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Subject[PATCH 2/2] rust: time: Use wrapping_sub() for Ktime::sub()
Date
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.

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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