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SubjectRe: [PATCH 2/2] rust: time: Use wrapping_sub() for Ktime::sub()
On Thu, Apr 11, 2024 at 04:08:01PM -0700, Boqun Feng wrote:
> Currently since Rust code is compiled with "-Coverflow-checks=y", so a
> normal substraction may be compiled as an overflow checking and panic
> if overflow happens:
>
> subq %rsi, %rdi
> jo .LBB0_2
> movq %rdi, %rax
> retq
> .LBB0_2:
> pushq %rax
> leaq str.0(%rip), %rdi
> leaq .L__unnamed_1(%rip), %rdx
> movl $33, %esi
> callq *core::panicking::panic::h59297120e85ea178@GOTPCREL(%rip)
>
> although overflow detection is nice to have, however this makes
> `Ktime::sub()` behave differently than `ktime_sub()`, moreover it's not
> clear that the overflow checking is helpful, since for example, the
> current binder usage[1] doesn't have the checking.
>

Ping. Thomas, John and Stepthen. Could you take a look at this, and the
discussion between Miguel and me? The key question is the behavior when
ktime_sub() hits a overflow, I think. Thanks!

(Cc Kees as well)

Regards,
Boqun

> Therefore make `Ktime::sub()` have the same semantics as `ktime_sub()`:
> overflow behaves like 2s-complement wrapping sub.
>
> Link: https://lore.kernel.org/lkml/5ac8c0d09392290be789423f0dd78a520b830fab.1682333709.git.zhangchuang3@xiaomi.com/ [1]
> Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
> ---
> rust/kernel/time.rs | 4 +++-
> 1 file changed, 3 insertions(+), 1 deletion(-)
>
> diff --git a/rust/kernel/time.rs b/rust/kernel/time.rs
> index e3bb5e89f88d..3cb15d3079f4 100644
> --- a/rust/kernel/time.rs
> +++ b/rust/kernel/time.rs
> @@ -77,7 +77,9 @@ impl core::ops::Sub for Ktime {
> #[inline]
> fn sub(self, other: Ktime) -> Ktime {
> Self {
> - inner: self.inner - other.inner,
> + // Mirrors `ktime_sub()`, kernel defines signed overflow to behave like 2s-complement,
> + // hence `wrapping_sub()` is used.
> + inner: self.inner.wrapping_sub(other.inner),
> }
> }
> }
> --
> 2.44.0
>

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