> > On Tue, 23 Dec 2025 12:06:17 +0200
> > Kari Argillander <kari.argillander@gmail.com> 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).
> >>
> >> No functional change intended.
> >>
> >> Reviewed-by: Dirk Behme <dirk.behme@de.bosch.com>
> >> Reviewed-by: Alexandre Courbot <acourbot@nvidia.com>
> >> Signed-off-by: Kari Argillander <kari.argillander@gmail.com>
> >> ---
> >> Changes in v2:
> >> - Added MSVR todo (Dirk Behme)
> >> - Link to v1: https://lore.kernel.org/r/20251219-rust-page-check-v1-1-df2e52fa3bd5@gmail.com
> >> ---
> >>  rust/kernel/page.rs | 19 +++++++++----------
> >>  1 file changed, 9 insertions(+), 10 deletions(-)
> >>
> >> diff --git a/rust/kernel/page.rs b/rust/kernel/page.rs
> >> index 432fc0297d4a..cd2af7e4c357 100644
> >> --- a/rust/kernel/page.rs
> >> +++ b/rust/kernel/page.rs
> >> @@ -239,17 +239,16 @@ 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)
> >> +        // TODO: Replace `map_or` with `is_none_or` once the MSRV is >= 1.82.  
> > 
> > I was about to suggest just enable the feature gate, but turns out it was
> > only added in 1.81. That's a fast one to stabilize!  
> 
> 
> With Alexandre's `matches!` proposal I was thinking that we could drop
> the TODO comment and we won't need to touch this file again once MSRV
> is >= 1.82. Opinions.

I personally would prefer `map_or` to match. The `matches!` approach is a
bit verbose to read to me.

Keeping a TODO comment about MSRV is good IMO as they're the data sources
that could justify bumping MSRV (also to remind people of cleaning up
after MSRV is bumped).

Best,
Gary

> 
> Cheers
> 
> Dirk
> 
> 
> > Reviewed-by: Gary Guo <gary@garyguo.net>
> > 
> > Best,
> > Gary
> >   
> >> +        if off.checked_add(len).map_or(true, |end| end > PAGE_SIZE) {
> >> +            return Err(EINVAL);
> >>          }