linux-kernel - Re: [PATCH v6 1/5] slab: Introduce kmalloc

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Message-ID: <a144cd1e-8bfc-4380-8f1b-071db0af0b2c@suse.cz>
Date: Fri, 9 Jan 2026 18:32:06 +0100
From: Vlastimil Babka <vbabka@...e.cz>
To: Kees Cook <kees@...nel.org>
Cc: Christoph Lameter <cl@...ux.com>, Pekka Enberg <penberg@...nel.org>,
 David Rientjes <rientjes@...gle.com>, Joonsoo Kim <iamjoonsoo.kim@....com>,
 Andrew Morton <akpm@...ux-foundation.org>,
 Roman Gushchin <roman.gushchin@...ux.dev>,
 Hyeonggon Yoo <42.hyeyoo@...il.com>,
 "Gustavo A . R . Silva" <gustavoars@...nel.org>,
 Bill Wendling <morbo@...gle.com>, Justin Stitt <justinstitt@...gle.com>,
 Jann Horn <jannh@...gle.com>, Przemek Kitszel
 <przemyslaw.kitszel@...el.com>, Marco Elver <elver@...gle.com>,
 Linus Torvalds <torvalds@...ux-foundation.org>,
 Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
 Sasha Levin <sashal@...nel.org>, linux-mm@...ck.org,
 Randy Dunlap <rdunlap@...radead.org>, Miguel Ojeda <ojeda@...nel.org>,
 Matthew Wilcox <willy@...radead.org>, John Hubbard <jhubbard@...dia.com>,
 Joe Perches <joe@...ches.com>, Vegard Nossum <vegard.nossum@...cle.com>,
 Harry Yoo <harry.yoo@...cle.com>, Nathan Chancellor <nathan@...nel.org>,
 Peter Zijlstra <peterz@...radead.org>,
 Nick Desaulniers <nick.desaulniers+lkml@...il.com>,
 Jonathan Corbet <corbet@....net>, Jakub Kicinski <kuba@...nel.org>,
 Yafang Shao <laoar.shao@...il.com>, Tony Ambardar <tony.ambardar@...il.com>,
 Alexander Lobakin <aleksander.lobakin@...el.com>,
 Jan Hendrik Farr <kernel@...rr.cc>, Alexander Potapenko <glider@...gle.com>,
 linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org,
 linux-doc@...r.kernel.org, llvm@...ts.linux.dev
Subject: Re: [PATCH v6 1/5] slab: Introduce kmalloc_obj() and family

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
>