kaniko/docs/design_proposals/filesystem-resolution-proposal-01.md

Filesystem Resolution 01

• Author(s): cgwippern@google.com
• Reviewers:
• Date: 2020-02-12
• Status: [Reviewed/Cancelled/Under implementation/Complete]

Here is a brief explanation of the Statuses

1. Reviewed: The proposal PR has been accepted, merged and ready for implementation.
2. Under implementation: An accepted proposal is being implemented by actual work. Note: The design might change in this phase based on issues during implementation.
3. Cancelled: During or before implementation the proposal was cancelled. It could be due to:
   • other features added which made the current design proposal obsolete.
   • No longer a priority.
4. Complete: This feature/change is implemented.

Background

Kaniko builds Docker image layers as overlay filesystem layers; specifically it creates a tar file which contains the entire content of a given layer in the overlay filesystem. Each overlay layer corresponds to one image layer.

Overlay filesystems should only contain the objects changed in each layer; meaning that if only one file changes between some layer A and some B, layer B would only contain a single file (the one that changed).

To accomplish this, Kaniko walks the entire filesystem to discover every object. Some of these objects may actually be a symlink to another object in the filesystem; in these cases we must consider both the link and the target object.

Kaniko also maintains a set of whitelisted (aka ignored) filepaths. Any object which matches one of these filepaths should be ignored by kaniko.

This results in a 3 dimensional search space

• changed relative to previous layer
• symlink
• whitelisted

Kaniko must also track which objects are referred to by multiple stages; this functionality is out of scope for this proposal.

This search space is currently managed in an inconsistent and somewhat ad-hoc way; code that manages the various search dimensions is spread out and duplicated. There are also a number of poorly handled edge cases which continue to cause bugs.

The search space dimensions cannot be reduced or substituted.

Currently there are a number of bugs around symlinks incorrectly resolved, whitelists not respected, and unchanged files added to layers.

Design

Resolution of the filesystem and the objects it contains should be handled consistently and with a single API.

• Callers of this API should not be concerned with the type of object at a given filepath (e.g. symlink or not).
• Callers of this API should not be concerned with whether a given path is whitelisted.
• This API should return a set of filepaths which should be added to the layer without further processing.

The API should take a limited set of arguments

• The root path in the filesystem
• The whitelist

The API should return only two arguments

• A set of filepaths which should be added to the layer without further processing
• error

Open Issues/Questions

<Ignore symlinks targeting whitelisted paths?>

Resolution: Not Yet Resolved

Implementation plan

• Write the new API
• Write tests for the new API
• Integrate the new API into existing code

Integration test plan

Add integration tests to the existing suite which cover the known bugs

Notes

Given some path /usr/lib/foo which is a link to /etc/foo/

And /etc/foo contains /etc/foo/bar.txt

Adding a link /usr/lib/foo/bar.txt => /etc/foo/bar.txt will break the image

In a linux shell this raises an error

$ ls /usr/lib/bar
=> /usr/lib/bar/foo.txt
$ ln -s /usr/lib/bar barlink
$ ln -s /usr/lib/bar/foo.txt barlink/foo.txt
=> ERROR

Given some path /usr/foo/bar which is a link to /dev/null, and /dev is whitelisted /dev/null should not be added to the image.