linux-kernel - Re: [PATCH v2 2/3] rust: alloc: add Vec::resize method

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [day] [month] [year] [list]

Message-ID: <CAJ-ks9=mbqMdguFmnRP+t-P2OC+oYGVDU2oLtT1dZzY=USESEg@mail.gmail.com>
Date: Wed, 19 Mar 2025 13:22:25 -0400
From: Tamir Duberstein <tamird@...il.com>
To: Miguel Ojeda <miguel.ojeda.sandonis@...il.com>
Cc: Benno Lossin <benno.lossin@...ton.me>, Andrew Ballance <andrewjballance@...il.com>, dakr@...nel.org, 
	airlied@...il.com, simona@...ll.ch, maarten.lankhorst@...ux.intel.com, 
	mripard@...nel.org, tzimmermann@...e.de, corbet@....net, ojeda@...nel.org, 
	alex.gaynor@...il.com, boqun.feng@...il.com, gary@...yguo.net, 
	bjorn3_gh@...tonmail.com, a.hindborg@...nel.org, aliceryhl@...gle.com, 
	tmgross@...ch.edu, acourbot@...dia.com, nouveau@...ts.freedesktop.org, 
	dri-devel@...ts.freedesktop.org, linux-doc@...r.kernel.org, 
	linux-kernel@...r.kernel.org, rust-for-linux@...r.kernel.org
Subject: Re: [PATCH v2 2/3] rust: alloc: add Vec::resize method

On Wed, Mar 19, 2025 at 12:43 PM Miguel Ojeda
<miguel.ojeda.sandonis@...il.com> wrote:
>
> On Wed, Mar 19, 2025 at 5:13 PM Tamir Duberstein <tamird@...il.com> wrote:
> >
> > No, I meant avoiding the check. The existing code already explicitly
> > checks `new_len > self.len()` before evaluating `new_len -
> > self.len()`. This means the check occurs twice. `checked_sub` reduces
> > the number of checks by 1. Perhaps my wording could have been clearer
> > ("avoid *an* underflow check").
>
> Ah, you mean in the function you suggested, I see.
>
> I think it they all may compile down to the same thing, whether
> overflows checks are enabled or not, and whether the version in the
> patch or `checked_sub` is used or not. At least in a quick Compiler
> Explorer test it seems so, but I didn't check in an actual kernel
> build.
>
> The implicit check is gated behind the other one, so that one can be
> removed, even if values are unknown -- we always have optimizations
> enabled, even under "debug" builds (assuming "debug" means overflow
> checking enabled).

Yeah, this elision is what I was trying to allude to in the original
wording. I still think the suggested code is simpler, but gave my RB
either way.