landlock: Optimize stack usage when !CONFIG_AUDIT

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 32 bytes for the
path_link and path_rename hooks, and by 20 bytes for most other
filesystem hooks.

Here is a summary of scripts/stackdelta before and after this change
when CONFIG_AUDIT is disabled:

  current_check_refer_path    560  320  -240
  current_check_access_path   328  184  -144
  hook_file_open              328  184  -144
  is_access_to_paths_allowed  376  360  -16

Also, add extra pointer checks to be more future-proof.

Cc: Günther Noack <gnoack@google.com>
Reported-by: Tingmao Wang <m@maowtm.org>
Closes: https://lore.kernel.org/r/eb86863b-53b0-460b-b223-84dd31d765b9@maowtm.org
Fixes: 2fc80c69df82 ("landlock: Log file-related denials")
Link: https://lore.kernel.org/r/20251219142302.744917-2-mic@digikod.net
Reviewed-by: Günther Noack <gnoack3000@gmail.com>
[mic: Improve stack usage measurement accuracy with scripts/stackdelta]
Signed-off-by: Mickaël Salaün <mic@digikod.net>

diff --git a/security/landlock/fs.c b/security/landlock/fs.c
index e3c3a8a9ac27baa8ebaffb0236ca570f8691182f..8205673c8b1c41fe52ee9cd1d87d80cce884c82d 100644 (file)
--- a/security/landlock/fs.c
+++ b/security/landlock/fs.c
@@ -939,7 +939,12 @@ jump_up:
        }
        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 @@ jump_up:
                        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 @@ jump_up:
                log_request_parent2->layer_masks_size =
                        ARRAY_SIZE(*layer_masks_parent2);
        }
+#endif /* CONFIG_AUDIT */
+
        return allowed_parent1 && allowed_parent2;
 }