linux-kernel - Re: [PATCH v6 4/5] slab: Introduce kmalloc

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Message-ID: <202601080908.398D2129A@keescook>
Date: Thu, 8 Jan 2026 09:12:09 -0800
From: Kees Cook <kees@...nel.org>
To: Vlastimil Babka <vbabka@...e.cz>
Cc: Jonathan Corbet <corbet@....net>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Christoph Lameter <cl@...two.org>,
	David Rientjes <rientjes@...gle.com>,
	Roman Gushchin <roman.gushchin@...ux.dev>,
	Harry Yoo <harry.yoo@...cle.com>,
	"Gustavo A. R. Silva" <gustavoars@...nel.org>,
	workflows@...r.kernel.org, linux-doc@...r.kernel.org,
	linux-mm@...ck.org, linux-hardening@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Randy Dunlap <rdunlap@...radead.org>,
	Miguel Ojeda <ojeda@...nel.org>,
	Przemek Kitszel <przemyslaw.kitszel@...el.com>,
	Matthew Wilcox <willy@...radead.org>,
	John Hubbard <jhubbard@...dia.com>, Joe Perches <joe@...ches.com>,
	Christoph Lameter <cl@...ux.com>, Marco Elver <elver@...gle.com>,
	Vegard Nossum <vegard.nossum@...cle.com>,
	Pekka Enberg <penberg@...nel.org>,
	Joonsoo Kim <iamjoonsoo.kim@....com>,
	Bill Wendling <morbo@...gle.com>,
	Justin Stitt <justinstitt@...gle.com>, Jann Horn <jannh@...gle.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Sasha Levin <sashal@...nel.org>,
	Nathan Chancellor <nathan@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Nick Desaulniers <nick.desaulniers+lkml@...il.com>,
	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, llvm@...ts.linux.dev
Subject: Re: [PATCH v6 4/5] slab: Introduce kmalloc_flex() and family

On Thu, Jan 08, 2026 at 03:06:31PM +0100, Vlastimil Babka wrote:
> On 12/4/25 00:30, Kees Cook wrote:
> > As done for kmalloc_obj*(), introduce a type-aware allocator for flexible
> > arrays, which may also have "counted_by" annotations:
> > 
> > 	ptr = kmalloc(struct_size(ptr, flex_member, count), gfp);
> > 
> > becomes:
> > 
> > 	ptr = kmalloc_flex(*ptr, flex_member, count, gfp);
> > 
> > The internal use of __flex_counter() allows for automatically setting
> > the counter member of a struct's flexible array member when it has
> > been annotated with __counted_by(), avoiding any missed early size
> > initializations while __counted_by() annotations are added to the
> > kernel. Additionally, this also checks for "too large" allocations based
> > on the type size of the counter variable. For example:
> > 
> > 	if (count > type_max(ptr->flex_counter))
> > 		fail...;
> > 	size = struct_size(ptr, flex_member, count);
> > 	ptr = kmalloc(size, gfp);
> > 	ptr->flex_counter = count;
> > 
> > becomes (n.b. unchanged from earlier example):
> > 
> > 	ptr = kmalloc_flex(*ptr, flex_member, count, gfp);
> > 	ptr->flex_count = count;
> 
>              ^ flex_counter ?
> 
> But if it was "too large", ptr is NULL so this will oops?

Oops, yes, typo in the example. I will fix that. As for NULL, I dropped
the NULL checking on both sides of the example just to focus on the
differences.

-- 
Kees Cook