LICENSES: Clearly mark dual license only licenses

Just like the CDDL the Apache license and the MPL must only be used as
a choice in additional to an GPL2 compatible license.  Copy over the
boilerplate from the CDDL file to the other two after fixing it up to
make it clear the licenses need to be GPL2 compatible, not just the
more generic GPL compatible.  For example the Apache 2 license is GPL3
compatible, but that doesn't matter for the kernel.

Also move these licenses to a separate directory and document the rules
in license-rules.rst.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/process/license-rules.rst

@@ -281,7 +281,56 @@ kernel, can be broken down into:
 
 |
 
-3. _`Exceptions`:
+3. Dual Licensing Only
+
+   These licenses should only be used to dual license code with another
+   license in addition to a preferred license.  These licenses are available
+   from the directory::
+
+      LICENSES/dual/
+
+   in the kernel source tree.
+
+   The files in this directory contain the full license text and
+   `Metatags`_.  The file names are identical to the SPDX license
+   identifier which shall be used for the license in source files.
+
+   Examples::
+
+      LICENSES/dual/MPL-1.1
+
+   Contains the Mozilla Public License version 1.1 license text and the
+   required metatags::
+
+      LICENSES/dual/Apache-2.0
+
+   Contains the Apache License version 2.0 license text and the required
+   metatags.
+
+   Metatags:
+
+   The metatag requirements for 'other' licenses are identical to the
+   requirements of the `Preferred licenses`_.
+
+   File format example::
+
+      Valid-License-Identifier: MPL-1.1
+      SPDX-URL: https://spdx.org/licenses/MPL-1.1.html
+      Usage-Guide:
+        Do NOT use. The MPL-1.1 is not GPL2 compatible. It may only be used for
+        dual-licensed files where the other license is GPL2 compatible.
+        If you end up using this it MUST be used together with a GPL2 compatible
+        license using "OR".
+        To use the Mozilla Public License version 1.1 put the following SPDX
+        tag/value pair into a comment according to the placement guidelines in
+        the licensing rules documentation:
+      SPDX-License-Identifier: MPL-1.1
+      License-Text:
+        Full license text
+
+|
+
+4. _`Exceptions`:
 
    Some licenses can be amended with exceptions which grant certain rights
    which the original license does not.  These exceptions are available

LICENSES/dual/Apache-2.0

@@ -1,6 +1,10 @@
 Valid-License-Identifier: Apache-2.0
 SPDX-URL: https://spdx.org/licenses/Apache-2.0.html
 Usage-Guide:
+  Do NOT use. The Apache-2.0 is not GPL2 compatible. It may only be used
+  for dual-licensed files where the other license is GPL2 compatible.
+  If you end up using this it MUST be used together with a GPL2 compatible
+  license using "OR".
   To use the Apache License version 2.0 put the following SPDX tag/value
   pair into a comment according to the placement guidelines in the
   licensing rules documentation:

LICENSES/dual/CDDL-1.0

@@ -1,8 +1,8 @@
 Valid-License-Identifier: CDDL-1.0
 SPDX-URL: https://spdx.org/licenses/CDDL-1.0.html
 Usage-Guide:
-  Do NOT use. The CDDL-1.0 is not GPL compatible. It may only be used for
-  dual-licensed files where the other license is GPL compatible.
+  Do NOT use. The CDDL-1.0 is not GPL2 compatible. It may only be used for
+  dual-licensed files where the other license is GPL2 compatible.
   If you end up using this it MUST be used together with a GPL2 compatible
   license using "OR".
   To use the Common Development and Distribution License 1.0 put the

LICENSES/dual/MPL-1.1

@@ -1,6 +1,10 @@
 Valid-License-Identifier: MPL-1.1
 SPDX-URL: https://spdx.org/licenses/MPL-1.1.html
 Usage-Guide:
+  Do NOT use. The MPL-1.1 is not GPL2 compatible. It may only be used for
+  dual-licensed files where the other license is GPL2 compatible.
+  If you end up using this it MUST be used together with a GPL2 compatible
+  license using "OR".
   To use the Mozilla Public License version 1.1 put the following SPDX
   tag/value pair into a comment according to the placement guidelines in
   the licensing rules documentation: