Re: [PATCH v1] landlock: Optimize stack usage when !CONFIG_AUDIT

["Mickaël Salaün" <mic@digikod.net>]

On Fri, Dec 19, 2025 at 03:22:59PM +0100, Mickaël Salaün wrote:
> Until now, each landlock_request struct were allocated on the stack, even
> if not really used, because is_access_to_paths_allowed() unconditionally
> modified the passed references.  Even if the changed landlock_request
> variables are not used, the compiler is not smart enough to detect this
> case.
> 
> To avoid this issue, explicitly disable the related code when
> CONFIG_AUDIT is not set, which enables elision of log_request_parent*
> and associated caller's stack variables thanks to dead code elimination.
> This makes it possible to reduce the stack frame by 192 bytes for the
> path_link and path_rename hooks, and by 96 bytes for most other
> filesystem hooks.
> 
> Here is a summary of scripts/checkstack.pl before and after this change
> when CONFIG_AUDIT is disabled:
> 
>   Function                       Old size   New size   Diff
>   ----------------------------------------------------------
>   current_check_refer_path       384        208        -176
>   current_check_access_path      192        112        -80
>   hook_file_open                 208        128        -80
>   is_access_to_paths_allowed     240        224        -16
> 
> Also, add extra pointer checks to be more future-proof.
> 

Reported-by: Tingmao Wang <m@maowtm.org>
Link: https://lore.kernel.org/r/eb86863b-53b0-460b-b223-84dd31d765b9@maowtm.org

> Fixes: 2fc80c69df82 ("landlock: Log file-related denials")
> Signed-off-by: Mickaël Salaün <mic@digikod.net>
> ---
>  security/landlock/fs.c | 11 +++++++++--
>  1 file changed, 9 insertions(+), 2 deletions(-)
> 
> diff --git a/security/landlock/fs.c b/security/landlock/fs.c
> index fe794875ad46..722f950307f6 100644
> --- a/security/landlock/fs.c
> +++ b/security/landlock/fs.c
> @@ -939,7 +939,12 @@ static bool is_access_to_paths_allowed(
>  	}
>  	path_put(&walker_path);
>  
> -	if (!allowed_parent1) {
> +	/*
> +	 * Check CONFIG_AUDIT to enable elision of log_request_parent* and
> +	 * associated caller's stack variables thanks to dead code elimination.
> +	 */
> +#ifdef CONFIG_AUDIT
> +	if (!allowed_parent1 && log_request_parent1) {
>  		log_request_parent1->type = LANDLOCK_REQUEST_FS_ACCESS;
>  		log_request_parent1->audit.type = LSM_AUDIT_DATA_PATH;
>  		log_request_parent1->audit.u.path = *path;
> @@ -949,7 +954,7 @@ static bool is_access_to_paths_allowed(
>  			ARRAY_SIZE(*layer_masks_parent1);
>  	}
>  
> -	if (!allowed_parent2) {
> +	if (!allowed_parent2 && log_request_parent2) {
>  		log_request_parent2->type = LANDLOCK_REQUEST_FS_ACCESS;
>  		log_request_parent2->audit.type = LSM_AUDIT_DATA_PATH;
>  		log_request_parent2->audit.u.path = *path;
> @@ -958,6 +963,8 @@ static bool is_access_to_paths_allowed(
>  		log_request_parent2->layer_masks_size =
>  			ARRAY_SIZE(*layer_masks_parent2);
>  	}
> +#endif /* CONFIG_AUDIT */
> +
>  	return allowed_parent1 && allowed_parent2;
>  }
>  
> -- 
> 2.52.0
>