linux-kernel - RE: [PATCH 00/13] [RFC] Rust support

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Date:   Thu, 15 Apr 2021 08:26:21 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Kees Cook' <keescook@...omium.org>,
        Miguel Ojeda <miguel.ojeda.sandonis@...il.com>
CC:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Miguel Ojeda <ojeda@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "rust-for-linux@...r.kernel.org" <rust-for-linux@...r.kernel.org>,
        "Linux Kbuild mailing list" <linux-kbuild@...r.kernel.org>,
        "open list:DOCUMENTATION" <linux-doc@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH 00/13] [RFC] Rust support

...
> Besides just FP, 128-bit, etc, I remain concerned about just basic
> math operations. C has no way to describe the intent of integer
> overflow, so the kernel was left with the only "predictable" result:
> wrap around. Unfortunately, this is wrong in most cases, and we're left
> with entire classes of vulnerability related to such overflows.

I'm not sure any of the alternatives (except perhaps panic)
are much better.
Many years ago I used a COBOL system that skipped the assignment
if ADD X to Y (y += x) would overflow.
That gave a very hard to spot error when the sump of a long list
way a little too large.
If it had wrapped the error would be obvious.

There are certainly places where saturate is good.
Mostly when dealing with analogue samples.

I guess the problematic code is stuff that checks:
	if (foo->size + constant > limit) goto error;
instead of:
	if (foo->size > limit - constant) goto error;

    David

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