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

[Kari Argillander]

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@xxxxxxxxxxxx>
Reviewed-by: Alexandre Courbot <acourbot@xxxxxxxxxx>
Signed-off-by: Kari Argillander <kari.argillander@xxxxxxxxx>
---
Changes in v2:
- Added MSVR todo (Dirk Behme)
- Link to v1: https://lore.kernel.org/r/20251219-rust-page-check-v1-1-df2e52fa3bd5@xxxxxxxxx
---
 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.
+        if off.checked_add(len).map_or(true, |end| end > PAGE_SIZE) {
+            return Err(EINVAL);
         }
+
+        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) })
+        })
     }
 
     /// Maps the page and reads from it into the given buffer.

---
base-commit: cc3aa43b44bdb43dfbac0fcb51c56594a11338a8
change-id: 20251219-rust-page-check-819ccc39c53a

Best regards,
-- 
Kari Argillander <kari.argillander@xxxxxxxxx>