Re: [PATCH v2 2/5] overflow: Expand check_add_overflow() for pointer addition

[Kees Cook <keescook@chromium.org>]

On Thu, Feb 01, 2024 at 10:19:15AM +0100, Przemek Kitszel wrote:
> On 1/30/24 23:06, Kees Cook wrote:
> > The check_add_overflow() helper is mostly a wrapper around
> > __builtin_add_overflow(), but GCC and Clang refuse to operate on pointer
> > arguments that would normally be allowed if the addition were open-coded.
> > 
> > For example, we have many places where pointer overflow is tested:
> > 
> > 	struct foo *ptr;
> > 	...
> > 	/* Check for overflow */
> > 	if (ptr + count < ptr) ...
> > 
> > And in order to avoid running into the overflow sanitizers in the
> > future, we need to rewrite these "intended" overflow checks:
> > 
> > 	if (check_add_overflow(ptr, count, &result)) ...
> > 
> > Frustratingly the argument type validation for __builtin_add_overflow()
> > is done before evaluating __builtin_choose_expr(), so for arguments to
> > be valid simultaneously for sizeof(*p) (when p may not be a pointer),
> > and __builtin_add_overflow(a, ...) (when a may be a pointer), we must
> > introduce wrappers that always produce a specific type (but they are
> > only used in the places where the bogus arguments will be ignored).
> > 
> > To test whether a variable is a pointer or not, introduce the __is_ptr()
> > helper, which uses __builtin_classify_type() to find arrays and pointers
> > (via the new __is_ptr_or_array() helper), and then decays arrays into
> > pointers (via the new __decay() helper), to distinguish pointers from
> > arrays.
> 
> This is (not just commit msg but together with impl), at first glance, too
> complicated for regular developers to grasp (that is perhaps fine),
> but could we make it simpler by, say _Generic() or other trick?

I haven't been able to find a way to do this, unfortunately. :( I would
*love* to find something simpler, but it eludes me.

-- 
Kees Cook