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

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Message-ID: <Zhk0q-0eM4z5JHqn@Boquns-Mac-mini.home>
Date: Fri, 12 Apr 2024 06:18:35 -0700
From: Boqun Feng <boqun.feng@...il.com>
To: Alice Ryhl <aliceryhl@...gle.com>
Cc: Thomas Gleixner <tglx@...utronix.de>, 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>,
	rust-for-linux@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] rust: time: Use wrapping_sub() for Ktime::sub()

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@...il.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@...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.
> 
> 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