Messages in this thread | | | From | Alice Ryhl <> | Date | Fri, 12 Apr 2024 10:36:05 +0200 | Subject | Re: [PATCH 2/2] rust: time: Use wrapping_sub() for Ktime::sub() |
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On Fri, Apr 12, 2024 at 1:08 AM Boqun Feng <boqun.feng@gmail.com> 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.
I don't think this is a good idea at all. Any code that triggers an overflow in Ktime::sub is wrong, and anyone who enables CONFIG_RUST_OVERFLOW_CHECKS does so because they want such bugs to be caught. You may have been able to find one example of a subtraction that doesn't have a risk of overflow, but overflow bugs really do happen in the real world. I have seen real examples of bugs in Rust code, where overflow checks were the reason the bug was not a security vulnerability.
> Therefore make `Ktime::sub()` have the same semantics as `ktime_sub()`: > overflow behaves like 2s-complement wrapping sub.
From Miguel's reply, it sounds like 2s-complement wrapping is not even the semantics of ktime_sub. The semantics are just UB.
Alice
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