linux-kernel - Re: [PATCH 2/2] memblock: annotate struct memblock_type with __counted_by

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Message-ID: <202511241117.CBE8ADC2@keescook>
Date: Mon, 24 Nov 2025 11:19:02 -0800
From: Kees Cook <kees@...nel.org>
To: Andrew Morton <akpm@...ux-foundation.org>
Cc: Bill Wendling <morbo@...gle.com>, linux-kernel@...r.kernel.org,
	"Gustavo A. R. Silva" <gustavoars@...nel.org>,
	Nathan Chancellor <nathan@...nel.org>,
	Nick Desaulniers <nick.desaulniers+lkml@...il.com>,
	Justin Stitt <justinstitt@...gle.com>,
	Miguel Ojeda <ojeda@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Heiko Carstens <hca@...ux.ibm.com>,
	Marc Herbert <Marc.Herbert@...ux.intel.com>,
	Uros Bizjak <ubizjak@...il.com>, Tejun Heo <tj@...nel.org>,
	Jeff Xu <jeffxu@...omium.org>,
	Michal Koutný <mkoutny@...e.com>,
	Shakeel Butt <shakeel.butt@...ux.dev>,
	Thomas Weißschuh <thomas.weissschuh@...utronix.de>,
	John Stultz <jstultz@...gle.com>,
	Christian Brauner <brauner@...nel.org>,
	Randy Dunlap <rdunlap@...radead.org>,
	Brian Gerst <brgerst@...il.com>,
	Masahiro Yamada <masahiroy@...nel.org>,
	Mike Rapoport <rppt@...nel.org>, linux-mm@...ck.org,
	linux-hardening@...r.kernel.org, llvm@...ts.linux.dev
Subject: Re: [PATCH 2/2] memblock: annotate struct memblock_type with
 __counted_by_ptr

On Sat, Nov 22, 2025 at 02:16:14PM -0800, Andrew Morton wrote:
> On Fri, 21 Nov 2025 16:30:43 -0800 Kees Cook <kees@...nel.org> wrote:
> 
> > On Fri, Nov 21, 2025 at 07:39:44PM +0000, Bill Wendling wrote:
> > > Add the '__counted_by_ptr' attribute to the 'regions' field of 'struct
> > > memblock_type'. The 'regions' field is an array of 'struct
> > > memblock_region' and its size is tracked by the 'max' field, which
> > > represents the total number of allocated regions.
> > 
> > As part of any counted_by annotation patch, there needs to be discussion
> > in the commit log about how it's been shown to be a safe annotation
> > to make. e.g. in this case, if all allocations of "regions" have a
> > corresponding "max" assignment, etc. If just "git grep" can't find them
> > all, using something like Coccinelle or CodeQL to search for struct
> > memblock_type::regions assignments can work.
> 
> How is anyone to know these things?  I can't find anything about this
> in include/ or Documentation/ or in the relevant commits.
> 
> There should be a comment at the __counted_by() definition site, please.
> 
> And possibly write a Documentation/ file then change checkpatch to
> direct people to that file if they add a counted_by?

This is a fair point, yes. The documentation and discussions around
counted_by are very big in my mind (and for Bill), so it was mostly a
consolidation/reminder and some extra detail on prior solutions, but
for anyone new to that annotation, we should have collected common
guidance. I will write something up.

-- 
Kees Cook