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

[Andreas Hindborg <nmi@metaspace.dk>]

Alice Ryhl <aliceryhl@google.com> writes:

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

I would agree. If users do not want panics on overflow, they disable
RUST_OVERFLOW_CHECKS. If the config is enabled, overflows in ktime sub
should panic, even if it does not do so in equivalent C code.

BR Andreas