Messages in this thread | | | Date | Fri, 12 Apr 2024 06:18:35 -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 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.
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
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