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

[Alice Ryhl]

On Fri, Apr 12, 2024 at 3:18 PM Boqun Feng <boqun.feng@xxxxxxxxx> wrote:
>
> 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.

What the CONFIG_RUST_OVERFLOW_CHECKS option does is enable panics on
overflow. So I don't understand how "it panics on overflow" is an
argument for removing the overflow check. That's what you asked for!
One could perhaps argue about whether CONFIG_RUST_OVERFLOW_CHECKS is a
good idea (I think it is), but that is orthogonal. When
CONFIG_RUST_OVERFLOW_CHECKS is enabled, you should respect the flag.

Alice