module: Cure the MODULE_LICENSE "GPL" vs. "GPL v2" bogosity

The original MODULE_LICENSE string for kernel modules licensed under the
GPL v2 (only / or later) was simply "GPL", which was - and still is -
completely sufficient for the purpose of module loading and checking
whether the module is free software or proprietary.

In January 2003 this was changed with commit 3344ea3ad4b7 ("[PATCH]
MODULE_LICENSE and EXPORT_SYMBOL_GPL support"). This commit can be found in
the history git repository which holds the 1:1 import of Linus' bitkeeper
repository:

  https://git.kernel.org/pub/scm/linux/kernel/git/tglx/history.git/commit/?id=3344ea3ad4b7c302c846a680dbaeedf96ed45c02

The main intention of the patch was to refuse linking proprietary modules
against symbols exported with EXPORT_SYMBOL_GPL() at module load time.

As a completely undocumented side effect it also introduced the distinction
between "GPL" and "GPL v2" MODULE_LICENSE() strings:

 *      "GPL"                           [GNU Public License v2 or later]
 *      "GPL v2"                        [GNU Public License v2]
 *      "GPL and additional rights"     [GNU Public License v2 rights and more]
 *      "Dual BSD/GPL"                  [GNU Public License v2
 *                                       or BSD license choice]
 *      "Dual MPL/GPL"                  [GNU Public License v2
 *                                       or Mozilla license choice]

This distinction was and still is wrong in several aspects:

 1) It broke all modules which were using the "GPL" string in the
    MODULE_LICENSE() already and were licensed under GPL v2 only.

    A quick license scan over the tree at that time shows that at least 480
    out of 1484 modules have been affected by this change back then. The
    number is probably way higher as this was just a quick check for
    clearly identifiable license information.

    There was exactly ONE instance of a "GPL v2" module license string in
    the kernel back then - drivers/net/tulip/xircom_tulip_cb.c which
    otherwise had no license information at all. There is no indication
    that the change above is any way related to this driver. The change
    happend with the 2.4.11 release which was on Oct. 9 2001 - so quite
    some time before the above commit. Unfortunately there is no trace on
    the intertubes to any discussion of this.

 2) The dual licensed strings became ill defined as well because following
    the "GPL" vs. "GPL v2" distinction all dual licensed (or additional
    rights) MODULE_LICENSE strings would either require those dual licensed
    modules to be licensed under GPL v2 or later or just be unspecified for
    the dual licensing case. Neither choice is coherent with the GPL
    distinction.

Due to the lack of a proper changelog and no real discussion on the patch
submission other than a few implementation details, it's completely unclear
why this distinction was introduced at all. Other than the comment in the
module header file exists no documentation for this at all.

From a license compliance and license scanning POV this distinction is a
total nightmare.

As of 5.0-rc2 2873 out of 9200 instances of MODULE_LICENSE() strings are
conflicting with the actual license in the source code (either SPDX or
license boilerplate/reference). A comparison between the scan of the
history tree and a scan of current Linus tree shows to the extent that the
git rename detection over Linus tree grafted with the history tree is
halfways complete that almost none of the files which got broken in 2003
have been cleaned up vs. the MODULE_LICENSE string. So subtracting those
480 known instances from the conflicting 2800 of today more than 25% of the
module authors got it wrong and it's a high propability that a large
portion of the rest just got it right by chance.

There is no value for the module loader to convey the detailed license
information as the only decision to be made is whether the module is free
software or not.

The "and additional rights", "BSD" and "MPL" strings are not conclusive
license information either. So there is no point in trying to make the GPL
part conclusive and exact. As shown above it's already non conclusive for
dual licensing and incoherent with a large portion of the module source.

As an unintended side effect this distinction causes a major headache for
license compliance, license scanners and the ongoing effort to clean up the
license mess of the kernel.

Therefore remove the well meant, but ill defined, distinction between "GPL"
and "GPL v2" and document that:

  - "GPL" and "GPL v2" both express that the module is licensed under GPLv2
    (without a distinction of 'only' and 'or later') and is therefore kernel
    license compliant.

  - None of the MODULE_LICENSE strings can be used for expressing or
    determining the exact license

  - Their sole purpose is to decide whether the module is free software or
    not.

Add a MODULE_LICENSE subsection to the license rule documentation as well.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Acked-by: Philippe Ombredanne <pombredanne@nexb.com>
Acked-by: Joe Perches <joe@perches.com>
[jc: Did s/merily/merely/ ]
Acked-by: Jessica Yu <jeyu@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

2 files changed, 79 insertions, 1 deletions

diff --git a/Documentation/process/license-rules.rst b/Documentation/process/license-rules.rst
index 2bb8c0fc2238..43b6a1ee0193 100644
--- a/Documentation/process/license-rules.rst
+++ b/Documentation/process/license-rules.rst
@@ -372,3 +372,65 @@ in the LICENSE subdirectories. This is required to allow tool
 verification (e.g. checkpatch.pl) and to have the licenses ready to read
 and extract right from the source, which is recommended by various FOSS
 organizations, e.g. the `FSFE REUSE initiative <https://reuse.software/>`_.
+
+_`MODULE_LICENSE`
+-----------------
+
+   Loadable kernel modules also require a MODULE_LICENSE() tag. This tag is
+   neither a replacement for proper source code license information
+   (SPDX-License-Identifier) nor in any way relevant for expressing or
+   determining the exact license under which the source code of the module
+   is provided.
+
+   The sole purpose of this tag is to provide sufficient information
+   whether the module is free software or proprietary for the kernel
+   module loader and for user space tools.
+
+   The valid license strings for MODULE_LICENSE() are:
+
+    ============================= =============================================
+    "GPL"			  Module is licensed under GPL version 2. This
+				  does not express any distinction between
+				  GPL-2.0-only or GPL-2.0-or-later. The exact
+				  license information can only be determined
+				  via the license information in the
+				  corresponding source files.
+
+    "GPL v2"			  Same as "GPL". It exists for historic
+				  reasons.
+
+    "GPL and additional rights"   Historical variant of expressing that the
+				  module source is dual licensed under a
+				  GPL v2 variant and MIT license. Please do
+				  not use in new code.
+
+    "Dual MIT/GPL"		  The correct way of expressing that the
+				  module is dual licensed under a GPL v2
+				  variant or MIT license choice.
+
+    "Dual BSD/GPL"		  The module is dual licensed under a GPL v2
+				  variant or BSD license choice. The exact
+				  variant of the BSD license can only be
+				  determined via the license information
+				  in the corresponding source files.
+
+    "Dual MPL/GPL"		  The module is dual licensed under a GPL v2
+				  variant or Mozilla Public License (MPL)
+				  choice. The exact variant of the MPL
+				  license can only be determined via the
+				  license information in the corresponding
+				  source files.
+
+    "Proprietary"		  The module is under a proprietary license.
+				  This string is solely for proprietary third
+				  party modules and cannot be used for modules
+				  which have their source code in the kernel
+				  tree. Modules tagged that way are tainting
+				  the kernel with the 'P' flag when loaded and
+				  the kernel module loader refuses to link such
+				  modules against symbols which are exported
+				  with EXPORT_SYMBOL_GPL().
+    ============================= =============================================
+
+
+
diff --git a/include/linux/module.h b/include/linux/module.h
index 9a21fe3509af..3a2402b8d790 100644
--- a/include/linux/module.h
+++ b/include/linux/module.h
@@ -172,7 +172,7 @@ extern void cleanup_module(void);
  * The following license idents are currently accepted as indicating free
  * software modules
  *
- *	"GPL"				[GNU Public License v2 or later]
+ *	"GPL"				[GNU Public License v2]
  *	"GPL v2"			[GNU Public License v2]
  *	"GPL and additional rights"	[GNU Public License v2 rights and more]
  *	"Dual BSD/GPL"			[GNU Public License v2
@@ -186,6 +186,22 @@ extern void cleanup_module(void);
  *
  *	"Proprietary"			[Non free products]
  *
+ * Both "GPL v2" and "GPL" (the latter also in dual licensed strings) are
+ * merely stating that the module is licensed under the GPL v2, but are not
+ * telling whether "GPL v2 only" or "GPL v2 or later". The reason why there
+ * are two variants is a historic and failed attempt to convey more
+ * information in the MODULE_LICENSE string. For module loading the
+ * "only/or later" distinction is completely irrelevant and does neither
+ * replace the proper license identifiers in the corresponding source file
+ * nor amends them in any way. The sole purpose is to make the
+ * 'Proprietary' flagging work and to refuse to bind symbols which are
+ * exported with EXPORT_SYMBOL_GPL when a non free module is loaded.
+ *
+ * In the same way "BSD" is not a clear license information. It merely
+ * states, that the module is licensed under one of the compatible BSD
+ * license variants. The detailed and correct license information is again
+ * to be found in the corresponding source files.
+ *
  * There are dual licensed components, but when running with Linux it is the
  * GPL that is relevant so this is a non issue. Similarly LGPL linked with GPL
  * is a GPL combined work.