Messages in this thread | | | Date | Fri, 12 Apr 2024 06:34:10 -0700 | From | Boqun Feng <> | Subject | Re: [PATCH 2/2] rust: time: Use wrapping_sub() for Ktime::sub() |
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On Fri, Apr 12, 2024 at 09:14:03AM +0200, Miguel Ojeda 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 > > Nit: it is enabled by default, but configurable (`CONFIG_RUST_OVERFLOW_CHECKS`). >
Ok, I will change it accordingly.
> > 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. > > > > Therefore make `Ktime::sub()` have the same semantics as `ktime_sub()`: > > overflow behaves like 2s-complement wrapping sub. > > If `ktime_sub()`'s callers rely on wrapping in some cases, then an > alternative we should consider is having a method for explicitly > wrapping, like the integers. This would allow callers to decide and it
That works for me, although I would prefer `Ktime::sub()` is wrapping sub and we have another function doing a safe version of sub.
> would make the expected semantics clear since the beginning (which is > the easiest time to add this kind of thing) for Rust code. > > Otherwise, I agree we should at least document the preconditions clearly. > > Having said that, I see a `ktime_add_unsafe()` too, which was added > due to a UBSAN report for `ktime_add()` in commit 979515c56458 ("time: > Avoid undefined behaviour in ktime_add_safe()"). There is also a > private `ktime_add_safe()` too, which is a saturating one. >
Exactly, ktime_add_safe() doesn't panic if overflow happens, right? I think that's pretty clear on how time subsystem wants to handle overflow (saturating it, or zeroing it instead of panicing).
> So, given that, can callers actually rely on wrapping for these > functions, or not? The documentation on the C side could perhaps be > clarified here (including the mention of UB in `ktime_add_unsafe()` -- > we use `-fno-strict-overflow`) and perhaps using the `wrapping_*()` C > functions too. >
I must defer this to Thomas.
> In addition, Binder calls `ktime_ms_delta()`, not `ktime_sub()`, > right? In that case the arguments are called `later` and `earlier`, > perhaps those have a different expectation even if `ktime_sub()` is > allowed to overflow and thus it would make sense to check in that > function only instead? (and document accordingly) >
Maybe, however neither of this function probably shouldn't have the panic-on-overflow behavior. So I agree that overflow checking is not a bad thing, but when to check and how to handle overflow should be controlled by the users, and making the default behavior panic-on-overflow doesn't look reasonable to me.
Regards, Boqun
> Thanks! > > Cheers, > Miguel
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