Re: [PATCH v6 1/5] slab: Introduce kmalloc_obj() and family

Kees Cook
2026-01-09T17:31:08Z

On Thu, Jan 08, 2026 at 03:01:00PM +0100, Vlastimil Babka wrote:
> On 12/4/25 00:30, Kees Cook wrote:
> > [...]
> > +/**
> > + * __alloc_objs - Allocate objects of a given type using
> > + * @KMALLOC: which size-based kmalloc wrapper to allocate with.
> > + * @GFP: GFP flags for the allocation.
> > + * @TYPE: type to allocate space for.
> > + * @COUNT: how many @TYPE objects to allocate.
> > + *
> > + * Returns: Newly allocated pointer to (first) @TYPE of @COUNT-many
> > + * allocated @TYPE objects, or NULL on failure.
> > + */
> > +#define __alloc_objs(KMALLOC, GFP, TYPE, COUNT) \
> > +({ \
> > + const size_t __obj_size = size_mul(sizeof(TYPE), COUNT); \
>
> I assume with the hardcoded 1 for COUNT, this size_mul() will be eliminated
> by the compiler and not add unnecessary runtime overhead? Otherwise we
> should have two core #define variants.

You're correct: the compiler completely collapses it with 0 runtime
overhead; a variant is not needed.

> I also noted that the existing kmalloc_array() and kvmalloc_array() do
> check_mul_overflow() and return NULL silently on overflow. This AFAIU will
> make SIZE_MAX passed to the underlying kmalloc/kvmalloc and thus will cause
> a warning. That's IMHO a good thing.

Right -- I prefer seeing the SIZE_MAX yelling from the allocator. Should
we change how k*malloc_array() behaves?

-Kees

--
Kees Cook