I second this and additionally to klorax’s pointers to the FAQ will post these snippets right from the GPLv3 license. v3 is the latest version so I’ll refer to that. Remember that GPL requires to redistribute with the same GPL. You legally can not change a program licensed with GPL to any other license even when redistributing. But this is not the concern of this thread. So let’s dig in:
Snippet from Preamble:
For the developers’ and authors’ protection, the GPL clearly explains
that there is no warranty for this free software. For both users’ and
authors’ sake, the GPL requires that modified versions be marked as
changed, so that their problems will not be attributed erroneously to
authors of previous versions.
This paragraph makes it very clear that you should at the very least make it apparent in the documentation that your modified version “be marked as changed”.
GPLv3 also makes your responsibilities clear when redistributing without changing anything in 4. Conveying Verbatim Copies:
You may convey verbatim copies of the Program’s source code as you
receive it, in any medium, provided that you conspicuously and
appropriately publish on each copy an appropriate copyright notice;
keep intact all notices stating that this License and any
non-permissive terms added in accord with section 7 apply to the code;
keep intact all notices of the absence of any warranty; and give all
recipients a copy of this License along with the Program.
This is pretty straight forward and from my understanding is the legal reasoning for the FAQ sections pointed out by klorax.
Modified code is covered additionally by 5. Conveying Modified Source Versions:
You may convey a work based on the Program, or the modifications to
produce it from the Program, in the form of source code under the
terms of section 4, provided that you also meet all of these conditions:
a) The work must carry prominent notices stating that you modified
it, and giving a relevant date.
b) The work must carry prominent notices stating that it is
released under this License and any conditions added under section
7. This requirement modifies the requirement in section 4 to
“keep intact all notices”.
Of special interest is mostly 5. a) here.
So you already know that any copyright notice made in the license must be carried over. Additionally comes in Section 7. Additional Terms:
Notwithstanding any other provision of this License, for material you
add to a covered work, you may (if authorized by the copyright holders of
that material) supplement the terms of this License with terms:
b) Requiring preservation of specified reasonable legal notices or
author attributions in that material or in the Appropriate Legal
Notices displayed by works containing it; or
c) Prohibiting misrepresentation of the origin of that material, or
requiring that modified versions of such material be marked in
reasonable ways as different from the original version; or
There are six pre-defined optional “supplements” to GPLv3. The two shown here are the ones commonly of most interest. c) is rather similar to the above paragraph but with different wording and slightly different intention. b) is what most people would want to look for.
Since these are optional you need to make sure that you include this term or an indicating notice pointing to them in every source file that you want to license. This is usually achieved by the typical copyright notice when stating: “You should have received a copy of the GNU General Public License along with [PROGRAM]. If not, see https://www.gnu.org/licenses”. We all know this sentence very well. The following is the legal portion referring to this sentence:
If you add terms to a covered work in accord with this section, you
must place, in the relevant source files, a statement of the
additional terms that apply to those files, or a notice indicating
where to find the applicable terms.
Also of noticeable mentioning is this term:
All other non-permissive additional terms are considered “further
restrictions” within the meaning of section 10. If the Program as you
received it, or any part of it, contains a notice stating that it is
governed by this License along with a term that is a further
restriction, you may remove that term.
This is a complex form of saying: whatever other nonsense you might write into the licensing terms or a separate license along with GPL is legally not applicable to the program licensed under GPL.
9. Acceptance Not Required for Having Copies:
You are not required to accept this License in order to receive or
run a copy of the Program. […] However,
nothing other than this License grants you permission to propagate or
modify any covered work. These actions infringe copyright if you do
not accept this License. Therefore, by modifying or propagating a
covered work, you indicate your acceptance of this License to do so.
This is to say that you legally bind to these terms when propagating or modifying the work licensed under this license and therefore fulfill the above stated requirements when doing so.
YES you HAVE TO keep copyright notices in GPLv3.
I have written several times already about this issue on opendesktop. Everybody knows this is a ridiculous discussion. Those who ignore common sense will not change their mind by pointing out digital licenses. Only the @opendesktop team can try to moderate this. And each and every one of us is free to adhere to their model or not. That being said I hope this gets even more attention by the team in the future.
Personally I have given up. I just don’t care anymore. It is NOT WORTH MY TIME to try to fight people who want to make a quick penny and successfully do so, some of them even generating quite some money. I DON’T CARE. I will still care for merge requests and comments/user requests to my products pushing updates but I’ll definitely not try to gain some income for very quality work through this platform until something changes on this matter. I’ll still be a recurring donator giving back some of that money but I’m contributing with my favorite Linux distro in my free time and will focus on my life instead.