* [PATCH v2] rust: time: fix as_micros_ceil() rounding near i64::MAX
@ 2026-08-07 13:05 ` FUJITA Tomonori
2026-08-10 10:47 ` Andreas Hindborg
0 siblings, 1 reply; 2+ messages in thread
From: FUJITA Tomonori @ 2026-08-07 13:05 UTC (permalink / raw)
To: a.hindborg, ojeda
Cc: acourbot, aliceryhl, anna-maria, bjorn3_gh, boqun, dakr,
daniel.almeida, frederic, gary, jstultz, lossin, lyude, sboyd,
tamird, tglx, tmgross, work, rust-for-linux, FUJITA Tomonori,
Miguel Ojeda
From: FUJITA Tomonori <fujita.tomonori@gmail.com>
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 can yield a result one microsecond too small.
Fixes: fae0cdc12340 ("rust: time: Introduce Delta type")
Reported-by: Miguel Ojeda <miguel.ojeda.sandonis@gmail.com>
Closes: https://lore.kernel.org/rust-for-linux/CANiq72mtS0ABA2JnT5tpz6J9c_mnxY+vyPvghV_ukngWvN8F2w@mail.gmail.com/
Signed-off-by: FUJITA Tomonori <fujita.tomonori@gmail.com>
---
v2
- Add the comment; no functional change
v1: https://lore.kernel.org/all/20260723120907.209643-1-tomo@flapping.org/
---
rust/kernel/time.rs | 17 ++++++++++-------
1 file changed, 10 insertions(+), 7 deletions(-)
diff --git a/rust/kernel/time.rs b/rust/kernel/time.rs
index b8463823aed9..54fc46d460b6 100644
--- a/rust/kernel/time.rs
+++ b/rust/kernel/time.rs
@@ -441,22 +441,25 @@ pub const fn as_nanos(self) -> i64 {
/// to the value in the [`Delta`].
#[inline]
pub fn as_micros_ceil(self) -> i64 {
+ // Only positive values need to be rounded up: truncating division already
+ // rounds towards zero, i.e. up, for negative values.
+ //
+ // The usual `(nanos + d - 1) / d` is not used because the addition overflows
+ // once `nanos` exceeds `i64::MAX - (d - 1)`; saturating the addition instead
+ // would drop the rounding bias and return a result one unit too small.
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: 075b74841bd0065a3bda3440873c747938e69b68
--
2.43.0
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