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

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Message-ID: <CAGG=3QVwhA8yca4hQD1q7c6TgPL0dT5RODpdpBHSw2yuh1x7oQ@mail.gmail.com>
Date: Mon, 24 Nov 2025 12:15:54 -0800
From: Bill Wendling <morbo@...gle.com>
To: Kees Cook <kees@...nel.org>
Cc: Andrew Morton <akpm@...ux-foundation.org>, 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 Mon, Nov 24, 2025 at 11:19 AM Kees Cook <kees@...nel.org> wrote:
>
> 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.
>
Good point. I'll add documentation for these attributes both in
Documentation/ and at the macro site. The frustrating thing is that
we're likely to have at least one other macro flavor (something like
"__counted_by_expr"), though that's the only foreseeable one. All of
these macros are wrappers around the same attribute because of
compiler skew.

-bw