RE: [PATCH v23 08/22] richacl: Compute maximum file masks from an acl

quoted

+ * Note: functions like richacl_allowed_to_who(),
+richacl_group_class_allowed(),
+ * and richacl_compute_max_masks() iterate through the entire acl in
+reverse
+ * order as an optimization.
+ *
+ * In the standard algorithm, aces are considered in forward order.
+When a
+ * process matches an ace, the permissions in the ace are either
+allowed or
+ * denied depending on the ace type.  Once a permission has been
+allowed or
+ * denied, it is no longer considered in further aces.
+ *
+ * By iterating through the acl in reverse order, we can compute the
+same
+ * result without having to keep track of which permissions have been
+allowed
+ * and denied already.
+ */

Clever!

Hmm, but does that result in examining the whole ACL for most access checks, at least for files where most of the accesses are by the owner, or a member of a specific group (with perhaps a ton of special case users added on the end)?

Frank

quoted

+
+/**
+ * richacl_allowed_to_who  -  permissions allowed to a specific who
+value
+ *
+ * Compute the maximum mask values allowed to a specific who value,
+taking
+ * everyone@ aces into account.
+ */
+static unsigned int richacl_allowed_to_who(struct richacl *acl,
+					   struct richace *who)
+{
+	struct richace *ace;
+	unsigned int allowed = 0;
+
+	richacl_for_each_entry_reverse(ace, acl) {
+		if (richace_is_inherit_only(ace))
+			continue;
+		if (richace_is_same_identifier(ace, who) ||
+		    richace_is_everyone(ace)) {
+			if (richace_is_allow(ace))
+				allowed |= ace->e_mask;
+			else if (richace_is_deny(ace))
+				allowed &= ~ace->e_mask;
+		}
+	}
+	return allowed;
+}
+
+/**
+ * richacl_group_class_allowed  -  maximum permissions of the group
+class
+ *
+ * Compute the maximum mask values allowed to a process in the group
+class
+ * (i.e., a process which is not the owner but is in the owning group
+or
+ * matches a user or group acl entry).  This includes permissions
+granted or
+ * denied by everyone@ aces.
+ *
+ * See richacl_compute_max_masks().
+ */
+static unsigned int richacl_group_class_allowed(struct richacl *acl)
+{
+	struct richace *ace;
+	unsigned int everyone_allowed = 0, group_class_allowed = 0;
+	int had_group_ace = 0;
+
+	richacl_for_each_entry_reverse(ace, acl) {
+		if (richace_is_inherit_only(ace) ||
+		    richace_is_owner(ace))
+			continue;
+
+		if (richace_is_everyone(ace)) {
+			if (richace_is_allow(ace))
+				everyone_allowed |= ace->e_mask;
+			else if (richace_is_deny(ace))
+				everyone_allowed &= ~ace->e_mask;
+		} else {
+			group_class_allowed |=
+				richacl_allowed_to_who(acl, ace);
+
+			if (richace_is_group(ace))
+				had_group_ace = 1;
+		}
+	}
+	/*
+	 * If the acl doesn't contain any group@ aces,

richacl_allowed_to_who()

quoted

+	 * wasn't called for the owning group.  We could make that call now,

but

quoted

+	 * we already know the result (everyone_allowed).
+	 */
+	if (!had_group_ace)
+		group_class_allowed |= everyone_allowed;
+	return group_class_allowed;
+}
+
+/**
+ * richacl_compute_max_masks  -  compute upper bound masks
+ *
+ * Computes upper bound owner, group, and other masks so that none of
+the
+ * permissions allowed by the acl are disabled.
+ *
+ * We don't make assumptions about who the owner is so that the owner
+can
+ * change with no effect on the file masks or file mode permission
+bits; this
+ * means that we must assume that all entries can match the owner.
+ */
+void richacl_compute_max_masks(struct richacl *acl) {
+	unsigned int gmask = ~0;
+	struct richace *ace;
+
+	/*
+	 * @gmask contains all permissions which the group class is ever
+	 * allowed.  We use it to avoid adding permissions to the group mask
+	 * from everyone@ allow aces which the group class is always denied
+	 * through other aces.  For example, the following acl would

otherwise

quoted

+	 * result in a group mask of rw:
+	 *
+	 *	group@:w::deny
+	 *	everyone@:rw::allow
+	 *
+	 * Avoid computing @gmask for acls which do not include any group

class

quoted

+	 * deny aces: in such acls, the group class is never denied any
+	 * permissions from everyone@ allow aces, and the group class

cannot

quoted

+	 * have fewer permissions than the other class.
+	 */
+
+restart:
+	acl->a_owner_mask = 0;
+	acl->a_group_mask = 0;
+	acl->a_other_mask = 0;
+
+	richacl_for_each_entry_reverse(ace, acl) {
+		if (richace_is_inherit_only(ace))
+			continue;
+
+		if (richace_is_owner(ace)) {
+			if (richace_is_allow(ace))
+				acl->a_owner_mask |= ace->e_mask;
+			else if (richace_is_deny(ace))
+				acl->a_owner_mask &= ~ace->e_mask;
+		} else if (richace_is_everyone(ace)) {
+			if (richace_is_allow(ace)) {
+				acl->a_owner_mask |= ace->e_mask;
+				acl->a_group_mask |= ace->e_mask &

gmask;

quoted

+				acl->a_other_mask |= ace->e_mask;
+			} else if (richace_is_deny(ace)) {
+				acl->a_owner_mask &= ~ace->e_mask;
+				acl->a_group_mask &= ~ace->e_mask;
+				acl->a_other_mask &= ~ace->e_mask;
+			}
+		} else {
+			if (richace_is_allow(ace)) {
+				acl->a_owner_mask |= ace->e_mask &

gmask;

quoted

+				acl->a_group_mask |= ace->e_mask &

gmask;

quoted

+			} else if (richace_is_deny(ace) && gmask == ~0) {
+				gmask = richacl_group_class_allowed(acl);
+				if (likely(gmask != ~0))
+					/* should always be true */
+					goto restart;
+			}
+		}
+	}
+
+	acl->a_flags &= ~(RICHACL_WRITE_THROUGH | RICHACL_MASKED); }
+EXPORT_SYMBOL_GPL(richacl_compute_max_masks);

diff --git a/include/linux/richacl.h b/include/linux/richacl.h index

9102ef0..3559b2c 100644

--- a/include/linux/richacl.h
+++ b/include/linux/richacl.h

@@ -178,5 +178,6 @@ extern void richace_copy(struct richace *, const

struct richace *);
 extern int richacl_masks_to_mode(const struct richacl *);
 extern unsigned int richacl_mode_to_mask(umode_t);
 extern unsigned int richacl_want_to_mask(unsigned int);
+extern void richacl_compute_max_masks(struct richacl *);

 #endif /* __RICHACL_H */

Reviewed-by: Jeff Layton <redacted>
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