Re: [PATCH 2/2] rust: time: Use wrapping_sub() for Ktime::sub()

[Boqun Feng <boqun.feng@gmail.com>]

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
>