Add an API so users can request /tmp to be a tmpfs

[slp]

This is required to handle crun's --tmpfs: containers/crun#1952

[mz-pdm]

What kind of API do you have on mind? If the aim is to support podman's --tmpfs, it'd be natural to use mounts entry from /.krun_config.json. Which can look like:

  "mounts": [
    {
      "destination": "/tmp",
      "type": "tmpfs",
      "source": "tmpfs",
      "options": [
        "rprivate",
        "nosuid",
        "nodev",
        "tmpcopyup"
      ]
    },
    {
      "destination": "/proc",
      "type": "proc",
      "source": "proc",
      "options": [
        "nosuid",
        "noexec",
        "nodev"
      ]
    },
    {
      "destination": "/dev",
      "type": "tmpfs",
      "source": "tmpfs",
      "options": [
        "nosuid",
        "strictatime",
        "mode=755",
        "size=65536k"
      ]
    },
    ...
  ]

Looking there, there are at least the following questions coming to mind:

• Do we want to support different options, or to ignore them and just mount the tmpfs in "our" way?
• There can be multiple tmpfs entries, how do we identify the right one to handle?
• Or should we, if the config file is provided, use all the mounts from the file instead of those hardwired in init.c?

Or do we want to take some simpler approach like introducing a different mechanism in crun to indicate to libkrun we want a specific tmpfs directory?

[mz-pdm]

An initial minimum PoC: mz-pdm@a8fa6c6

[mz-pdm]

• Do we want to support different options, or to ignore them and just mount the tmpfs in "our" way?

Note that tmpcopyup is enabled by default in podman run. Copying the files is a non-trivial piece of code. It's available in crun but under a different license (LGPL) and I suppose we don't want complex changes before rewriting init.c to Rust.