[PATCH v5] rust: Return Option from page_align and ensure no usize overflow

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

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>
---
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.
- Link to v1 (and v2): https://lore.kernel.org/rust-for-linux/aSheTh-T1oroAUHR@fedora/T/#t

Changes in v3:
- Fix documentation layout for better rustdoc rendering
- Add doc examples and doctest
- Ensure function is always inlined for performance optimisation
- Restructure function so that early return is the None case and the
  default is the happy path.

Changes in v4:
- Fix rustdoc missing comment (//) prefix
- Rebase on master
- Link to v3: https://lore.kernel.org/rust-for-linux/aSoY31U3uDI2y7V1@fedora/T/#u

Changes in v5:
- Use kernel `PAGE_SIZE` for all doctest examples
- Backtick the backtickable works in example comment
- Add new example for `usize::MAX` input value
- Newline before # Examples
- Link to v4: https://lore.kernel.org/rust-for-linux/aSzDj1htLp11eCWF@fedora/T/#t

rust/kernel/page.rs | 36 ++++++++++++++++++++++++++++++------
 1 file changed, 30 insertions(+), 6 deletions(-)

diff --git a/rust/kernel/page.rs b/rust/kernel/page.rs
index 432fc0297d4a..4a0cfa32a5d6 100644
--- a/rust/kernel/page.rs
+++ b/rust/kernel/page.rs
@@ -27,12 +27,36 @@

 /// 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
+/// Returns a page aligned [`usize`] in cases where the value can be aligned. Otherwise, returns [`None`]
+/// if the aligned size will overflow a [`usize`].
+///
+/// # Examples
+///
+/// ```rust
+/// use kernel::page::{page_align, PAGE_SIZE};
+/// // Case 1: Already aligned
+/// assert_eq!(page_align(0x0), Some(0x0));
+/// assert_eq!(page_align(PAGE_SIZE), Some(PAGE_SIZE));
+///
+/// // Case 2: Needs alignment up
+/// assert_eq!(page_align(0x1), Some(PAGE_SIZE));
+/// assert_eq!(page_align(PAGE_SIZE + 1), Some(2 * PAGE_SIZE));
+///
+/// // Case 3: Requested address causes overflow (returns None)
+/// // The check asserts that `None` is returned when a value is requested within one `PAGE_SIZE` of
+/// // `usize::MAX`.
+/// let overflow_addr = usize::MAX - (PAGE_SIZE / 2);
+/// assert_eq!(page_align(overflow_addr), None);
+///
+/// // Case 4: Requested address size of `usize::MAX` would overflow, and subsequently returns `None`.
+/// assert_eq!(page_align(usize::MAX), None);
+/// ```
+#[inline(always)]
+pub const fn page_align(addr: usize) -> Option<usize> {
+    let Some(sum) = addr.checked_add(PAGE_SIZE - 1) else {
+        return None;
+    };
+    Some(sum & PAGE_MASK)
 }

 /// Representation of a non-owning reference to a [`Page`].
--
2.51.1