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rust: time: fix as_micros_ceil() rounding near i64::MAX

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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: fae0cdc123 ("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>
Acked-by: Andreas Hindborg <a.hindborg@kernel.org>
Cc: stable@vger.kernel.org
Link: https://patch.msgid.link/20260807130531.1056209-1-tomo@flapping.org
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
This commit is contained in:
FUJITA Tomonori
2026-08-07 22:05:31 +09:00
committed by Miguel Ojeda
parent 409d09194c
commit ec90dfcf05
1 changed files with 10 additions and 7 deletions
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17
rust/kernel/time.rs
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@@ -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
}
}