Modifying Patches

If you are a subsystem or branch maintainer, sometimes you need to slightly modify patches you receive in order to merge them, because the code is not exactly the same in your tree and the submitters’. If you stick strictly to rule (c) of the developers certificate of origin, you should ask the submitter to rediff, but this is a totally counter-productive waste of time and energy. Rule (b) allows you to adjust the code, but then it is very impolite to change one submitters code and make him endorse your bugs. To solve this problem, it is recommended that you add a line between the last Signed-off-by header and yours, indicating the nature of your changes. While there is nothing mandatory about this, it seems like prepending the description with your mail and/or name, all enclosed in square brackets, is noticeable enough to make it obvious that you are responsible for last-minute changes. Example:

Signed-off-by: Random J Developer <random@developer.example.org>
[lucky@maintainer.example.org: struct foo moved from foo.c to foo.h]
Signed-off-by: Lucky K Maintainer <lucky@maintainer.example.org>

This practice is particularly helpful if you maintain a stable branch and want at the same time to credit the author, track changes, merge the fix, and protect the submitter from complaints. Note that under no circumstances can you change the author’s identity (the From header), as it is the one which appears in the changelog.

Special note to back-porters: It seems to be a common and useful practice to insert an indication of the origin of a patch at the top of the commit message (just after the subject line) to facilitate tracking. For instance, here’s what we see in a 3.x-stable release:

Date:   Tue Oct 7 07:26:38 2014 -0400

  libata: Un-break ATA blacklist

  commit 1c40279960bcd7d52dbdf1d466b20d24b99176c8 upstream.

And here’s what might appear in an older kernel once a patch is backported:

Date:   Tue May 13 22:12:27 2008 +0200

    wireless, airo: waitbusy() won't delay

    [backport of 2.6 commit b7acbdfbd1f277c1eb23f344f899cfa4cd0bf36a]

Whatever the format, this information provides a valuable help to people tracking your trees, and to people trying to troubleshoot bugs in your tree.