linux-kernel - Re: [PATCH 2/2] rust: time: Use wrapping

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Date: Mon, 13 May 2024 07:06:19 -0700
From: Boqun Feng <boqun.feng@...il.com>
To: Thomas Gleixner <tglx@...utronix.de>
Cc: Miguel Ojeda <ojeda@...nel.org>, John Stultz <jstultz@...gle.com>,
	Stephen Boyd <sboyd@...nel.org>,
	Alex Gaynor <alex.gaynor@...il.com>,
	Wedson Almeida Filho <wedsonaf@...il.com>,
	Gary Guo <gary@...yguo.net>, bjorn3_gh@...tonmail.com,
	Benno Lossin <benno.lossin@...ton.me>,
	Andreas Hindborg <a.hindborg@...sung.com>,
	Alice Ryhl <aliceryhl@...gle.com>, rust-for-linux@...r.kernel.org,
	linux-kernel@...r.kernel.org, Kees Cook <keescook@...omium.org>
Subject: Re: [PATCH 2/2] rust: time: Use wrapping_sub() for Ktime::sub()

On Thu, May 09, 2024 at 02:14:16PM +0200, Thomas Gleixner wrote:
> On Tue, Apr 23 2024 at 14:11, 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@...PCREL(%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!
> 
> 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! Looks to me, a reasonable implementation would be calling
`overflowing_sub`[1] and print a warning if overflow happens:

	<in the implementation of Sub trait of Ktime>

	let (ret, overflow) = self.inner.overflowing_sub(other.inner);

	if overflow {
		pr_err!("Overflow on Ktime: {} - {}\n", self.inner, other.inner);
	}

	return ret;

I will use this as a new version for more reviews.

[1]: https://doc.rust-lang.org/std/primitive.i64.html#method.overflowing_sub

Regards,
Boqun

> Thanks,
> 
>         tglx
> 
>