[PATCH v8 03/12] landlock: Replace union access_masks_all with helper functions

[Günther Noack]

* Stop using a union for access_masks_all.
* Expose helper functions for intersection checks and union operations.

The memory layout of bitfields is only loosely defined by the C
standard, so our static assertion that expects a fixed size was
brittle, and it broke on some compilers when we attempted to add a
17th file system access right.

Reported-by: kernel test robot <lkp at intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202603261438.jBx2DGNe-lkp@intel.com/
Signed-off-by: Günther Noack <gnoack3000 at gmail.com>
---
 security/landlock/access.h  | 21 ++++++++++++++-------
 security/landlock/cred.h    | 10 ++--------
 security/landlock/ruleset.h | 13 ++++---------
 3 files changed, 20 insertions(+), 24 deletions(-)

diff --git a/security/landlock/access.h b/security/landlock/access.h
index 42c95747d7bd..277b6ed7f7bb 100644
--- a/security/landlock/access.h
+++ b/security/landlock/access.h
@@ -52,14 +52,21 @@ struct access_masks {
 	access_mask_t scope : LANDLOCK_NUM_SCOPE;
 };
 
-union access_masks_all {
-	struct access_masks masks;
-	u32 all;
-};
+/* Checks whether two access masks have any common bit set. */
+static inline bool access_masks_intersect(const struct access_masks a,
+					  const struct access_masks b)
+{
+	return (a.fs & b.fs) || (a.net & b.net) || (a.scope & b.scope);
+}
 
-/* Makes sure all fields are covered. */
-static_assert(sizeof(typeof_member(union access_masks_all, masks)) ==
-	      sizeof(typeof_member(union access_masks_all, all)));
+/* ORs the bits of @src into @dst. */
+static inline void access_masks_merge(struct access_masks *dst,
+				      const struct access_masks src)
+{
+	dst->fs |= src.fs;
+	dst->net |= src.net;
+	dst->scope |= src.scope;
+}
 
 /**
  * struct layer_access_masks - A boolean matrix of layers and access rights
diff --git a/security/landlock/cred.h b/security/landlock/cred.h
index f287c56b5fd4..207a6db1c086 100644
--- a/security/landlock/cred.h
+++ b/security/landlock/cred.h
@@ -123,9 +123,6 @@ landlock_get_applicable_subject(const struct cred *const cred,
 				const struct access_masks masks,
 				size_t *const handle_layer)
 {
-	const union access_masks_all masks_all = {
-		.masks = masks,
-	};
 	const struct landlock_ruleset *domain;
 	ssize_t layer_level;
 
@@ -138,11 +135,8 @@ landlock_get_applicable_subject(const struct cred *const cred,
 
 	for (layer_level = domain->num_layers - 1; layer_level >= 0;
 	     layer_level--) {
-		union access_masks_all layer = {
-			.masks = domain->access_masks[layer_level],
-		};
-
-		if (layer.all & masks_all.all) {
+		if (access_masks_intersect(domain->access_masks[layer_level],
+					   masks)) {
 			if (handle_layer)
 				*handle_layer = layer_level;
 
diff --git a/security/landlock/ruleset.h b/security/landlock/ruleset.h
index 889f4b30301a..9f8b33815c2c 100644
--- a/security/landlock/ruleset.h
+++ b/security/landlock/ruleset.h
@@ -229,18 +229,13 @@ static inline void landlock_get_ruleset(struct landlock_ruleset *const ruleset)
 static inline struct access_masks
 landlock_union_access_masks(const struct landlock_ruleset *const domain)
 {
-	union access_masks_all matches = {};
+	struct access_masks matches = {};
 	size_t layer_level;
 
-	for (layer_level = 0; layer_level < domain->num_layers; layer_level++) {
-		union access_masks_all layer = {
-			.masks = domain->access_masks[layer_level],
-		};
+	for (layer_level = 0; layer_level < domain->num_layers; layer_level++)
+		access_masks_merge(&matches, domain->access_masks[layer_level]);
 
-		matches.all |= layer.all;
-	}
-
-	return matches.masks;
+	return matches;
 }
 
 static inline void
-- 
2.53.0