linux-hardening - Re: [RFC PATCH] mm/damon/sysfs: add __counted

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Date:   Fri, 25 Aug 2023 14:32:34 -0700
From:   Kees Cook <keescook@...omium.org>
To:     SeongJae Park <sj@...nel.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>, damon@...ts.linux.dev,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Qing Zhao <qing.zhao@...cle.com>,
        linux-hardening@...r.kernel.org
Subject: Re: [RFC PATCH] mm/damon/sysfs: add __counted_by() annotation

On Fri, Aug 25, 2023 at 02:04:41AM +0000, SeongJae Park wrote:
> Commit dd06e72e68bc ("Compiler Attributes: Add __counted_by macro")
> added __counted_by() annotation.  The underlying attribute for the
> feature is still under development, but adopting it can improve the
> readability of the code.  Apply it to four dynamic arrays in DAMON sysfs
> code.
> 
> Signed-off-by: SeongJae Park <sj@...nel.org>
> ---
>  mm/damon/sysfs.c | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/mm/damon/sysfs.c b/mm/damon/sysfs.c
> index 808a8efe0523..36a176f38726 100644
> --- a/mm/damon/sysfs.c
> +++ b/mm/damon/sysfs.c
> @@ -93,7 +93,7 @@ static const struct kobj_type damon_sysfs_region_ktype = {
> 
>  struct damon_sysfs_regions {
>  	struct kobject kobj;
> -	struct damon_sysfs_region **regions_arr;
> +	struct damon_sysfs_region **regions_arr __counted_by(nr);
>  	int nr;
>  };

Unfortunately this isn't supported yet for non-flexible-array-members:

mm/damon/sysfs.c:96:14: error: 'counted_by' attribute may not be specified for a non-array field
   96 |         struct damon_sysfs_region **regions_arr __counted_by(nr);
      |                                     ^~~~~

We'll have to wait a bit before we can add this patch. Thank you for
getting it ready, though! This support is next in line. :)

The FAM support is likely ready to land, though[1]. I just tested it
today.

-Kees

[1] https://gcc.gnu.org/pipermail/gcc-patches/2023-August/628459.html

-- 
Kees Cook