linux-hardening - Re: [RFC] Mitigating unexpected arithmetic overflow

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Date: Thu, 9 May 2024 12:15:40 -0700
From: Linus Torvalds <torvalds@...ux-foundation.org>
To: Al Viro <viro@...iv.linux.org.uk>
Cc: "Theodore Ts'o" <tytso@....edu>, Kees Cook <keescook@...omium.org>, 
	Justin Stitt <justinstitt@...gle.com>, Peter Zijlstra <peterz@...radead.org>, 
	Mark Rutland <mark.rutland@....com>, linux-hardening@...r.kernel.org, 
	linux-kernel@...r.kernel.org, llvm@...ts.linux.dev
Subject: Re: [RFC] Mitigating unexpected arithmetic overflow

On Thu, 9 May 2024 at 11:48, Al Viro <viro@...iv.linux.org.uk> wrote:
>
> FWIW, the thing that somewhat worries me about having a helper along
> the lines of combine_to_u64(low, high) is that
>         foo->splat = combine_to_u64(something, something_else);
> would be inviting hard-to-catch brainos -
>         foo->splat = combine_to_u64(something_else, something);

Yeah, we'd have to be very clear about naming and ordering. So it
would probably have to be something like

        result = combine_to_u64_hi_lo(high, low);

to be easy to use.

The good news is that if you *do* get it wrong despite clear naming,
the resulting value will be so obviously wrong that it's generally a
"Duh!" thing if you do any testing what-so-ever.

Of course, I say that as somebody who always points out that I haven't
tested my own patches at all, and they are "something like this,
perhaps?".

But having "hi_lo" kind of naming would hopefully make it really
obvious even when just looking at the source code.

              Linus