Legal Team/only-operator-proposal

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Introduction to Issue

Historical Background

Originally, the SPDX License List listed variations of licenses as separate "line items". Notably, various versions of L/GPL had two listed items: one for a specific version-only, and one indicating a version-or-later. This was indicated in the list via the inclusion of the words "only" and "or later" in the full name; and via the short identifier: GPL-2.0 and GPL-2.0+ respectively. (examples throughout this page use GPL-2.0, but this issue applies to the entire family of GNU licenses, i.e., GPL, LGPL, FDL, APGL).

For example, the pre-version-2.0 SPDX License List looked like this:

  GNU General Public License v2.0 only	  GPL-2.0	
  GNU General Public License v2.0 or later	  GPL-2.0+

When the license expression syntax was introduced in version 2.0 of SPDX, licenses with the "or later" indication were removed as separate listed licenses ("deprecated"), as the "or later" option could now be provided for via using the + operator. This enabled the use of the + operator with other licenses, as applicable.

The ability to create license expressions using the + operator along with the with operator also solved the issue of under-representation of license-exception combinations.

However, this also created a new issue whereby the standard header which is usually where the "or later" or "only" option is indicated in practice could no longer be differentiated or captured directly. This also resulted in the full license name having "only" with no real way to modify that to "or later" in the full license name where/when the +.

The original argument for the + operator was that it could be used with other licenses (not just GNU family). However, to anyone's memory we did, not at that time, conduct a full analysis of the other licenses with "or later" language or how they work in practice.

Other License with "or later" Clauses

Not all licenses that provide for later versions treat their application in the same way or as explicitly as the GNU family of licenses does. A list of the licenses with text related to later versions, relevant text, and an analysis of the licenses that have "or later version" type language are listed on this page:

Notably, most licenses that reference the possibility of later versions can be read to say you can take and redistribute the work under the license you found it with or any other later version with no explicit option of 'this version only.'

The CDDL family is slightly different in that the CDDL is “or later” by default and “only” with an explicit notice prohibiting later versions. There's currently no only operator for the CDDL, although some code (e.g. parts of FreeBSD's ZFS implementation) does declare that prohibition while other code (e.g. some illumos userspace tools) does not (and is therefore presumably CDDL-1.0+).


In practice, practitioners are not always explicit about whether a license is a version only or or later option. Also, because the "only" or "or later" distinction often is found outside the actual license text (e.g., in a license notice or header in the source files), machines (and humans) need a way to identify the license file itself without determining (yet) if the package is "only" or "or later". In other words, there is currently no way to express 'I just found this license text and am not sure what the package license is' using only license expressions.

The SPDX specification provides a way to distinguish between what is detected license files and the concluded file license as well as similar fields at the package level when creating an SPDX document. But before getting to that point, tools must have ways to correctly identify what is precisely found in the files without having to draw a conclusion. Also, as discussed in the 2017-08-08 meeting, some ecosystems restrict themselves to license expressions (e.g. npm allows SPDX 2.0 license expressions), and some tools attempt to detect package licenses by looking only at the license files and not at license-grant blurbs (e.g. Licensee, which is used by the GitHub license API).

Examples / Challenges

The scenarios below map out various examples and how one would identify the License Information in File and then the Concluded License for the package using SPDX short identifiers and bearing in mind machine reading for this task. Some of these scenarios have been discussed on the various calls on this topic as to how one would identify the license short identifier for each file currently:

  1. you find: 1 text file with the license text of GPLv2; and 4 source files with the standard header for GPLv2, which include the language "any later version"
    1. 4.6 License Information in File:
      1. 1 text file with license text of GPLv2 = as GPL-2.0 means "only" - this is hard to determine, and different tools may handle this in different ways, including NOASSERTION
      2. 4 source files with standard header for GPLv2, which include the language "any later version" = GPL-2.0+
    2. 3.15 Declared License: same issue as for determining identifier for text file with license text of GPLv2 as above
    3. 3.13 Concluded License at package level = GPL-2.0+
    4. 4.5 Concluded License at file level - same as for package level due to notices
    5. Note: in the current reality, GPL-2.0 for the license text file would mean "only", yet one cannot determine from that file alone if the copyright holder intends to use the version only or the or later option (see example 4). Also note, that a machine can positively identify the "or later" option via the use of recommended standard license header. This would be the same scenario, but with GPL-2.0 if the license header did not evoke the "any later version" language.
  1. you find: no license file; 4 source files with the statement, “This is licensed under GPL"
    1. 4.6 License Information in File: 4 source files = NOASSERTION - because the information doesn't match anything on the SPDX License List or standard headers.
    2. 3.15 Declared License (for package) = NONE (because no license file was found); NOASSERTION would also be an option
    3. 4.5 Concluded License at file level = GPL-1.0+
    4. 3.13 Concluded License package = GPL-1.0+
    5. Note: The determination of GPL-1.0+ would be made by a human. But given the language of the GPL stating, "If the Program does not specify a version number of the license, you may choose any version ever published by the Free Software Foundation." and the short identifier GPL-1.0+ covers any version.
  1. you find: 1 license file with GPLv2 license text; 4 source files with no license information whatsoever (aka, the Github example)
    1. 4.6 License Information in File:
      1. 1 text file with license text of GPLv2 = as GPL-2.0 means "only" - this is hard to determine, and different tools may handle this in different ways, including NOASSERTION (same as scenario 1)
      2. 4 source files with no license information whatsoever = NONE (as per SPDX specification)
    2. 4.5 Concluded License at file level = some interpretation will be made here, based on the text of the license
    3. 3.13 Concluded License package = some interpretation will be made here, based on the text of the license
    4. different conclusions might include GPL-1.0+, GPL-2.0, GPL-2.0+
      1. Is including a copy of a particular version of the license "the Program specif[ying] a version number of the license which applies to it" and thus we can confidently conclude the package is GPL-2.0? Is the "or later" option triggered due to the language or would that be too far of a step to take? See one interpretation from mailing list here.
    5. Answer: John Sullivan from the FSF was going to check for a FSF position on this.


  • Provide way to get to "GPL-2.0-only" (or the like) identifier to enable better clarity for that option/scenario
  • Ensure we accommodate the reality of other licenses with language re: later versions such that all options can be represented appropriately
  • Ensure that there is some amount of consistency with what the short identifiers mean / Don't completely break the meaning for current users

Proposed Solution: add only operator

Based on the various legal and tech calls and including representatives from the LF (some notes in Legal Team/or-later-vs-unclear-disambiguation), we coalesced around the following proposal:

Create an "only" operator defined as to be used to indicate 'only the version of the license is intended for use'

For licenses that may include “only” or “or later” (or both) clauses in the license text, having explicit + and only operators to use in SPDX identifiers reduces the chance that an unfamiliar user mis-uses or misinterprets the identifier and provides a way to be explicit and accurate that was arguably not available before. This solution allows the license text to speak for itself (via the "plain" license identifier) and uses the only and + operators to be explicit in intent.

This leaves the ability to use the "plain" license identifier (without an operator) to indicate the existence of the license text itself or other scenarios where it is not clear whether the "or later" or "only" options are articulated by the copyright holder. This avoids SPDX having to make an interpretation as to what the licenses mean or intend. This would also bring into consistency the use of "plain" identifiers for all other licenses that have "or later" clauses.

The + operator would remain with the same definition.

Necessary/associated changes:

  • add new "only" operator to Appendix IV: SPDX License Expressions of spec and explanatory language as to how/when to use it.
  • Remove "only" from full name of GNU family licenses on SPDX License List
  • clarify that the license ID used by itself would indicate that the license text itself should be used to determine if later versions of the license could be used (in Appendix IV or V?)

Potential issues:

  • Backwards compatibility for GPL-2.0 meaning 'only' to now meaning 'whatever the license says' creates some inconsistency as the default position of the license text with no other info is "or later" or "any version", not "only". However, it was pointed out on the call that those people conscientious enough to be using GPL-2.0 correctly, will be more likely to be conscientious enough to add the only operator as needed once it's available. All seemed to agree that use of GPL-2.0 is probably not thought through as to "or later" or "only", and thus in these situations, there is no different meaning.
  • Does not make clear indication of "ambiguous" situation to push people towards using 'only' and '+' operators to be explicit (preferred). However, we don't have a clear indication of ambiguity now either (is ambiguity ever clear?). You can clearly detect the ambiguous situations in tooling if you add metadata to license definitions recording whether they are compatible with the various version-related operators (as discussed here and here).
  • GPL language is clear that it is one or the other, so this could create more confusion and not reflect the intention of the license. However, all agreed that when you have a machine scanning code with no other license info other than a file with the license text of GPL-2.0, this is not entirely clear situation.