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

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Message-ID: <20251204085435.GC2528459@noisy.programming.kicks-ass.net>
Date: Thu, 4 Dec 2025 09:54:35 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: Kees Cook <kees@...nel.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 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?

Because the moment we have __counted_by() on pointers (remember, you
promised to rename __counted_by_ptr() to __counted_by() once GCC-16
releases) using __flex_counter() on a ptr thing makes absolutely no
sense.

So perhaps just name this thing __counter_of() or something?