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This patch adds a section pointing out how permissions should be done within Github workflows. I believe all of our workflows are currently compliant with this, but it helps to have something to point to documenting the practice and especially the motivation.
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======================
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LLVM CI Best Practices
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======================
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Overview
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========
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This document contains a list of guidelines and best practices to use when
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working on LLVM's CI systems. These are intended to keep our actions reliable,
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consistent, and secure.
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Github Actions Best Practices
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=============================
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This section contains information on best practices/guidelines when working on
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LLVM's github actions workflows.
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Disabling Jobs In Forks
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-----------------------
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There are many LLVM forks that exist, and we currently default to preventing
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actions from running outside of the LLVM organization to prevent them from
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running in forks. We default to this as actions running in forks are usually
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not desired and only run by accident. In addition, many of our workflows
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assume that they are operating within the main LLVM repository and break
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otherwise.
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Adhering to this best practice looks like adding the following to each of the
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jobs specified within a workflow:
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.. code-block:: yaml
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jobs:
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<job name>:
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if: github.repository_owner == 'llvm'
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We choose to use ``github.repository_owner`` rather than ``github.repository``
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to enable these workflows to run in forks inside the LLVM organization such as
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the ClangIR fork.
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There are some exceptions to this rule where ``github.repository`` might be
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used when it makes sense to limit a workflow to only running in the main
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monorepo repository. These include things like the issue subscriber and
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release tasks, which should not run anywhere else.
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Hash Pinning Dependencies
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-------------------------
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Github Actions allows the use of actions from other repositories as steps in
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jobs. We take advantage of various actions for a variety of different tasks,
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but especially tasks like checking out the repository, and
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downloading/uploading build caches. These actions are typically versioned with
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just a release, which looks like the following:
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.. code-block:: yaml
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steps:
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- name: Checkout LLVM
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uses: actions/checkout@v4
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However, it is best practice to specify an exact commit SHA from which to pull
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the action from, noting the version in a comment:
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We plan on revisting this reccomendation once Github's immutable actions have
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been rolled out as GA.
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.. code-block:: yaml
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steps:
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- name: Checkout LLVM
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uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
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This is beneficial for two reasons: reliability and security. Specifying an
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exact SHA rather than just a major version ensures we end up running the same
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action originally specified when the workflow as authored and/or updated,
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and that no breaking changes sneak in from new versions of a workflow being
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released. However, this effect could also be achieved by specifying an exact
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dot release. The biggest reason to prefer hash pinned dependencies is security.
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Release assets on Github are mutable, allowing an attacker to change the code
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within a specific version of an action after the fact, potentially stealing
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sensitive tokens and credentials. Hash pinning the dependencies prevents this
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as the hash would change with the code.
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Using Versioned Runner Images
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-----------------------------
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Github actions allows the use of either specifically versioned runner images
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(e.g., ``ubuntu-22.04``), or just the latest runner image
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(e.g., ``ubuntu-latest``). It is best practice to use explicitly versioned
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runner images. This prevents breakages when Github rolls the latest runner
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image to a new version with potentially breaking changes, instead allowing us
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to explicitly opt-in to using the new image when we have done sufficient
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testing to ensure that our existing workflows work as expected in the new
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environment.
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Top Level Read Permissions
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--------------------------
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The top of every workflow should specify that the job only has read
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permissions:
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.. code-block:: yaml
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permissions:
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contents: read
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If specific jobs within the workflow need additional permissions, those
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permissions should be added within the specific job. This practice locks down
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all permissions by default and only enables them when needed, better enforcing
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the principle of least privilege.
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