Backporting and conflict resolution

Author:

Vegard Nossum <vegard.nossum@oracle.com>

Introduction

Some developers may never really have to deal with backporting patches, merging branches, or resolving conflicts in their day-to-day work, so when a merge conflict does pop up, it can be daunting. Luckily, resolving conflicts is a skill like any other, and there are many useful techniques you can use to make the process smoother and increase your confidence in the result.

This document aims to be a comprehensive, step-by-step guide to backporting and conflict resolution.

Applying the patch to a tree

Sometimes the patch you are backporting already exists as a git commit, in which case you just cherry-pick it directly using git cherry-pick. However, if the patch comes from an email, as it often does for the Linux kernel, you will need to apply it to a tree using git am.

If you’ve ever used git am, you probably already know that it is quite picky about the patch applying perfectly to your source tree. In fact, you’ve probably had nightmares about .rej files and trying to edit the patch to make it apply.

It is strongly recommended to instead find an appropriate base version where the patch applies cleanly and then cherry-pick it over to your destination tree, as this will make git output conflict markers and let you resolve conflicts with the help of git and any other conflict resolution tools you might prefer to use. For example, if you want to apply a patch that just arrived on LKML to an older stable kernel, you can apply it to the most recent mainline kernel and then cherry-pick it to your older stable branch.

It’s generally better to use the exact same base as the one the patch was generated from, but it doesn’t really matter that much as long as it applies cleanly and isn’t too far from the original base. The only problem with applying the patch to the “wrong” base is that it may pull in more unrelated changes in the context of the diff when cherry-picking it to the older branch.

A good reason to prefer git cherry-pick over git am is that git knows the precise history of an existing commit, so it will know when code has moved around and changed the line numbers; this in turn makes it less likely to apply the patch to the wrong place (which can result in silent mistakes or messy conflicts).

If you are using b4. and you are applying the patch directly from an email, you can use b4 am with the options -g/--guess-base and -3/--prep-3way to do some of this automatically (see the b4 presentation for more information). However, the rest of this article will assume that you are doing a plain git cherry-pick.

Once you have the patch in git, you can go ahead and cherry-pick it into your source tree. Don’t forget to cherry-pick with -x if you want a written record of where the patch came from!

Note that if you are submitting a patch for stable, the format is slightly different; the first line after the subject line needs to be either:

commit <upstream commit> upstream

or:

[ Upstream commit <upstream commit> ]

Resolving conflicts

Uh-oh; the cherry-pick failed with a vaguely threatening message:

CONFLICT (content): Merge conflict

What to do now?

In general, conflicts appear when the context of the patch (i.e., the lines being changed and/or the lines surrounding the changes) doesn’t match what’s in the tree you are trying to apply the patch to.

For backports, what likely happened was that the branch you are backporting from contains patches not in the branch you are backporting to. However, the reverse is also possible. In any case, the result is a conflict that needs to be resolved.

If your attempted cherry-pick fails with a conflict, git automatically edits the files to include so-called conflict markers showing you where the conflict is and how the two branches have diverged. Resolving the conflict typically means editing the end result in such a way that it takes into account these other commits.

Resolving the conflict can be done either by hand in a regular text editor or using a dedicated conflict resolution tool.

Many people prefer to use their regular text editor and edit the conflict directly, as it may be easier to understand what you’re doing and to control the final result. There are definitely pros and cons to each method, and sometimes there’s value in using both.

We will not cover using dedicated merge tools here beyond providing some pointers to various tools that you could use:

• Emacs Ediff mode

• vimdiff/gvimdiff

• KDiff3

• TortoiseMerge

• Meld

• P4Merge

• Beyond Compare

• IntelliJ

• VSCode

To configure git to work with these, see git mergetool --help or the official git-mergetool documentation.

Prerequisite patches

Most conflicts happen because the branch you are backporting to is missing some patches compared to the branch you are backporting from. In the more general case (such as merging two independent branches), development could have happened on either branch, or the branches have simply diverged -- perhaps your older branch had some other backports applied to it that themselves needed conflict resolutions, causing a divergence.

It’s important to always identify the commit or commits that caused the conflict, as otherwise you cannot be confident in the correctness of your resolution. As an added bonus, especially if the patch is in an area you’re not that familiar with, the changelogs of these commits will often give you the context to understand the code and potential problems or pitfalls with your conflict resolution.

git log

A good first step is to look at git log for the file that has the conflict -- this is usually sufficient when there aren’t a lot of patches to the file, but may get confusing if the file is big and frequently patched. You should run git log on the range of commits between your currently checked-out branch (HEAD) and the parent of the patch you are picking (<commit>), i.e.:

git log HEAD..<commit>^ -- <path>

Even better, if you want to restrict this output to a single function (because that’s where the conflict appears), you can use the following syntax:

git log -L:'\<function\>':<path> HEAD..<commit>^

Note

The \< and \> around the function name ensure that the matches are anchored on a word boundary. This is important, as this part is actually a regex and git only follows the first match, so if you use -L:thread_stack:kernel/fork.c it may only give you results for the function try_release_thread_stack_to_cache even though there are many other functions in that file containing the string thread_stack in their names.

Another useful option for git log is -G, which allows you to filter on certain strings appearing in the diffs of the commits you are listing:

git log -G'regex' HEAD..<commit>^ -- <path>

This can also be a handy way to quickly find when something (e.g. a function call or a variable) was changed, added, or removed. The search string is a regular expression, which means you can potentially search for more specific things like assignments to a specific struct member:

git log -G'\->index\>.*='

git blame

Another way to find prerequisite commits (albeit only the most recent one for a given conflict) is to run git blame. In this case, you need to run it against the parent commit of the patch you are cherry-picking and the file where the conflict appeared, i.e.:

git blame <commit>^ -- <path>

This command also accepts the -L argument (for restricting the output to a single function), but in this case you specify the filename at the end of the command as usual:

git blame -L:'\<function\>' <commit>^ -- <path>

Navigate to the place where the conflict occurred. The first column of the blame output is the commit ID of the patch that added a given line of code.

It might be a good idea to git show these commits and see if they look like they might be the source of the conflict. Sometimes there will be more than one of these commits, either because multiple commits changed different lines of the same conflict area or because multiple subsequent patches changed the same line (or lines) multiple times. In the latter case, you may have to run git blame again and specify the older version of the file to look at in order to dig further back in the history of the file.

Prerequisite vs. incidental patches

Having found the patch that caused the conflict, you need to determine whether it is a prerequisite for the patch you are backporting or whether it is just incidental and can be skipped. An incidental patch would be one that touches the same code as the patch you are backporting, but does not change the semantics of the code in any material way. For example, a whitespace cleanup patch is completely incidental -- likewise, a patch that simply renames a function or a variable would be incidental as well. On the other hand, if the function being changed does not even exist in your current branch then this would not be incidental at all and you need to carefully consider whether the patch adding the function should be cherry-picked first.

If you find that there is a necessary prerequisite patch, then you need to stop and cherry-pick that instead. If you’ve already resolved some conflicts in a different file and don’t want to do it again, you can create a temporary copy of that file.

To abort the current cherry-pick, go ahead and run git cherry-pick --abort, then restart the cherry-picking process with the commit ID of the prerequisite patch instead.

Understanding conflict markers

Combined diffs

Let’s say you’ve decided against picking (or reverting) additional patches and you just want to resolve the conflict. Git will have inserted conflict markers into your file. Out of the box, this will look something like:

<<<<<<< HEAD
this is what's in your current tree before cherry-picking
=======
this is what the patch wants it to be after cherry-picking
>>>>>>> <commit>... title

This is what you would see if you opened the file in your editor. However, if you were to run git diff without any arguments, the output would look something like this:

$ git diff
[...]
++<<<<<<<< HEAD
 +this is what's in your current tree before cherry-picking
++========
+ this is what the patch wants it to be after cherry-picking
++>>>>>>>> <commit>... title

When you are resolving a conflict, the behavior of git diff differs from its normal behavior. Notice the two columns of diff markers instead of the usual one; this is a so-called “combined diff”, here showing the 3-way diff (or diff-of-diffs) between

1. the current branch (before cherry-picking) and the current working directory, and

2. the current branch (before cherry-picking) and the file as it looks after the original patch has been applied.

Better diffs

3-way combined diffs include all the other changes that happened to the file between your current branch and the branch you are cherry-picking from. While this is useful for spotting other changes that you need to take into account, this also makes the output of git diff somewhat intimidating and difficult to read. You may instead prefer to run git diff HEAD (or git diff --ours) which shows only the diff between the current branch before cherry-picking and the current working directory. It looks like this:

$ git diff HEAD
[...]
+<<<<<<<< HEAD
 this is what's in your current tree before cherry-picking
+========
+this is what the patch wants it to be after cherry-picking
+>>>>>>>> <commit>... title

As you can see, this reads just like any other diff and makes it clear which lines are in the current branch and which lines are being added because they are part of the merge conflict or the patch being cherry-picked.

Merge styles and diff3

The default conflict marker style shown above is known as the merge style. There is also another style available, known as the diff3 style, which looks like this:

<<<<<<< HEAD
this is what is in your current tree before cherry-picking
||||||| parent of <commit> (title)
this is what the patch expected to find there
=======
this is what the patch wants it to be after being applied
>>>>>>> <commit> (title)

As you can see, this has 3 parts instead of 2, and includes what git expected to find there but didn’t. It is highly recommended to use this conflict style as it makes it much clearer what the patch actually changed; i.e., it allows you to compare the before-and-after versions of the file for the commit you are cherry-picking. This allows you to make better decisions about how to resolve the conflict.

To change conflict marker styles, you can use the following command:

git config merge.conflictStyle diff3

There is a third option, zdiff3, introduced in Git 2.35, which has the same 3 sections as diff3, but where common lines have been trimmed off, making the conflict area smaller in some cases.

Iterating on conflict resolutions

The first step in any conflict resolution process is to understand the patch you are backporting. For the Linux kernel this is especially important, since an incorrect change can lead to the whole system crashing -- or worse, an undetected security vulnerability.

Understanding the patch can be easy or difficult depending on the patch itself, the changelog, and your familiarity with the code being changed. However, a good question for every change (or every hunk of the patch) might be: “Why is this hunk in the patch?” The answers to these questions will inform your conflict resolution.

Resolution process

Sometimes the easiest thing to do is to just remove all but the first part of the conflict, leaving the file essentially unchanged, and apply the changes by hand. Perhaps the patch is changing a function call argument from 0 to 1 while a conflicting change added an entirely new (and insignificant) parameter to the end of the parameter list; in that case, it’s easy enough to change the argument from 0 to 1 by hand and leave the rest of the arguments alone. This technique of manually applying changes is mostly useful if the conflict pulled in a lot of unrelated context that you don’t really need to care about.

For particularly nasty conflicts with many conflict markers, you can use git add or git add -i to selectively stage your resolutions to get them out of the way; this also lets you use git diff HEAD to always see what remains to be resolved or git diff --cached to see what your patch looks like so far.

Dealing with file renames

One of the most annoying things that can happen while backporting a patch is discovering that one of the files being patched has been renamed, as that typically means git won’t even put in conflict markers, but will just throw up its hands and say (paraphrased): “Unmerged path! You do the work...”

There are generally a few ways to deal with this. If the patch to the renamed file is small, like a one-line change, the easiest thing is to just go ahead and apply the change by hand and be done with it. On the other hand, if the change is big or complicated, you definitely don’t want to do it by hand.

As a first pass, you can try something like this, which will lower the rename detection threshold to 30% (by default, git uses 50%, meaning that two files need to have at least 50% in common for it to consider an add-delete pair to be a potential rename):

git cherry-pick -strategy=recursive -Xrename-threshold=30

Sometimes the right thing to do will be to also backport the patch that did the rename, but that’s definitely not the most common case. Instead, what you can do is to temporarily rename the file in the branch you’re backporting to (using git mv and committing the result), restart the attempt to cherry-pick the patch, rename the file back (git mv and committing again), and finally squash the result using git rebase -i (see the rebase tutorial) so it appears as a single commit when you are done.

Gotchas

Function arguments

Pay attention to changing function arguments! It’s easy to gloss over details and think that two lines are the same but actually they differ in some small detail like which variable was passed as an argument (especially if the two variables are both a single character that look the same, like i and j).

Error handling

If you cherry-pick a patch that includes a goto statement (typically for error handling), it is absolutely imperative to double check that the target label is still correct in the branch you are backporting to. The same goes for added return, break, and continue statements.

Error handling is typically located at the bottom of the function, so it may not be part of the conflict even though could have been changed by other patches.

A good way to ensure that you review the error paths is to always use git diff -W and git show -W (AKA --function-context) when inspecting your changes. For C code, this will show you the whole function that’s being changed in a patch. One of the things that often go wrong during backports is that something else in the function changed on either of the branches that you’re backporting from or to. By including the whole function in the diff you get more context and can more easily spot problems that might otherwise go unnoticed.

Refactored code

Something that happens quite often is that code gets refactored by “factoring out” a common code sequence or pattern into a helper function. When backporting patches to an area where such a refactoring has taken place, you effectively need to do the reverse when backporting: a patch to a single location may need to be applied to multiple locations in the backported version. (One giveaway for this scenario is that a function was renamed -- but that’s not always the case.)

To avoid incomplete backports, it’s worth trying to figure out if the patch fixes a bug that appears in more than one place. One way to do this would be to use git grep. (This is actually a good idea to do in general, not just for backports.) If you do find that the same kind of fix would apply to other places, it’s also worth seeing if those places exist upstream -- if they don’t, it’s likely the patch may need to be adjusted. git log is your friend to figure out what happened to these areas as git blame won’t show you code that has been removed.

If you do find other instances of the same pattern in the upstream tree and you’re not sure whether it’s also a bug, it may be worth asking the patch author. It’s not uncommon to find new bugs during backporting!

Verifying the result

colordiff

Having committed a conflict-free new patch, you can now compare your patch to the original patch. It is highly recommended that you use a tool such as colordiff that can show two files side by side and color them according to the changes between them:

colordiff -yw -W 200 <(git diff -W <upstream commit>^-) <(git diff -W HEAD^-) | less -SR

Here, -y means to do a side-by-side comparison; -w ignores whitespace, and -W 200 sets the width of the output (as otherwise it will use 130 by default, which is often a bit too little).

The rev^- syntax is a handy shorthand for rev^..rev, essentially giving you just the diff for that single commit; also see the official git rev-parse documentation.

Again, note the inclusion of -W for git diff; this ensures that you will see the full function for any function that has changed.

One incredibly important thing that colordiff does is to highlight lines that are different. For example, if an error-handling goto has changed labels between the original and backported patch, colordiff will show these side-by-side but highlighted in a different color. Thus, it is easy to see that the two goto statements are jumping to different labels. Likewise, lines that were not modified by either patch but differ in the context will also be highlighted and thus stand out during a manual inspection.

Of course, this is just a visual inspection; the real test is building and running the patched kernel (or program).

Build testing

We won’t cover runtime testing here, but it can be a good idea to build just the files touched by the patch as a quick sanity check. For the Linux kernel you can build single files like this, assuming you have the .config and build environment set up correctly:

make path/to/file.o

Note that this won’t discover linker errors, so you should still do a full build after verifying that the single file compiles. By compiling the single file first you can avoid having to wait for a full build in case there are compiler errors in any of the files you’ve changed.

Runtime testing

Even a successful build or boot test is not necessarily enough to rule out a missing dependency somewhere. Even though the chances are small, there could be code changes where two independent changes to the same file result in no conflicts, no compile-time errors, and runtime errors only in exceptional cases.

One concrete example of this was a pair of patches to the system call entry code where the first patch saved/restored a register and a later patch made use of the same register somewhere in the middle of this sequence. Since there was no overlap between the changes, one could cherry-pick the second patch, have no conflicts, and believe that everything was fine, when in fact the code was now scribbling over an unsaved register.

Although the vast majority of errors will be caught during compilation or by superficially exercising the code, the only way to really verify a backport is to review the final patch with the same level of scrutiny as you would (or should) give to any other patch. Having unit tests and regression tests or other types of automatic testing can help increase the confidence in the correctness of a backport.

Submitting backports to stable

As the stable maintainers try to cherry-pick mainline fixes onto their stable kernels, they may send out emails asking for backports when encountering conflicts, see e.g. <https://lore.kernel.org/stable/2023101528-jawed-shelving-071a@gregkh/>. These emails typically include the exact steps you need to cherry-pick the patch to the correct tree and submit the patch.

One thing to make sure is that your changelog conforms to the expected format:

<original patch title>

[ Upstream commit <mainline rev> ]

<rest of the original changelog>
[ <summary of the conflicts and their resolutions> ]
Signed-off-by: <your name and email>

The “Upstream commit” line is sometimes slightly different depending on the stable version. Older version used this format:

commit <mainline rev> upstream.

It is most common to indicate the kernel version the patch applies to in the email subject line (using e.g. git send-email --subject-prefix='PATCH 6.1.y'), but you can also put it in the Signed-off-by:-area or below the --- line.

The stable maintainers expect separate submissions for each active stable version, and each submission should also be tested separately.

A few final words of advice

1. Approach the backporting process with humility.

2. Understand the patch you are backporting; this means reading both the changelog and the code.

3. Be honest about your confidence in the result when submitting the patch.

4. Ask relevant maintainers for explicit acks.

Examples

The above shows roughly the idealized process of backporting a patch. For a more concrete example, see this video tutorial where two patches are backported from mainline to stable: Backporting Linux Kernel Patches.