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Package managers, the lazy alternative to git submodules

The problem

I'm not lazy, I'm just saving my energy

Package managers have always looked strange to me. Why would you want to use one? It is actually not weird to have that point of view, especially if your background comes from programming languages like C or C++, which don't have any well known package manager (even though people keep trying to build some) and developers resort to using the system libraries/packages or git submodules to manage their software builds. On the other hand, people mostly coming from interpreted languages are so used to the idea that not using a package manager seems like crazy advice. Today, software development uses some form of source content management, and it seems like git has won the war, especially since it is used by GitHub, possibly the most popular free repository choice for open source and free software. As such, many developers are exposed to git, but not many know about submodules, a command which allows you to keep track of external sources in a secure and deterministic way.

Package managers are more user friendly than git submodule, and the requirements to get started using one tend to be reduced to writing a simple text file and running one or at most two commands to download all the dependencies. Package managers have thus exploded in circles where software distribution is preferably done in source code form, requiring end users to build the source or run it (in the case of interpreted languages). Rather than asking users to install dependencies one by one into their system, package managers typically allow downloading source or binary dependencies into the project, usually without requiring a system install, allowing users to build the project without administrator rights on a system (quite useful for build servers, own or rented).

However, this user friendliness towards developers and users is starting to catch developers unguarded of malicious intent. In the case of the npm ecosystem, malicious actors have sometimes gained access to the accounts of certain developers and uploaded malicious versions of their packages. Recently Homebrew, the so called missing package manager for macOs, suffered a supply chain attack, where people blindly trusting and downloading Homebrew packages could have gotten extra unwanted code. Using git submodule doesn't offer any protection against malicious developers uploading libraries with hidden malicious behavior, but it can help to prevent the supplantation of an existing good package with a bad one (unless you are capable of subverting git hashes, of course).

Package managers keep references to external packages through a textual version requirement number, usually in the form of a string like 2.3.4 or 2.*/2.+ (which are even worse from a security point of view because not even you are sure what you will get from the dependency!). The local package manager fetches data from an external repository and gets the specified number or the latest matching the version pseudo regular expression. That's where the attack comes, you are trusting this package to be the one you mean. On the other hand, using git submodule forces you to refer to a specific git hash, and that can't be subverted.

I'm used to the popular package managers for mobile development (Cocoapods for iOS and gradle/maven for Android) and in terms of security they offer none by default for supply chain attacks. In the case of gradle, software dependencies are specified as version numbers (ie. com.google.guava:guaga:23.0). Cocoapods does essentially the same, which means that both iOS and Android developers are subject to supply chain attacks if somebody manages to hijack the repository and replace the dependency. The security conscious developer could fetch the dependencies and commit the fetched version into their repository, getting the worst cross between a package manager and git submodule: potentially big repository increase, versioning noise and manual dependencies.

In order to be secure and get exactly what you want, for Cocoapods you need to specify a git repository and a specific commit (oh my, doesn't that look like a git submodule thing). Recent versions of gradle are starting to allow specifying git repositories and branches, so being able to specify a specific commit will hopefully come along. Same thing applies to the nimble package manager for the Nim programming language. Most certainly the majority of users will specify a string version number, but it also offers the possibility of specifying a specific hash (note that git tags or branches aren't safe either, they can be changed!). Extract from the documentation:

$ nimble install nimgame@#head

This is of course Git-specific, for Mercurial, use tip instead of head. A branch, tag, or commit hash may also be specified in the place of head.

At this point the package manager is just a fancy wrapper around the git submodule command, so is it really needed? It is true that package managers can offer more features, like search of packages by keywords or automatic build/testing features, so maybe there is a valid reason for using package managers if they reduce the amount of work a developer has to do, but in terms of supply chain attacks and build reproducibility they are just a ticking bomb waiting to explode in your face. Plus I'd argue those other features are best serviced somewhere else, or in different separate commands/tools (the last thing you want for user friendliness is a package manager with more command line switches than git!).

Luckily for package managers the solution is trivial: add an option to specify the hash of the downloaded package. Just normalize the hacks which allow you to specify a git commit into a generic package hash, allowing for normal binary/zip distribution to also be safe, and make that hash not dependant on the protocol for people with other source versioning software.

Even if developers are lazy and use an arbitrary version number to get a package into their system because they are exploring solutions, as long as they validate and verify it is what they were looking for, they could then freeze the local hash into their build specification. With this simple change, supply chain attacks would be prevented, because changing a good 2.3.4 package into an evil 2.3.4 would necessarily alter the hash of the final package, and the package manager could flag it and prevent including malicious code into a project (pretty important for automated continuous integration build machines distributing nightly snapshots of a development branch to end users!).

One more feature I find very useful of not using package managers is the specification of different or conflicting versions of a dependency compared to one installed globally on the system and making sure nothing in the build environment affects the build. In order to have reproducible builds you want the compiler to read the version you want and nothing else. Yet it happened to me in the past that I was expecting the build of a program to use a library installed in a directory, but the default compiler search path was getting the same named library from another place, thus getting a different version for a hilarious waste of time. For Nim code this has lead me to always use the noNimblePath compiler switch which prevents any global or pseudo global modules to be included in the project and forces you to specify the local path to the modules you want to use. This is more important for people building libraries for others, so I guess it's not so important for the majority of developers. People like solving problems with big guns, so you end up seeing teams provisioning virtual machines to make reproducible builds, rather than asking their tools to be able to deal with the environment properly. Talk about priorities.

Did you see that? Adding an optional hash parameter will make us look good again in the cataratic eyes of a few picky programmers

Still prefer git? Keep calm and commit bugfixes

Most developers think that the URLs baked into git submodules can't be changed, but this would defeat the purpose of a decentralized source control management tool. In order to see how flexible git submodules are without suffering from supply chain attacks, we will go through a multi repository scenario. Digression: why do many git tutorials and documentation present the scenario of programming on an airplane (search for the word airplane)? It is confusing, I thought programmers were meant to never leave their parents' basement? What are programmers now, some kind of idols travelling to places and getting harassed at airports by reporters asking them what their latest commit was? I'm so confused Internet, get your stereotypes right!

Anyway, we will fix a bug in a project dependency completely offline across packages, which will require changing the remote repositories to local ones where the work will be done for a while, then upload for others to check. The magic of commit hashes will allow us to orchestrate offline a series of related commits without having to push to a public repository. In fact, since reviews are so common, we will make changes in separate branches for entangled pull requests. All offline. In an airplane. With freaking snakes.

Before we step on the airplane, however, we need to construct our public repositories to verify this is all working. I'm going to use GitLab for the example but any other host will work. By going to https://gitlab.com/projects/new I create new gsm_lib_module and gsm_miner projects, both public. Let's create some local code to fill those awesome repositories with Python:

[~]$ cd /tmp/

[/tmp]$ mkdir gsm_lib_module

[/tmp]$ cd gsm_lib_module/

[/tmp/gsm_lib_module]$ vim .gitignore

[/tmp/gsm_lib_module]$ cat .gitignore
*.pyc
*.swp
.DS_Store

[/tmp/gsm_lib_module]$ git init
Initialized empty Git repository in /private/tmp/gsm_lib_module/.git/

[/tmp/gsm_lib_module(master)]$ git add .gitignore

[/tmp/gsm_lib_module(master)]$ git commit -av -m "Starting repo"
[master (root-commit) 02e0f10] Starting repo
 1 file changed, 3 insertions(+)
 create mode 100644 .gitignore

[/tmp/gsm_lib_module(master)]$ vim lib_module.py

[/tmp/gsm_lib_module(master)]$ cat lib_module.py
def say_hello_lib():
    print("Hello lib")

if __name__ == "__main__":
    say_hello_lib()

[/tmp/gsm_lib_module(master)]$ vim __init__.py

[/tmp/gsm_lib_module(master)]$ cat __init__.py
from lib_module import say_hello_lib

[/tmp/gsm_lib_module(master)]$

[/tmp/gsm_lib_module(master)]$ git commit -av -m "Blockchain library"
[master ee19c05] Blockchain library
 2 files changed, 6 insertions(+)
 create mode 100644 __init__.py
 create mode 100644 lib_module.py

[/tmp/gsm_lib_module(master)]$ git remote add origin git@gitlab.com:gradha/gsm_lib_module.git

[/tmp/gsm_lib_module(master)]$ git push -u origin master
Counting objects: 7, done.
Delta compression using up to 8 threads.
Compressing objects: 100% (4/4), done.
Writing objects: 100% (7/7), 661 bytes | 661.00 KiB/s, done.
Total 7 (delta 0), reused 0 (delta 0)
To gitlab.com:gradha/gsm_lib_module.git
 * [new branch]      master -> master
Branch 'master' set up to track remote branch 'master' from 'origin'.

With that sequence of commands we will have a library project available at https://gitlab.com/gradha/gsm_lib_module. Your URLs will be different, of course, due to the username being different. Let's create now an awesome python blockchain thingy:

[~]$ cd /tmp/

[/tmp]$ mkdir gsm_miner

[/tmp]$ cd gsm_miner/

[/tmp/gsm_miner]$

[/tmp/gsm_miner]$ vim .gitignore

[/tmp/gsm_miner]$ cat .gitignore
*.pyc
*.swp
.DS_Store

[/tmp/gsm_miner]$ git init
Initialized empty Git repository in /private/tmp/gsm_miner/.git/

[/tmp/gsm_miner(master)]$ git add .gitignore

[/tmp/gsm_miner(master)]$ git commit -av -m "Starting repo"
[master (root-commit) 69f664f] Starting repo
 1 file changed, 3 insertions(+)
 create mode 100644 .gitignore

[/tmp/gsm_miner(master)]$ vim program.py

[/tmp/gsm_miner(master)]$ cat program.py
import gsm_lib_module

def main():
    print("Running main module")
    gsm_lib_module.say_hello_lib()

if __name__ == "__main__":
    main()

[/tmp/gsm_miner(master)]$ git submodule init

[/tmp/gsm_miner(master)]$ git submodule add https://gitlab.com/gradha/gsm_lib_module.git
Cloning into '/private/tmp/gsm_miner/gsm_lib_module'...
remote: Enumerating objects: 7, done.
remote: Counting objects: 100% (7/7), done.
remote: Compressing objects: 100% (4/4), done.
remote: Total 7 (delta 0), reused 0 (delta 0)
Unpacking objects: 100% (7/7), done.

[/tmp/gsm_miner(master)]$ python program.py
Running main module
Hello lib

[/tmp/gsm_miner(master)]$ git add program.py

[/tmp/gsm_miner(master)]$ git commit -av -m "Getting there"
[master da08e71] Getting there
 3 files changed, 12 insertions(+)
 create mode 100644 .gitmodules
 create mode 160000 gsm_lib_module
 create mode 100644 program.py

[/tmp/gsm_miner(master)]$ git remote add origin git@gitlab.com:gradha/gsm_miner.git

[/tmp/gsm_miner(master)]$ git push -u origin master
Counting objects: 7, done.
Delta compression using up to 8 threads.
Compressing objects: 100% (5/5), done.
Writing objects: 100% (7/7), 756 bytes | 756.00 KiB/s, done.
Total 7 (delta 0), reused 3 (delta 0)
To gitlab.com:gradha/gsm_miner.git
 * [new branch]      master -> master
Branch 'master' set up to track remote branch 'master' from 'origin'.

Offline hacking via the dangerous method

And there you go, our first friendly steps towards blockchain investors. A few minutes after pushing this repo we hear the phone ringing: investors are all lined up to pay zillions, but they want to have a personal presentation in some far away place which requires travelling by airplane. Minutes before embarking the investors call and request a change. Oh noes, now you have to work hard on the plane without internet. Once the airplane is off the ground you furiously start changing the library repository to add a new function:

[/tmp/gsm_miner(master)]$ cd /tmp/gsm_lib_module/

[/tmp/gsm_lib_module(master)]$ git co -b happy_investors
Switched to a new branch 'happy_investors'

[/tmp/gsm_lib_module(happy_investors)]$ vim lib_module.py

[/tmp/gsm_lib_module(happy_investors)]$ cat lib_module.py
def say_hello_lib():
    print("Hello lib")

def welcome_zillions():
    print("send moneys")

if __name__ == "__main__":
    say_hello_lib()

[/tmp/gsm_lib_module(happy_investors)]$ vim __init__.py

[/tmp/gsm_lib_module(happy_investors)]$ cat __init__.py
from lib_module import *

[/tmp/gsm_lib_module(happy_investors)]$ git commit -av -m "One step closer to nirvana"
[happy_investors da0578a] One step closer to nirvana
 2 files changed, 4 insertions(+), 1 deletion(-)

Now the repository is changed locally, but how are we going to reference that commit without being able to push it? There are two ways, so for the convenience of the tutorial let's create a copy of the main repository before touching it so we can do both methods and compare. The first method is easy but potentially dangerous:

[~]$ cd /tmp

[/tmp]$ cp -r gsm_miner gsm_miner_2

[/tmp]$ cd gsm_miner

[/tmp/gsm_miner(master)]$ cat .gitmodules
[submodule "gsm_lib_module"]
    path = gsm_lib_module
    url = https://gitlab.com/gradha/gsm_lib_module.git

[/tmp/gsm_miner(master)]$ vim .gitmodules

[/tmp/gsm_miner(master)]$ cat .gitmodules
[submodule "gsm_lib_module"]
    path = gsm_lib_module
    url = file:///tmp/gsm_lib_module

[/tmp/gsm_miner(master)]$ git submodule sync
Synchronizing submodule url for 'gsm_lib_module'

[/tmp/gsm_miner(master)]$ cd gsm_lib_module/

[/tmp/gsm_miner/gsm_lib_module(master)]$ git remote -v
origin	file:///tmp/gsm_lib_module (fetch)
origin	file:///tmp/gsm_lib_module (push)

[/tmp/gsm_miner/gsm_lib_module(master)]$ git pull
remote: Counting objects: 4, done.
remote: Compressing objects: 100% (3/3), done.
remote: Total 4 (delta 0), reused 0 (delta 0)
Unpacking objects: 100% (4/4), done.
From file:///tmp/gsm_lib_module
 * [new branch]      happy_investors -> origin/happy_investors
Already up to date.

[/tmp/gsm_miner/gsm_lib_module(master)]$ git checkout happy_investors
Branch 'happy_investors' set up to track remote branch 'happy_investors' from 'origin'.
Switched to a new branch 'happy_investors'

[/tmp/gsm_miner/gsm_lib_module(happy_investors)]$ cd ..

[/tmp/gsm_miner(master)]$ vim program.py

[/tmp/gsm_miner(master)]$ cat program.py
import gsm_lib_module

def main():
    print("Running main module")
    gsm_lib_module.say_hello_lib()
    gsm_lib_module.welcome_zillions()

if __name__ == "__main__":
    main()

[/tmp/gsm_miner(master)]$ python program.py
Running main module
Hello lib
send moneys

[/tmp/gsm_miner(master)]$ git status
On branch master
Your branch is up to date with 'origin/master'.

Changes not staged for commit:
  (use "git add <file>..." to update what will be committed)
  (use "git checkout -- <file>..." to discard changes in working directory)
    
    modified:   .gitmodules
    modified:   gsm_lib_module (new commits)
    modified:   program.py

no changes added to commit (use "git add" and/or "git commit -a")

[/tmp/gsm_miner(master)]$ git add gsm_lib_module program.py

[/tmp/gsm_miner(master)]$ git commit -m "I'm leet"
[master 2e653f5] I'm leet
 2 files changed, 2 insertions(+), 1 deletion(-)

[/tmp/gsm_miner(master)]$ git show
commit 2e653f562c69cdaf05c6b7c18655a59cbaf742fa (HEAD -> master)
Author: Grzegorz Adam Hankiewicz <gradha@imap.cc>
Date:   Sun Nov 18 22:52:00 2018 +0100
    
    I'm leet

diff --git a/gsm_lib_module b/gsm_lib_module
index ee19c05..da0578a 160000
--- a/gsm_lib_module
+++ b/gsm_lib_module
@@ -1 +1 @@
-Subproject commit ee19c0528e5ba8d375362ec557b4126ee916ce0d
+Subproject commit da0578a23ac4823a4164ebd37d1500f777e24128
diff --git a/program.py b/program.py
index 4b77e2b..c278166 100644
--- a/program.py
+++ b/program.py
@@ -3,6 +3,7 @@ import gsm_lib_module
 def main():
     print("Running main module")
     gsm_lib_module.say_hello_lib()
+    gsm_lib_module.welcome_zillions()
 
 if __name__ == "__main__":
     main()

OK, so what have we done here? The first step is to modify the .gitmodules file and replace the http URL with a local path. The git submodule sync takes the contents of .gitmodules and does whatever sorcery is needed to make the repository point to that local path instead of the internet. Next, as any programmer would do, we enter the submodule, check that it points to our local file, and pull changes in order to switch the submodule to the commit of the new branch not available online yet.

The dangerous part is changing files inside gsm_miner carefully, we want to commit everything except the .gitmodules file. If we were to include this file in a commit and push it to the public, everybody would get those changes and their online URL would be replaced by a path they likely won't have and thus break the program. Zillions of investment would be lost. Still, if you are careful avoiding to include the .gitmodules file this is a valid strategy. Once online, we could discard the local changes to .gitmodules, run git submodule sync and continue as if we had been all the time online.

Wait a second, why do I need to ignore changes to a file tracked by git?

Offline hacking via the icky method

Let's see an alternate way of doing the same without the dangers of commiting weird paths to our repository. The ugly part here is that we need to change the repository URL in internal configuration files which are only visible to us, and changing .git internal files is always icky:

[~]$ cd /tmp

[/tmp]$ cd gsm_miner_2/

[/tmp/gsm_miner_2(master)]$ cat .git/config
[core]
    repositoryformatversion = 0
    filemode = true
    bare = false
    logallrefupdates = true
    ignorecase = true
    precomposeunicode = true
[branch "master"]
[submodule "gsm_lib_module"]
    url = https://gitlab.com/gradha/gsm_lib_module.git
    active = true
[remote "origin"]
    url = git@gitlab.com:gradha/gsm_miner.git
    fetch = +refs/heads/*:refs/remotes/origin/*
[branch "master"]
    remote = origin
    merge = refs/heads/master

[/tmp/gsm_miner_2(master)]$ vim .git/config

[/tmp/gsm_miner_2(master)]$ cat .git/config
[core]
    repositoryformatversion = 0
    filemode = true
    bare = false
    logallrefupdates = true
    ignorecase = true
    precomposeunicode = true
[branch "master"]
[submodule "gsm_lib_module"]
    url = file:///tmp/gsm_lib_module
    active = true
[remote "origin"]
    url = git@gitlab.com:gradha/gsm_miner.git
    fetch = +refs/heads/*:refs/remotes/origin/*
[branch "master"]
    remote = origin
    merge = refs/heads/master

[/tmp/gsm_miner_2(master)]$ rm -Rf .git/modules/gsm_lib_module

[/tmp/gsm_miner_2(master)]$ rm -R gsm_lib_module/

[/tmp/gsm_miner_2(master)]$ git submodule init

[/tmp/gsm_miner_2(master)]$ git submodule update
Cloning into '/private/tmp/gsm_miner_2/gsm_lib_module'...
Submodule path 'gsm_lib_module': checked out 'ee19c0528e5ba8d375362ec557b4126ee916ce0d'

[/tmp/gsm_miner_2(master)]$ cd gsm_lib_module/

[/tmp/gsm_miner_2/gsm_lib_module((ee19c05...))]$ git checkout happy_investors
Previous HEAD position was ee19c05 Blockchain library
Switched to branch 'happy_investors'
Your branch is up to date with 'origin/happy_investors'.

[/tmp/gsm_miner_2/gsm_lib_module(happy_investors)]$ cd ..

[/tmp/gsm_miner_2(master)]$ git status
On branch master
Your branch is up to date with 'origin/master'.

Changes not staged for commit:
  (use "git add <file>..." to update what will be committed)
  (use "git checkout -- <file>..." to discard changes in working directory)
    
    modified:   gsm_lib_module (new commits)

no changes added to commit (use "git add" and/or "git commit -a")

I'll spare you the rest of the commands to replicate the whole example since they are the same. Note that we have just achieved our goal, a git status command doesn't show any other changes than the submodule change, so we can't propagate publicly any incorrect submodule URL. Instead of touching .gitmodules we did change the .git/config file to make it point to our local file path. After that, we removed both the module directory and its cached version inside .git/modules. Removing the cache is crucial, otherwise the next submodule commands (init and update) would use this cache, which itself points to the online URL and does not contain changes we recently made offline. By deleting the cached module we force git to fetch it from our local path. This method is ickier because we have to touch more files and directories internal to .git, but on the other hand we can replace a reference URL with whatever we want and not worry about messing up other people's repositories with careless changes.

Conclusions and relative paths FTW!

Both of these ways to replace the source of a submodule work for you locally. If somebody deleted a repository used as a submodule somewhere else, you would use the first method (changing .gitmodules) so that everybody else can have an updated version pointing to some new URL. But if you have to work with some repository and the network is bad, or there are firewall rules preventing the connection, maybe copying data through USBs and using the second method to refer to a local repository can save the day.

I dream of a day when git will be user friendly, will I be alive to see it myself?

While researching the commands and looking at how other people went around tweaking their repositories I found a very interesting piece about using relative paths instead of full paths for submodules. The only practical difference for the previous examples would be the way to acquire the external submodule using a relative path, which has to be done after we specify the remote origin:

[~]$ cd /tmp

[/tmp]$ mkdir gsm_miner_relative

[/tmp]$ cd gsm_miner_relative

[/tmp/gsm_miner_relative]$ vim .gitignore

[/tmp/gsm_miner_relative]$ git init
Initialized empty Git repository in /private/tmp/gsm_miner_relative/.git/

[/tmp/gsm_miner_relative(master)]$ git add .gitignore

[/tmp/gsm_miner_relative(master)]$ git commit -av -m "Starting repository."
[master (root-commit) 826cac6] Starting repository.
 1 file changed, 3 insertions(+)
 create mode 100644 .gitignore

[/tmp/gsm_miner_relative(master)]$ git remote add origin git@gitlab.com:gradha/gsm_miner_relative.git

[/tmp/gsm_miner_relative(master)]$ git submodule add ../gsm_lib_module
Cloning into '/private/tmp/gsm_miner_relative/gsm_lib_module'...
remote: Enumerating objects: 10, done.
remote: Counting objects: 100% (10/10), done.
remote: Compressing objects: 100% (7/7), done.
remote: Total 10 (delta 2), reused 0 (delta 0)
Receiving objects: 100% (10/10), done.
Resolving deltas: 100% (2/2), done.

[/tmp/gsm_miner_relative(master)]$ git commit -av -m "Using relative paths FTW"
[master 3e1d047] Using relative paths FTW
 2 files changed, 4 insertions(+)
 create mode 100644 .gitmodules
 create mode 160000 gsm_lib_module

[/tmp/gsm_miner_relative(master)]$ git push --set-upstream origin master
Counting objects: 6, done.
Delta compression using up to 8 threads.
Compressing objects: 100% (4/4), done.
Writing objects: 100% (6/6), 600 bytes | 600.00 KiB/s, done.
Total 6 (delta 0), reused 0 (delta 0)
To gitlab.com:gradha/gsm_miner_relative.git
 * [new branch]      master -> master
Branch 'master' set up to track remote branch 'master' from 'origin'.

Once the origin is set as remote, even before pushing changes, the submodule command works with the relative URL specification. Another cool feature as the source article states is that you don't have to worry about adding the submodule using a specific protocol. Usually people mess things by using ssh instead of http in their submodule absolute URL. Everything works for them, but won't for others, since they don't have write access to those repositories. This is also the mentioned drawback in the article: if you fork a repository you need to fork the submodules too. Or you can use the lessons learned here about replacing the URLs of submodules to checkout just one module and make others point to online public URLs instead of relative paths. But if you can fork a project, why wouldn't you be able to fork its relative submodules?

In any case I like the idea of relative paths, I'll play with it in future projects and see how it goes.

$ nimble search ouroboros
      Error No package found.

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Published on: 19/11/2018 00:30. Last update: 26/11/2018 18:31. rss feed
Copyright 2018 by Grzegorz Adam Hankiewicz.
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