Volumes of type tmpfs lacks support for the mode parameter

[rawtaz]

Docker Compose currently does not support the mode parameter for tmpfs mounts. It only support the size parameter.

This has been verified both empirically with Docker Compose version v2.6.0 with Docker Engine - Community version 20.10.17 and by looking in the source code.

Here are some references showing that mode is currently not supported:

• The buildTmpfsOptions function is missing it and even has a comment for it.
• The ServiceVolumeTmpfs struct is missing it.

Worth noting is that Docker does support it in the mount.TmpfsOptions struct that is returned by the buildTmpfsOptions function in Compose.

Also worth mentioning is that the official Docker Compose specification documentation for tmpfs volumes clearly states that mode should be supported for tmpfs. Despite this, however, the compose-go library does not support the mode parameter.

Here's an example of a tmpfs specification for a service that does not work (Compose throws an error services.foo.volumes.0.tmpfs Additional property mode is not allowed):

volumes:
  - type: tmpfs
    target: /test
    tmpfs:
      size: 128k
      mode: 0777

I'm not sure if I got all of the relationships between specifications, libraries and other software right, I figured that given the above mentioned pieces of mode throughout the source code, it made sense to open an issue about it in this repository.

It looks like it's rather straight forward to add support for this option, since Docker itself supports it, or am I missing something?

Either way, it's a problem because it's currently impossible to mount and use tmpfs volumes with unprivileged processes/containers.

[cavo789]

Same issue

I just met the same issue. I'm building a postgres image where I'm using a non root user and tmpfs storage for my PGDATA directory.

The error message I got is mkdir: can't create directory '/var/lib/postgresql/data': Permission denied and this is because I'm using a volume like this:

volumes:
      - type: tmpfs
        target: /var/lib/postgresql/data
        tmpfs:
          size: 536870912 # 512 MB

I don't have that error with this volume:

volumes:
      - my_postgres_data:/var/lib/postgresql/data:delegated

So linked to tmpfs.

After many searches, I've found some docs (like https://docs.docker.com/storage/tmpfs/#specify-tmpfs-options) suggesting to set the mode to 1770.

According this doc (https://github.com/compose-spec/compose-spec/blob/master/spec.md#long-syntax-4); I should be able to write the portion below in my docker-compose.yml file but docker compose config mention it's invalid. (The error message is then services.my_postgres.volumes.0.tmpfs Additional property mode is not allowed.)

volumes:
      - type: tmpfs
        target: /var/lib/postgresql/data
        tmpfs:
          mode: 1700
          size: 536870912 # 512 MB

My editor (vscode) is not protesting agains the schema (https://raw.githubusercontent.com/compose-spec/compose-spec/master/schema/compose-spec.json); seems well valid.

[cavo789]

Thank you @milas and @nicksieger 👍