[RFC 27/39] richacl: Apply the file masks to a richacl

[Andreas Gruenbacher <andreas.gruenbacher@gmail.com>]

Put all the pieces of the acl transformation puzzle together for
computing a richacl which has the file masks "applied" so that the
standard nfsv4 access check algorithm can be used on the richacl.

Signed-off-by: Andreas Gruenbacher <agruen@kernel.org>
---
 fs/richacl_compat.c     | 101 ++++++++++++++++++++++++++++++++++++++++++++++++
 include/linux/richacl.h |   3 ++
 2 files changed, 104 insertions(+)

diff --git a/fs/richacl_compat.c b/fs/richacl_compat.c
index 4f76cb9..981bd49 100644
--- a/fs/richacl_compat.c
+++ b/fs/richacl_compat.c
@@ -646,3 +646,104 @@ richacl_isolate_group_class(struct richacl_alloc *x)
 	}
 	return 0;
 }
+
+/**
+ * __richacl_apply_masks  -  apply the file masks to all aces
+ * @x:		acl and number of allocated entries
+ *
+ * Apply the owner mask to owner@ aces, the other mask to
+ * everyone@ aces, and the group mask to all other aces.
+ *
+ * The previous transformations have brought the acl into a
+ * form in which applying the masks will not lead to the
+ * accidental loss of permissions anymore.
+ */
+static int
+__richacl_apply_masks(struct richacl_alloc *x)
+{
+	struct richace *ace;
+
+	richacl_for_each_entry(ace, x->acl) {
+		unsigned int mask;
+
+		if (richace_is_inherit_only(ace) || !richace_is_allow(ace))
+			continue;
+		if (richace_is_owner(ace))
+			mask = x->acl->a_owner_mask;
+		else if (richace_is_everyone(ace))
+			mask = x->acl->a_other_mask;
+		else
+			mask = x->acl->a_group_mask;
+		if (richace_change_mask(x, &ace, ace->e_mask & mask))
+			return -1;
+	}
+	return 0;
+}
+
+/**
+ * richacl_apply_masks  -  apply the masks to the acl
+ *
+ * Transform @acl so that the standard NFSv4 permission check algorithm (which
+ * is not aware of file masks) will compute the same access decisions as the
+ * richacl permission check algorithm (which looks at the acl and the file
+ * masks).
+ *
+ * This algorithm is split into several steps:
+ *
+ *   - Move everyone@ aces to the end of the acl.  This simplifies the other
+ *     transformations, and allows the everyone@ allow ace at the end of the
+ *     acl to eventually allow permissions to the other class only.
+ *
+ *   - Propagate everyone@ permissions up the acl.  This transformation makes
+ *     sure that the owner and group class aces won't lose any permissions when
+ *     we apply the other mask to the everyone@ allow ace at the end of the acl.
+ *
+ *   - Apply the file masks to all aces.
+ *
+ *   - Make sure that the owner is not granted any permissions beyond the owner
+ *     mask from group class aces or from everyone@.
+ *
+ *   - Make sure that the group class is not granted any permissions from
+ *     everyone@.
+ *
+ * The algorithm is exact except for richacls which cannot be represented as an
+ * acl alone: for example, given this acl:
+ *
+ *    group@:rw::allow
+ *
+ * when file masks corresponding to mode 0600 are applied, the owner would only
+ * get rw access if he is a member of the owning group.  This algorithm would
+ * produce an empty acl in this case.  We fix this case by modifying
+ * richacl_permission() so that the group mask is always applied to group class
+ * aces.  With this fix, the owner would not have any access (beyond the
+ * implicit permissions always granted to owners).
+ *
+ * NOTE: Depending on the acl and file masks, this algorithm can increase the
+ * number of aces by almost a factor of three in the worst case. This may make
+ * the acl too large for some purposes.
+ */
+int
+richacl_apply_masks(struct richacl **acl)
+{
+	int retval = 0;
+
+	if ((*acl)->a_flags & RICHACL_MASKED) {
+		struct richacl_alloc x = {
+			.acl = *acl,
+			.count = (*acl)->a_count,
+		};
+
+		if (richacl_move_everyone_aces_down(&x) ||
+		    richacl_propagate_everyone(&x) ||
+		    __richacl_apply_masks(&x) ||
+		    richacl_isolate_owner_class(&x) ||
+		    richacl_isolate_group_class(&x))
+			retval = -ENOMEM;
+
+		x.acl->a_flags &= ~RICHACL_MASKED;
+		*acl = x.acl;
+	}
+
+	return retval;
+}
+EXPORT_SYMBOL_GPL(richacl_apply_masks);
diff --git a/include/linux/richacl.h b/include/linux/richacl.h
index c709345..1a07ea0 100644
--- a/include/linux/richacl.h
+++ b/include/linux/richacl.h
@@ -323,4 +323,7 @@ extern int richacl_equiv_mode(const struct richacl *, mode_t *);
 extern int richacl_permission(struct inode *, const struct richacl *, int);
 extern struct richacl *richacl_create(struct inode *, struct inode *);
 
+/* richacl_compat.c */
+extern int richacl_apply_masks(struct richacl **);
+
 #endif /* __RICHACL_H */
-- 
2.1.0