[PATCH v1] rust: time: fix as_micros_ceil() rounding near i64::MAX

From: FUJITA Tomonori <[email protected]>

The ceiling adjustment used saturating_add(NSEC_PER_USEC - 1) before
dividing. Once the nanosecond value gets within NSEC_PER_USEC - 1 of
i64::MAX the addition saturates to i64::MAX, which drops the ceiling
bias and yields a result one microsecond too small.

Fixes: fae0cdc12340 ("rust: time: Introduce Delta type")
Reported-by: Miguel Ojeda <[email protected]>
Closes: https://lore.kernel.org/rust-for-linux/CANiq72mtS0ABA2JnT5tpz6J9c_mnxY+vyPvghV_ukngWvN8F2w@mail.gmail.com/
Signed-off-by: FUJITA Tomonori <[email protected]>
---
 rust/kernel/time.rs | 11 ++++-------
 1 file changed, 4 insertions(+), 7 deletions(-)

diff --git a/rust/kernel/time.rs b/rust/kernel/time.rs
index b8463823aed9..3d2c8e3a2f80 100644
--- a/rust/kernel/time.rs
+++ b/rust/kernel/time.rs
@@ -442,21 +442,18 @@ pub const fn as_nanos(self) -> i64 {
     #[inline]
     pub fn as_micros_ceil(self) -> i64 {
         let n = self.as_nanos();
-        let n = if n >= 0 {
-            n.saturating_add(NSEC_PER_USEC - 1)
-        } else {
-            n
-        };
+
+        let (n, add) = if n > 0 { (n - 1, 1) } else { (n, 0) };
 
         #[cfg(CONFIG_64BIT)]
         {
-            n / NSEC_PER_USEC
+            n / NSEC_PER_USEC + add
         }
 
         #[cfg(not(CONFIG_64BIT))]
         // SAFETY: It is always safe to call `ktime_to_us()` with any value.
         unsafe {
-            bindings::ktime_to_us(n)
+            bindings::ktime_to_us(n) + add
         }
     }
 

base-commit: 880c43b185ca52239e75bc546cc4f4d9154d0fed
-- 
2.43.0