Re: [PATCH v5 2/4] slab: Introduce kmalloc_obj() and family

[Kees Cook <kees@kernel.org>]

On Mon, Nov 24, 2025 at 09:12:14PM +0000, Matthew Wilcox wrote:
> On Mon, Nov 24, 2025 at 12:38:57PM -0800, Kees Cook wrote:
> > For code like:
> > 
> > 	u8 size;
> > 	...
> > 	size = struct_size(ptr, flex_member, count);
> > 	ptr = kmalloc(size, gfp);
> > 
> > While struct_size() is designed to deal with overflows beyond SIZE_MAX,
> > it can't do anything about truncation of its return value since it has
> > no visibility into the lvalue type. So this code pattern happily
> > truncates, allocates too little memory, and then usually does stuff like
> > runs a for-loop based on "count" instead of "size" and walks right off
> > the end of the heap allocation, clobbering whatever follows it.
> 
> Have we investigated a compiler warning like
> -Wimplicit-arithmetic-truncation that would complain about this kind of
> thing and could be shut up by an explicit cast:
> 
> 	size = (u8)struct_size(ptr, flex_member, count);
> 
> or arithmetic that can be proven to not overflow:
> 	size = struct_size(ptr, flex_member, count) & 0xff;
> 
> Maybe such a warning already exists and it's just too noisy to even
> start thinking about turning it on?

Yes, -Wconversion (W=3) is mind-blowingly noisy, unfortunately.

-- 
Kees Cook