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

[Boqun Feng]

On Fri, Apr 12, 2024 at 10:36:05AM +0200, Alice Ryhl wrote:
> 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

The point is you won't panic the kernel because of an overflow. I
agree that overflow is something we want to catch, but currently
ktime_t doesn't panic if overflow happens.

Regards,
Boqun

> 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