Porting Case Study

As a non-trivial example of porting a Rust app, let's look at what was done to port gitoxide. This port was already done, so it is now much simpler, but perhaps some of these steps will apply to you.

The goal when porting is to capture all the necessary configuration in recipes and scripts, and to avoid requiring a fork of the repo or upstreaming changes. This is not always feasible, but forking/upstreaming should be avoided when it can be.

We are using full pathnames for clarity, you don't need to.

Build on Linux

Before we start, we need to build the software for our Linux system and make sure it works. This is not part of the porting, it's just to make sure our problems are not coming from the Linux version of the software. We follow the normal build instructions for the software we are porting.

cd ~

git clone https://github.com/Byron/gitoxide.git

cd gitoxide

cargo run --bin ein

Set up the working tree

We start with a fresh clone of the Redox repository. In a Terminal/Console/Command window:

mkdir -p ~/redox-gitoxide

cd ~/redox-gitoxide

git clone git@gitlab.redox-os.org:redox-os/redox.git --origin upstream --recursive

The new recipe will be part of the cookbook repository, so we need to fork then branch it. To fork the cookbook repo:

In the browser, go to Cookbook
Click the Fork button in the upper right part of the page
Create a public fork under your gitlab user name (it's the only option that's enabled)

Then we need to set up our local cookbook repo and create the branch. cookbook was cloned when we cloned redox, so we will just tweak that. In the Terminal window:

cd ~/redox-gitoxide/redox/cookbook

git remote rename origin upstream

git rebase upstream master

git remote add origin git@gitlab.redox-os.org:MY_USERNAME/cookbook.git

git checkout -b gitoxide-port

Create a Recipe

To create a recipe, we need to make a new directory in cookbook/recipes with the name the package will have, in this case gitoxide, and create a recipe.toml file with a first-draft recipe.

mkdir -p ~/redox-gitoxide/redox/cookbook/recipes/gitoxide

cd ~/redox-gitoxide/redox/cookbook/recipes/gitoxide

nano recipe.toml

Start with the following content in the recipe.toml file.

[source]
git = "https://github.com/Byron/gitoxide.git"
[build]
template = "cargo"

First Attempt

Next we attempt to build the recipe. Note that the first attempt may require the Redox toolchain to be updated, so we run make prefix, which may take quite a while.

cd ~/redox-gitoxide/redox

make prefix

make r.gitoxide |& tee gitoxide.log

We get our first round of errors (among other messages):

error[E0425]: cannot find value `POLLRDNORM` in crate `libc`
error[E0425]: cannot find value `POLLWRBAND` in crate `libc`

Make a Local Copy of libc

We suspect the problem is that these items have not been defined in the Redox edition of libc. libc is not a Redox crate, it is a rust-lang crate, but it has parts that are Redox-specific. We need to work with a local copy of libc, and then later ask someone with authority to upstream the required changes.

First, clone libc into our gitoxide directory.

cd ~/redox-gitoxide/redox/cookbook/recipes/gitoxide

git clone https://github.com/rust-lang/libc.git

Try to find the missing constants.

cd ~/redox-gitoxide/redox/cookbook/recipes/gitoxide/libc

grep -nrw "POLLRDNORM" --include "*.rs"

grep -nrw "POLLWRBAND" --include "*.rs"

Looks like the value is not defined for the Redox version of libc. Let's see if it's in relibc.

cd ~/redox-gitoxide/redox/relibc

grep -nrw "POLLRDNORM" --include "*.rs"

grep -nrw "POLLWRBAND" --include "*.rs"

Yes, both are already defined in relibc, and after a bit of poking around, it looks like they have an implementation. They just need to get published in libc. Let's do that.

Make Changes to libc

Let's add our constants to our local libc. We are not going to bother with git because these changes are just for debugging purposes. Copy the constant declarations from relibc, and paste them in the appropriate sections of libc/src/unix/redox/mod.rs. In addition to copying the constants, we have to change the type c_short to ::c_short to conform to libc style.

cd ~/redox-gitoxide/redox/cookbook/recipes/gitoxide

nano libc/src/unix/redox/mod.rs

We add the following lines to mod.rs:


#![allow(unused)]
fn main() {
pub const POLLRDNORM: ::c_short = 0x040;
pub const POLLRDBAND: ::c_short = 0x080;
pub const POLLWRNORM: ::c_short = 0x100;
pub const POLLWRBAND: ::c_short = 0x200;
}

In order to test our changes, we will have to modify our gitoxide clone for now. Once the changes to libc are upstreamed, we won't need a modified gitoxide clone. To avoid overwriting our work, we want to turn off future fetches of the gitoxide source during build, so change recipe.toml to comment out the source section: nano recipe.toml.

#[source]
#git = "https://github.com/Byron/gitoxide.git"
[build]
template = "cargo"

We edit gitoxide's Cargo.toml so we use our libc.

nano ~/redox-gitoxide/cookbook/recipes/gitoxide/source/Cargo.toml

After the [dependencies] section, but before the [profile] sections, add the following to Cargo.toml:

[patch.crates-io]
libc = { path = "../libc" }

Bump the version number on our libc, so it will take priority.

nano ~/redox-gitoxide/cookbook/recipes/gitoxide/libc/Cargo.toml

version = "0.2.143"

Update gitoxide's Cargo.lock.

cd ~/redox-gitoxide/redox/cookbook/recipes/gitoxide/source

cargo update

Make sure we have saved all the files we just edited, and let's try building.

cd ~/redox-gitoxide/redox

make r.gitoxide

Our libc errors are solved! Remember, these changes will need to upstreamed by someone with the authority to make changes to libc. Post a request on the chat's Redox OS/MRs room to add the constants to libc.

Creating a Custom Recipe

In looking at what is included in gitoxide, we see that it uses OpenSSL, which has some custom build instructions described in the docs. There is already a Redox fork of openssl to add Redox as a target, so we will set up our environment to use that.

In order to do this, we are going to need a custom recipe. Let's start with a simple custom recipe, just to get us going. Edit our previously created recipe, cookbook/recipes/gitoxide/recipe.toml, changing it to look like this.

#[source]
#git = "https://github.com/Byron/gitoxide.git"
[build]
template = "custom"
script = """
printenv
"""

In this version of our recipe, we are just going to print the environment variables during cook, so we can see what we might make use of in our custom script. We are not actually attempting to build gitoxide. Now, when we run make r.gitoxide in ~/redox-gitoxide/redox, we see some useful variables such as TARGET and COOKBOOK_ROOT.

Two key shell functions are provided by the custom script mechanism, cookbook_cargo and cookbook_configure. If you need a custom script for building a Rust program, your script should set up the environment, then call cookbook_cargo, which calls Redox's version of cargo. If you need a custom script for using a Makefile, your script should set up the environment, then call cookbook_configure. If you have a custom build process, or you have a patch-and-build script, you can just include that in the script section and not use either of the above functions. If you are interested in looking at the code that runs custom scripts, see the function build() in cookbook's cook.rs.

Adding a dependency on openssl ensures that the build of openssl will happen before attempting to build gitoxide, so we can trust that the library contents are in the target directory of the ssl package. And we need to set the environment variables as described in the OpenSSL bindings crate docs.

Our recipe now looks like this:

#[source]
#git = "https://github.com/Byron/gitoxide.git"
[build]
dependencies = [
    "openssl",
]
template = "custom"
script = """
export OPENSSL_DIR="${COOKBOOK_SYSROOT}"
export OPENSSL_STATIC="true"
cookbook_cargo
"""

Linker Errors

Now we get to the point where the linker is trying to statically link the program and libraries into the executable. This program, called ld, will report errors if there are any undefined functions or missing static variable definitions.

undefined reference to `tzset'
undefined reference to `cfmakeraw'

In our case we find we are missing tzset, which is a timezone function. We are also missing cfmakeraw from termios. Both of these functions are normally part of libc. In our case, they are defined in the libc crate, but they are not implemented by Redox's version of libc, which is called relibc. We need to add these functions.

Add Missing Functions to relibc

Let's set up to modify relibc. As with cookbook, we need a fork of relibc. Click on the Fork button and add a public fork. Then update our local relibc repo and branch.

cd ~/redox-gitoxide/redox/relibc

git remote rename origin upstream

git rebase upstream master

git remote add origin git@gitlab.redox-os.org:MY_USERNAME/relibc.git

git checkout -b gitoxide-port

Now we need to make our changes to relibc...

After a fair bit of work, which we omit here, the functions tzset and cfmakeraw are implemented in relibc. An important note is that in order to publish the functions, they need to be preceded with:

#[no_mangle]
extern "C" fn tzset() ...

Now let's build the system. The command touch relibc changes the timestamp on the relibc directory, which will cause the library to be updated. We then clean and rebuild gitoxide.

cd ~/redox-gitoxide/redox

cd relibc

cargo update

cd ..

touch relibc

make prefix

make cr.gitoxide

Testing in QEMU

Now we need to build a full Redox image and run it in QEMU. Let's make a configuration file.

cd ~/redox-gitoxide/redox/config/x86_64

cp desktop.toml my_desktop.toml

nano my_desktop.toml

Note that the prefix "my_" at the beginning of the config file name means that it is gitignore'd, so it is preferred that you prefix your config name with "my_".

In my_desktop.toml, at the end of the list of packages, after uutils = {}, add

gitoxide = {}

Now let's tell make about our new config definition, build the system, and test our new command.

cd ~/redox-gitoxide/redox

echo "CONFIG_NAME?=my_desktop.toml" >> .config

make rebuild

make qemu

We get some errors, but we are making progress.

Submitting the MRs

Before committing our new recipe, we need to uncomment the [source] section. Edit ~/redox-gitoxide/redox/cookbook/recipes/gitoxide/recipe.toml to remove the # from the start of the first two lines.
We commit our changes to cookbook to include the new gitoxide recipe and submit an MR, following the instructions Creating Proper Pull Requests.
We commit our changes to relibc. We need to rebuild the system and test it thoroughly in QEMU, checking anything that might be affected by our changes. Once we are confident in our changes, we can submit the MR.
We post links to both MRs on Redox OS/MRs to ensure they get reviewed.
After making our changes to libc and testing them, we need to request to have those changes upstreamed by posting a message on Redox OS/MRs. If the changes are complex, please create an issue here and include a link to it in your post.

The Redox Operating System