linux-kernel - Re: [PATCH v6 3/5] compiler_types: Introduce __flex

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite for Android: free password hash cracker in your pocket

[<prev] [next>] [<thread-prev] [day] [month] [year] [list]

Message-ID: <202512041215.44484FCACD@keescook>
Date: Thu, 4 Dec 2025 12:25:55 -0800
From: Kees Cook <kees@...nel.org>
To: Peter Zijlstra <peterz@...radead.org>
Cc: Vlastimil Babka <vbabka@...e.cz>, Miguel Ojeda <ojeda@...nel.org>,
	"Gustavo A. R. Silva" <gustavoars@...nel.org>,
	Nathan Chancellor <nathan@...nel.org>,
	Nick Desaulniers <ndesaulniers@...gle.com>,
	Marco Elver <elver@...gle.com>,
	Przemek Kitszel <przemyslaw.kitszel@...el.com>,
	linux-hardening@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Randy Dunlap <rdunlap@...radead.org>,
	Matthew Wilcox <willy@...radead.org>,
	John Hubbard <jhubbard@...dia.com>, Joe Perches <joe@...ches.com>,
	Christoph Lameter <cl@...ux.com>,
	Vegard Nossum <vegard.nossum@...cle.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>,
	Harry Yoo <harry.yoo@...cle.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>, linux-mm@...ck.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-doc@...r.kernel.org,
	llvm@...ts.linux.dev
Subject: Re: [PATCH v6 3/5] compiler_types: Introduce __flex_counter() and
 family

On Thu, Dec 04, 2025 at 09:54:35AM +0100, Peter Zijlstra wrote:
> On Wed, Dec 03, 2025 at 03:30:33PM -0800, Kees Cook wrote:
> > Introduce __flex_counter() which wraps __builtin_counted_by_ref(),
> > as newly introduced by GCC[1] and Clang[2]. Use of __flex_counter()
> > allows access to the counter member of a struct's flexible array member
> > when it has been annotated with __counted_by().
> > 
> > Introduce typeof_flex_counter(), overflows_flex_counter_type(), and
> > __set_flex_counter() to provide the needed _Generic() wrappers to get
> > sane results out of __flex_counter().
> > 
> > For example, with:
> > 
> > 	struct foo {
> > 		int counter;
> > 		short array[] __counted_by(counter);
> > 	} *p;
> > 
> > __flex_counter(p->array) will resolve to: &p->counter
> > 
> > typeof_flex_counter(p->array) will resolve to "int". (If p->array was not
> > annotated, it would resolve to "size_t".)
> 
> Uhh, how will this interact with the proposed extension of
> __counted_by() to normal pointer types?

Good point -- at present __builtin_counted_by_ref() only works on flex
arrays. I will ask the compiler devs their thoughts on expanding it to
the pointer references too...

-- 
Kees Cook