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

[Boqun Feng]

On Mon, May 13, 2024 at 05:04:43PM +0200, Miguel Ojeda wrote:
> On Thu, May 9, 2024 at 2:14 PM Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote:
> >
> > In principle ktime_sub() should not overflow for regular use cases.
> >
> > If the binder example overflows the substraction, then something is
> > seriously wrong. Though in that case as it's only for debug purposes
> > panicing would be totally counter productive. A warning might be
> > appropriate though.
> 
> Thanks for the clarification Thomas.
> 
> Alice and I pinged about this in the RustNL Unconf, i.e. about having
> a way to customize what happens on integer overflow (and, in general,
> other panics too), which I had in:
> 
>     https://github.com/Rust-for-Linux/linux/issues/354
> 
> e.g. like UBSan that allows a "report and continue" option.
> 
> We chatted with Mara and Amanieu, who were receptive. In particular,
> Mara posted right away:
> 
>     https://github.com/rust-lang/rfcs/pull/3632
> 
> as a possible first step to eventually have that ability, mentioning
> Rust for Linux as a user.
> 
> If we get the ability to customize those, then this could simply be a
> normal arithmetic operation, i.e. like any other. That is, considered
> to be a bug if it overflows.
> 

That is nice to have, thank you guys!

> Meanwhile, one alternative is going with Boqun's approach, but I would
> just use the standard operator (i.e. what we do elsewhere) and try to
> get the customization happen as soon as possible since we will need it
> for everything else (plus we avoid to make the code uglier with
> changes that we will need to revert anyway; and it also allows us to
> easily test any new customization feature from the compiler/library).
> 

However, I must point out that it needs more than a customized panic
handler to work: we also need to change the code generation (or adding
a different flag similar to -Coverflow-checks), because the current code
generation is Rust panic when overflow happens, which means the
subsequent code is unreachable.

Regards,
Boqun

> Cheers,
> Miguel