linux-kernel - Re: [PATCH v4 1/2] compiler_types: Introduce __flex

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Message-ID: <03d69156-1cee-43bc-901b-5f85f3aa7575@infradead.org>
Date: Fri, 14 Mar 2025 21:53:41 -0700
From: Randy Dunlap <rdunlap@...radead.org>
To: Kees Cook <kees@...nel.org>, Vlastimil Babka <vbabka@...e.cz>
Cc: Miguel Ojeda <ojeda@...nel.org>,
 "Gustavo A. R. Silva" <gustavoars@...nel.org>,
 Nathan Chancellor <nathan@...nel.org>, Peter Zijlstra
 <peterz@...radead.org>, Nick Desaulniers <nick.desaulniers+lkml@...il.com>,
 Marco Elver <elver@...gle.com>,
 Przemek Kitszel <przemyslaw.kitszel@...el.com>,
 linux-hardening@...r.kernel.org, Christoph Lameter <cl@...ux.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>,
 Hyeonggon Yoo <42.hyeyoo@...il.com>, Bill Wendling <morbo@...gle.com>,
 Justin Stitt <justinstitt@...gle.com>, Jann Horn <jannh@...gle.com>,
 Linus Torvalds <torvalds@...ux-foundation.org>,
 Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
 Sasha Levin <sashal@...nel.org>, 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-mm@...ck.org, linux-doc@...r.kernel.org,
 llvm@...ts.linux.dev
Subject: Re: [PATCH v4 1/2] compiler_types: Introduce __flex_counter() and
 family

Hi Kees,

On 3/14/25 8:15 PM, Kees Cook wrote:

> diff --git a/include/linux/overflow.h b/include/linux/overflow.h
> index 0c7e3dcfe867..e2b81cb5576e 100644
> --- a/include/linux/overflow.h
> +++ b/include/linux/overflow.h
> @@ -440,4 +440,40 @@ static inline size_t __must_check size_sub(size_t minuend, size_t subtrahend)
>  #define DEFINE_FLEX(TYPE, NAME, MEMBER, COUNTER, COUNT)	\
>  	_DEFINE_FLEX(TYPE, NAME, MEMBER, COUNT, = { .obj.COUNTER = COUNT, })
>  
> +/**
> + * typeof_flex_counter() - Return the type of the counter variable of a given
> + *                         flexible array member annotated by __counted_by().
> + * @FAM: Pointer to the flexible array member within a given struct.
> + *
> + * Returns "size_t" if no annotation exists.

Please use
 * Returns: <text>
instead so that kernel-doc can make a special doc section for it.

Same for patch 2/2.

> + */
> +#define typeof_flex_counter(FAM)				\
> +	typeof(_Generic(__flex_counter(FAM),			\
> +			void *: (size_t)0,			\
> +			default: *__flex_counter(FAM)))
> +
> +/** can_set_flex_counter() - Check if the counter associated with the given

Needs a newline between /** and the function name, as in set_flex_counter() below.

> + *                           flexible array member can represent a value.
> + * @FAM: Pointer to the flexible array member within a given struct.
> + * @COUNT: Value to check against the __counted_by annotated @FAM's counter.
> + */
> +#define can_set_flex_counter(FAM, COUNT)			\
> +	(!overflows_type(COUNT, typeof_flex_counter(FAM)))
> +
> +/**
> + * set_flex_counter() - Set the counter associated with the given flexible
> + *                      array member that has been annoated by __counted_by().
> + * @FAM: Pointer to the flexible array member within a given struct.
> + * @COUNT: Value to store to the __counted_by annotated @FAM's counter.
> + *
> + * This is a no-op if no annotation exists. Count needs to be checked with
> + * can_set_flex_counter(FAM, COUNT) before using this function.
> + */
> +#define set_flex_counter(FAM, COUNT)				\
> +({								\
> +	*_Generic(__flex_counter(FAM),				\
> +		  void *:  &(size_t){ 0 },			\
> +		  default: __flex_counter(FAM)) = (COUNT);	\
> +})
> +
>  #endif /* __LINUX_OVERFLOW_H */

-- 
~Randy