Messages in this thread | | | From | Alice Ryhl <> | Date | Fri, 12 Apr 2024 15:51:12 +0200 | Subject | Re: [PATCH 2/2] rust: time: Use wrapping_sub() for Ktime::sub() |
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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.
Alice
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