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

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

Changed page_align() to return Option<usize> which allows for validation
of the provided addr value. This ensures that any value that is provided
within one PAGE_SIZE of usize::MAX will not panic and instead page_align
will return None.

Callers of page_align() should raise a EINVAL when they receive None
from page_align().

Signed-off-by: Brendan Shephard <bshephar@bne-home.net>
---
 rust/kernel/page.rs | 13 ++++++-------
 1 file changed, 6 insertions(+), 7 deletions(-)

diff --git a/rust/kernel/page.rs b/rust/kernel/page.rs
index 432fc0297d4a..b78473b67003 100644
--- a/rust/kernel/page.rs
+++ b/rust/kernel/page.rs
@@ -26,13 +26,12 @@
 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 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