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

[Peter Zijlstra]

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"
> 	return -1;
> }
> 
> (Isn't this just an implicit "try"?)

So I like this implicit try with a default label, and mostly I expect
this will be fine.

But as Linus already mentioned, sometimes you might want more. Could we
perhaps also have an explicit version, something along the lines of:

int func()
{
	int __ob_trap size;

	size = try(count * flex_size, __mul_overflow);
	size = try(size + base_size, __add_overflow);

	obj = kzalloc(size,...);

}

where we have something like:

#define try(stmt, _label) ({		\
	__label __overflow; 		\
	if (0) {			\
__overflow:				\
		goto _label;		\
	}				\
	stmt; })

That is, have the overflow trapped and confined in the
statement-expression by using the overflow label as a local label and
use this little trampoline to re-direct to a custom label.