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

Vlastimil Babka
2026-01-09T17:32:12Z

On 1/9/26 18:31, Kees Cook wrote:
> 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?

Yeah it should best behave the same I think.

> -Kees
>