Re: [PATCH v2] rust: Return Option from page_align and ensure no usize overflow

[Brendan Shephard <bshephar@bne-home.net>]

Changes in v2:
  - Reworded commit message to follow the imperative form.
  - Expanded the documentation to explain the `Some` and `None` return cases.
  - Added a period at the end of the documentation comment.

Change `page_align()` to return `Option<usize>` to allow validation
of the provided `addr` value. This ensures that any value that is
within one `PAGE_SIZE` of `usize::MAX` will not panic, and instead
returns `None` to indicate overflow.

Signed-off-by: Brendan Shephard <bshephar@bne-home.net>
---
 rust/kernel/page.rs | 14 +++++++-------
 1 file changed, 7 insertions(+), 7 deletions(-)

diff --git a/rust/kernel/page.rs b/rust/kernel/page.rs
index 432fc0297d4a..d5cc340c0689 100644
--- a/rust/kernel/page.rs
+++ b/rust/kernel/page.rs
@@ -26,13 +26,13 @@
 pub const PAGE_MASK: usize = !(PAGE_SIZE - 1);
 
 /// Round up the given number to the next multiple of [`PAGE_SIZE`].
-///
-/// It is incorrect to pass an address where the next multiple of [`PAGE_SIZE`] doesn't fit in a
-/// [`usize`].
-pub const fn page_align(addr: usize) -> usize {
-    // Parentheses around `PAGE_SIZE - 1` to avoid triggering overflow sanitizers in the wrong
-    // cases.
-    (addr + (PAGE_SIZE - 1)) & PAGE_MASK
+/// Return Some [`usize`] that is page aligned. Or None in cases where the next multiple of
+/// [`PAGE_SIZE`] would overflow a [`usize`].
+pub const fn page_align(addr: usize) -> Option<usize> {
+    if let Some(sum) = addr.checked_add(PAGE_SIZE - 1) {
+        return Some(sum & PAGE_MASK);
+    }
+    None
 }
 
 /// Representation of a non-owning reference to a [`Page`].
-- 
2.51.1