Re: [PATCH 5/5] types: Add standard __ob_trap and __ob_wrap scalar types

[Kees Cook]

On Wed, Apr 01, 2026 at 10:57:06AM +0200, Peter Zijlstra wrote:
> On Tue, Mar 31, 2026 at 01:31:16PM -0700, Kees Cook wrote:
> 
> > int func()
> > {
> > 	...
> > 	u8 __ob_trap product = 5;
> > 	...
> > 	product = a * b; // if store is truncated, goto __overflow
> > 	...
> > 	return product;
> > 
> > __overflow:
> > 	pr_info("%u\n", product); // shows "5"
> 
> I'm confused by this 'product is still 5' thing. It seems to me that
> making this happen will, in general, require more instructions/registers
> than allowing the old value to be clobbered and have product be the
> truncated result of whatever overflow.

Yeah, that's fair. That's what happens to "out" already with the existing
check_{op}_overflow(a, b, &out) builtins, and what happens right now
with the "while (var--)" exception (var will wrap even as an __ob_trap).

-- 
Kees Cook