Bradley M. Kuhn

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Samba XP Keynote, Jeremy's GPLv3 talk, & GPLv2/LGPLv3

Tuesday 10 May 2011 by Bradley M. Kuhn

This morning, I gave the keynote talk at Samba XP. I was really honored to be invited to speak to Samba XP (the Samba Developers and Users Conference).

My talk, entitled Samba, GPL Enforcement, and the GPLv3 was about GPL enforcement, and how it relates to the Samba project and embedded devices. I've pushed my slides to my gitorious “talks” project. That's of course just the source code of the slides. Previously, some folks have complained that they have trouble building the slides because they don't have pandoc or other such dependencies installed. (I do, BTW, believe that my Installation Information is adequate, even though the talk isn't GPLv3'd, but it does have some dependencies :). Anyway, I've put up an installed version of my Samba XP slides as well.

Some have asked if there's a recording of the talk. I see video cameras and the like here at Samba XP, and I will try to get the audio for a future FaiF Cast.

Speaking of FaiFCast, Karen and I timed it (mostly by luck) so that, while I'm at Samba XP, we'd release FaiF 0x0F, which includes audio from Jeremy's Linux Collaboration Summit talk about why Samba chose to switch to GPLv3. BTW, I'm sorry I didn't do show notes this week, but because of being at Samba XP the last few days, I wasn't able to write detailed show notes. However, the main thing you need are Jeremy's slides, which are linked to from the show notes section.

Later this week, I'm giving the Friday keynote at Linux Tag, also on GPL enforcement (It's at 13:00 on Friday 2011-05-13). I hope those of you who can come to Berlin will come see my talk!

Finally, Ivo de Decker in the audience at Samba XP asked about LGPLv3/GPLv2 incompatibility. In my answer to the question, I noted the GPL Compatibility Matrix on the GNU site. Also, regarding the specific LGPLv3 compatibility issue, I mentioned post I made last year on the GNOME desktop-devel-list about the LGPLv3/GPLv2 issue. I promised that I'd also quote that post here in my blog, so that there was a stable URL that discussed the issue. I therefore quote the relevant parts of that email here:

The most important point [about GPLv2-only/LGPLv3-or-later incompatibility], I'd like to make is to suggest a possible compromise. Specifically, I suggest disjunctive licensing, (GPLv2|LGPLv3-or-later), which could be implemented like this:

This program's license gives you software freedom; you can copy, modify, convey, propagate, and/or redistribute this software under the terms of either:

• the GNU Lesser General Public License as published by the Free Software Foundation; either version 3 of the License, or (at your option) any later version.
OR
• the GNU General Public License, version 2 only, as published by the Free Software Foundation.

In addition, when you convey, distribute, and/or propagate this software and/or modified versions thereof, you may also preserve this notice so that recipients of such distributions will also have both licensing options described above.

A good moniker for this license is (GPLv2|LGPLv3-or-later). It actually gives 3+ licensing options to downstream: they can continue under the full (GPLv2|LGPLv3-or-later), or they can use GPLv2-only, or they can use LGPLv3 (or any later version of the LGPL).

Some folks will probably note this isn't that different from LGPLv2.1-or-later. The key difference, though, is that it removes LGPLv2.1 from the mix. If you've read the LGPLv2.1 lately, you've seen that it really shows its age. LGPLv3 is a much better implementation of the weak copyleft idea. If any license needs deprecation, it's LGPLv2.1. I thus personally believe upgrade to (GPLv2|LGPLv3-or-later) is something worth doing right away.

I note, BTW, that existing code licensed LGPLv2.1-or-later has also already given permission to migrate to the license (GPLv2|LGPLv3-or-later). Specifically, it's permitted by LGPLv2.1 to license the work under GPLv2 if you want to. Furthermore, LGPLv2.1-or-later permits you to license LGPLv3-or-later. Therefore, LGPLv2.1-or-later can, at anyone's option, be upgraded to (GPLv2|LGPLv3-or-later).

Note the incompatibility exists on both [GPLv2-only and LGPLv3] sides (it proverbially takes two to tango), but the incompatibility centers primarily around the strong copyleft on the GPLv2 side, not the weak copyleft on the LGPLv3 side. Specifically, GPLv2 requires that:

You may not copy, modify, sublicense, or distribute the Program except as expressly provided under this License.

and

You may not impose any further restrictions on the recipients' exercise of the rights granted herein.

This is part of the text that creates copyleft: making sure that other terms can't be imposed.

The problem occurs in interaction with another copyleft license (even a weak one). Usually, no two copyleft implementations are isomorphic and therefore there are different requirements in the details. LGPLv3, for its part, doesn't care much about additional restrictions imposed by another license (hence its weak copyleft nature). However, from the point of view of the GPLv2-side observer, any additional requirement, even minor ones imposed by LGPLv3, are merely “further restrictions”.

This is why copyleft licenses, when they want compatibility, have to explicitly permit relicensing (as LGPLv2 does for GPLv2/GPLv3 and as LGPLv3 does for GPLv3), by allowing you to “upgrade” to the another copyleft from the current copyleft. To be clear, from the point of view the LGPLv3 observer, it has no qualms about “upgrading” from LGPLv3 to GPLv2. The problem occurs from the GPLv2 side, specifically because the (relatively) minor things that LGPLv3 requires are written differently from the similar things asked for in GPLv2.

It's a common misconception that LGPL has no licensing requirements whatsoever on “works that use the library” (LGPLv2) or the “Application” (LGPLv3). That's not completely true; for example, in LGPLv3 § 4+5 (and LGPLv2.1 § 6+7), you find various requirements regarding licensing of such works. Those requirements aren't strict and are actually very easy to comply with. However, from GPLv2's point of view, they are “further restrictions” since they are not written exactly in the same fashion in GPLv2.

(BTW, note that LGPLv2.1's compatibility with GPLv2 and/or GPLv3 comes explicitly from LGPLv2.1's Section 3, which allows direct upgrade to GPLv2 or GPLv3, or to any later version published by FSF).

I hope the above helps some to clarify the GPLv2/LGPLv3 incompatibility.

Posted on Tuesday 10 May 2011 at 12:00 by Bradley M. Kuhn.

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