Messages in this thread | | | Date | Tue, 23 Apr 2024 16:37:17 -0700 | From | Kees Cook <> | Subject | Re: [PATCH 2/2] rust: time: Use wrapping_sub() for Ktime::sub() |
| |
On Tue, Apr 23, 2024 at 02:11:22PM -0700, Boqun Feng wrote: > On Thu, Apr 11, 2024 at 04:08:01PM -0700, Boqun Feng 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. > > > > Ping. Thomas, John and Stepthen. Could you take a look at this, and the > discussion between Miguel and me? The key question is the behavior when > ktime_sub() hits a overflow, I think. Thanks! > > (Cc Kees as well)
While working on the signed (and unsigned) integer overflow sanitizer support on the C side for the kernel, I've also run into timekeeping being a questionable area[1]. I *think* from what I can tell, it's always expected to have wrapping behavior.
Can we define the type itself to be wrapping? (This has been my plan on the C side, but we're still waiting on a finalized implementation of the "wraps" attribute.[2])
-Kees
[1] This is strictly WIP, as I think fixing the _types_ is going to be the more sustainable solution, but you can see some of what I was poking at: https://git.kernel.org/pub/scm/linux/kernel/git/kees/linux.git/commit/?h=devel/overflow/enable-unsigned-sanitizer&id=284464817a59b14f00d397bfbf1bf05683ed2f58 [2] https://github.com/llvm/llvm-project/pull/86618
-- Kees Cook
| |