linux-hardening - Re: [PATCH] landlock: Drop "const" argument qualifier to avoid GCC 4.9 warnings

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Date:   Mon, 13 Sep 2021 13:19:19 +0200
From:   Mickaël Salaün <mic@...ikod.net>
To:     Kees Cook <keescook@...omium.org>
Cc:     James Morris <jmorris@...ei.org>,
        "Serge E. Hallyn" <serge@...lyn.com>,
        linux-security-module@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: Re: [PATCH] landlock: Drop "const" argument qualifier to avoid GCC
 4.9 warnings


On 11/09/2021 00:36, Kees Cook wrote:
> When building under GCC 4.9, the compiler warns about const mismatches:
> 
> security/landlock/ruleset.c: In function 'insert_rule':
> security/landlock/ruleset.c:196:34: error: passing argument 2 of 'create_rule' from incompatible pointer type [-Werror]
>    new_rule = create_rule(object, &this->layers, this->num_layers,
>                                   ^
> security/landlock/ruleset.c:69:30: note: expected 'const struct landlock_layer ** const' but argument is of type 'struct landlock_layer (*)[]'
>  static struct landlock_rule *create_rule(
>                               ^
> security/landlock/ruleset.c: In function 'landlock_insert_rule':
> security/landlock/ruleset.c:240:38: error: passing argument 3 of 'insert_rule' from incompatible pointer type [-Werror]
>   return insert_rule(ruleset, object, &layers, ARRAY_SIZE(layers));
>                                       ^
> security/landlock/ruleset.c:144:12: note: expected 'const struct landlock_layer ** const' but argument is of type 'struct landlock_layer (*)[1]'
>  static int insert_rule(struct landlock_ruleset *const ruleset,

I guess this is a bug in GCC 4.9 (i.e. missing automatic const upgrade).
Couldn't we backport a fix to GCC 4.9 instead?

>             ^
> 
> Drop "const" from the function definition.
> 
> Cc: "Mickaël Salaün" <mic@...ikod.net>
> Cc: James Morris <jmorris@...ei.org>
> Cc: "Serge E. Hallyn" <serge@...lyn.com>
> Cc: linux-security-module@...r.kernel.org
> Signed-off-by: Kees Cook <keescook@...omium.org>
> ---
>  security/landlock/ruleset.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/security/landlock/ruleset.c b/security/landlock/ruleset.c
> index ec72b9262bf3..64c37af88ee7 100644
> --- a/security/landlock/ruleset.c
> +++ b/security/landlock/ruleset.c
> @@ -68,7 +68,7 @@ static void build_check_rule(void)
>  
>  static struct landlock_rule *create_rule(
>  		struct landlock_object *const object,
> -		const struct landlock_layer (*const layers)[],
> +		struct landlock_layer (*layers)[],

The "const layers" is not an issue, it should not be removed.

>  		const u32 num_layers,
>  		const struct landlock_layer *const new_layer)
>  {
> @@ -143,7 +143,7 @@ static void build_check_ruleset(void)
>   */
>  static int insert_rule(struct landlock_ruleset *const ruleset,
>  		struct landlock_object *const object,
> -		const struct landlock_layer (*const layers)[],
> +		struct landlock_layer (*layers)[],

Same here for the second const.

>  		size_t num_layers)
>  {
>  	struct rb_node **walker_node;
>