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landlock: Optimize stack usage when !CONFIG_AUDIT

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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: 2fc80c69df ("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>
This commit is contained in:
Mickaël Salaün
2025-12-19 15:22:59 +01:00
parent 6548fb5218
commit 602acfb541
1 changed files with 9 additions and 2 deletions
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11
security/landlock/fs.c
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@@ -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;
}