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[PATCH v22 14/22] richacl: Update the file masks in chmod()

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Doing a chmod() sets the file mode, which includes the file permission
bits.  When a file has a richacl, the permissions that the richacl
grants need to be limited to what the new file permission bits allow.

This is done by setting the file masks in the richacl to what the file
permission bits map to.  The richacl access check algorithm takes the
file masks into account, which ensures that the richacl cannot grant too
many permissions.

It is possible to explicitly add permissions to the file masks which go
beyond what the file permission bits can grant (like the
RICHACE_WRITE_ACL permission).  The POSIX.1 standard calls this an
alternate file access control mechanism.  A subsequent chmod() would
ensure that those permissions are disabled again.

Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
Reviewed-by: J. Bruce Fields <bfields@redhat.com>
---
 fs/richacl.c            | 71 +++++++++++++++++++++++++++++++++++++++++++++++++
 include/linux/richacl.h |  1 +
 2 files changed, 72 insertions(+)

diff --git a/fs/richacl.c b/fs/richacl.c
index b2a03c1..ba110a6 100644
--- a/fs/richacl.c
+++ b/fs/richacl.c
@@ -547,3 +547,74 @@ out:
 	return denied ? -EACCES : 0;
 }
 EXPORT_SYMBOL_GPL(richacl_permission);
+
+/**
+ * __richacl_chmod  -  update the file masks to reflect the new mode
+ * @acl:	access control list
+ * @mode:	new file permission bits including the file type
+ *
+ * Return a copy of @acl where the file masks have been replaced by the file
+ * masks corresponding to the file permission bits in @mode, or returns @acl
+ * itself if the file masks are already up to date.  Takes over a reference
+ * to @acl.
+ */
+static struct richacl *
+__richacl_chmod(struct richacl *acl, umode_t mode)
+{
+	unsigned int x = S_ISDIR(mode) ? 0 : RICHACE_DELETE_CHILD;
+	unsigned int owner_mask, group_mask, other_mask;
+	struct richacl *clone;
+
+	owner_mask = richacl_mode_to_mask(mode >> 6) & ~x;
+	group_mask = richacl_mode_to_mask(mode >> 3) & ~x;
+	other_mask = richacl_mode_to_mask(mode)      & ~x;
+
+	if (acl->a_owner_mask == owner_mask &&
+	    acl->a_group_mask == group_mask &&
+	    acl->a_other_mask == other_mask &&
+	    (acl->a_flags & RICHACL_MASKED) &&
+	    (acl->a_flags & RICHACL_WRITE_THROUGH))
+		return acl;
+
+	clone = richacl_clone(acl, GFP_KERNEL);
+	richacl_put(acl);
+	if (!clone)
+		return ERR_PTR(-ENOMEM);
+
+	clone->a_flags |= (RICHACL_WRITE_THROUGH | RICHACL_MASKED);
+	clone->a_owner_mask = owner_mask;
+	clone->a_group_mask = group_mask;
+	clone->a_other_mask = other_mask;
+
+	return clone;
+}
+
+/**
+ * richacl_chmod  -  filesystem chmod helper
+ * @inode:	inode whose file permission bits to change
+ * @mode:	new file permission bits including the file type
+ *
+ * Helper for filesystems to use to perform a chmod on the richacl of an inode.
+ */
+int
+richacl_chmod(struct inode *inode, umode_t mode)
+{
+	struct richacl *acl;
+	int retval;
+
+	if (S_ISLNK(mode))
+		return -EOPNOTSUPP;
+	if (!inode->i_op->set_richacl)
+		return -EOPNOTSUPP;
+	acl = get_richacl(inode);
+	if (IS_ERR_OR_NULL(acl))
+		return PTR_ERR(acl);
+	acl = __richacl_chmod(acl, mode);
+	if (IS_ERR(acl))
+		return PTR_ERR(acl);
+	retval = inode->i_op->set_richacl(inode, acl);
+	richacl_put(acl);
+
+	return retval;
+}
+EXPORT_SYMBOL(richacl_chmod);
diff --git a/include/linux/richacl.h b/include/linux/richacl.h
index 3e05c94..db82fab 100644
--- a/include/linux/richacl.h
+++ b/include/linux/richacl.h
@@ -190,5 +190,6 @@ 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 *);
 extern int richacl_permission(struct inode *, const struct richacl *, int);
+extern int richacl_chmod(struct inode *, umode_t);
 
 #endif /* __RICHACL_H */
-- 
2.5.5

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[PATCH v22 00/22] Richacls (Core and Ext4) Andreas Gruenbacher <agruenba@redhat.com> - 2016-05-31 14:00 +0200
  [PATCH v22 01/22] vfs: Add IS_ACL() and IS_RICHACL() tests Andreas Gruenbacher <agruenba@redhat.com> - 2016-05-31 14:00 +0200
  [PATCH v22 22/22] ext4: Add richacl feature flag Andreas Gruenbacher <agruenba@redhat.com> - 2016-05-31 14:00 +0200
  [PATCH v22 15/22] richacl: Check if an acl is equivalent to a file mode Andreas Gruenbacher <agruenba@redhat.com> - 2016-05-31 14:00 +0200
  [PATCH v22 18/22] richacl: xattr mapping functions Andreas Gruenbacher <agruenba@redhat.com> - 2016-05-31 14:00 +0200
  [PATCH v22 21/22] ext4: Add richacl support Andreas Gruenbacher <agruenba@redhat.com> - 2016-05-31 14:00 +0200
  [PATCH v22 12/22] vfs: Add get_richacl and set_richacl inode operations Andreas Gruenbacher <agruenba@redhat.com> - 2016-05-31 14:00 +0200
  [PATCH v22 17/22] richacl: Automatic Inheritance Andreas Gruenbacher <agruenba@redhat.com> - 2016-05-31 14:00 +0200
  [PATCH v22 14/22] richacl: Update the file masks in chmod() Andreas Gruenbacher <agruenba@redhat.com> - 2016-05-31 14:00 +0200
  [PATCH v22 10/22] posix_acl: Improve xattr fixup code Andreas Gruenbacher <agruenba@redhat.com> - 2016-05-31 14:00 +0200
  [PATCH v22 13/22] vfs: Cache richacl in struct inode Andreas Gruenbacher <agruenba@redhat.com> - 2016-05-31 14:00 +0200
  [PATCH v22 05/22] vfs: Add permission flags for setting file attributes Andreas Gruenbacher <agruenba@redhat.com> - 2016-05-31 14:00 +0200
  [PATCH v22 08/22] richacl: Compute maximum file masks from an acl Andreas Gruenbacher <agruenba@redhat.com> - 2016-05-31 14:10 +0200
  [PATCH v22 09/22] richacl: Permission check algorithm Andreas Gruenbacher <agruenba@redhat.com> - 2016-05-31 14:10 +0200

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