linux-hardening - Re: [PATCH v3 2/3] overflow: Introduce add_wrap(), sub_wrap(), and mul

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Date: Tue, 6 Feb 2024 09:42:26 +0100
From: Rasmus Villemoes <rasmus.villemoes@...vas.dk>
To: Kees Cook <kees@...nel.org>, Eric Biggers <ebiggers@...nel.org>,
 Kees Cook <keescook@...omium.org>
Cc: linux-hardening@...r.kernel.org, Mark Rutland <mark.rutland@....com>,
 "Gustavo A. R. Silva" <gustavoars@...nel.org>, Marco Elver
 <elver@...gle.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 2/3] overflow: Introduce add_wrap(), sub_wrap(), and
 mul_wrap()

On 06/02/2024 00.21, Kees Cook wrote:
> 
> 
> On February 5, 2024 11:17:12 PM GMT, Eric Biggers <ebiggers@...nel.org> wrote:
>> On Mon, Feb 05, 2024 at 02:44:14PM -0800, Kees Cook wrote:
>>> On Mon, Feb 05, 2024 at 12:21:45PM -0800, Eric Biggers wrote:
>>>> On Mon, Feb 05, 2024 at 01:12:30AM -0800, Kees Cook wrote:
>>>>> Subject: Re: [PATCH v3 2/3] overflow: Introduce add_wrap(), sub_wrap(), and mul_wrap()
>>>>
>>>> Maybe these should be called wrapping_add, wrapping_sub, and wrapping_mul?
>>>> Those names are more grammatically correct, and Rust chose those names too.
>>>
>>> Sure, that works for me. What bout the inc_wrap() and dec_wrap() names?
>>> I assume wrapping_inc() and wrapping_dec() ?
>>>
>>
>> Yes, though I'm not sure those should exist at all.  Maybe a += b should just
>> become a = wrapping_add(a, b), instead of wrapping_inc(a, b)?
>> wrapping_inc(a, b) isn't as self-explanatory.  Likewise for wrapping_dec.
> 
> It was to avoid repeating type information, as it would go from:
> 
> var_a += var_b;
> 
> to:
> 
> var_a = wrapping_add(typeof(var_a), var_a, var_b);
> 
> Which repeats "var_a" 3 times. :|

Yeah, I think that's a reasonable rationale. I'm fine with the
wrapping_* naming, and then the _inc and _dec helpers should follow.

However, I now wonder if those should really also return the new value.
Yes, that corresponds to the value of the expression (a += b), but
nobody would ever write c = (a += b) or otherwise make use of that
value, and the naming doesn't immediately imply whether one should think
of ++a or a++.

Rasmus