create volume/fsmap mountpoints properly

[bergwolf]

If there is a symlink in the middle, resolve it as if we were in a chroot jail.

e.g., following pod file will fail to start without this PR, because nginx image has /var/run symlink pointing to /run. Fixes hyperhq/hyperd#521

{
        "containers" : [{
            "image": "nginx",
            "command": ["/bin/sh"],
            "user": {
                "name":"nobody"
            },
            "volumes": [{
                "volume": "tmp",
                "path": "/var/run/secrets/kubernetes.io/serviceaccount",
                "readOnly": false
             }]
        }],
        "resource": {
            "vcpu": 1,
            "memory": 256
        },
        "files": [],
        "volumes": [{
            "name": "tmp",
            "source": "/tmp",
            "format": "vfs"
        }],
        "tty": true
}

[bergwolf]

hykins failed with �Failed to mkfs the block:exec: "mkfs.xfs": executable file not found in $PATH:.

[gnawux]

@bergwolf will check the hykins config. jimmy has fixed this for hyperd

[gnawux]

I updated the jenkins configuration, could ask retest this please.

[bergwolf]

retest this please, @hykins

[feiskyer]

@bergwolf Confirmed this PR works properly. LGTM.

[feiskyer]

@bergwolf Any updates on this?

[bergwolf]

@feiskyer working on an updated version that handles symlinks in symlink targets.

[bergwolf]

updated to handle following additional cases:

1. symlinks in symlink file contents.
2. .. in path components in which case chroot jail is always valid.

For a container created with below Dockerfile:

FROM busybox
RUN mkdir /data; ln -s /etc/ /data/foo; ln -s ../../data/foo /etc/bar

Following pod file can still work:

{
        "containers" : [{
            "image": "bergwolf/test",
            "command": ["/bin/sh"],
            "user": {
                "name":"nobody"
            },
            "volumes": [{
                "volume": "tmp",
                "path": "/etc/bar/haha",
                "readOnly": false
             }]
        }],
        "resource": {
            "vcpu": 1,
            "memory": 256
        },
        "files": [],
        "volumes": [{
            "name": "tmp",
            "source": "/tmp",
            "format": "vfs"
        }],
        "tty": true
}

[laijs]

need to check the case len==sizeof(buf)