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

[Alice Ryhl]

On Fri, Apr 12, 2024 at 1:08 AM Boqun Feng <boqun.feng@xxxxxxxxx> 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