Fix tmpfs mode opts when dir already exists

[cpuguy83]

When a directory already exists (or after a container is restarted) the perms of the directory being mounted to were being used even when a different permission is set on the tmpfs mount options.

This prepends the original directory perms to the mount options. If the perms were already set in the mount opts then those perms will win.
This eliminates the need to perform a chmod after mount entirely.

Fixes #3911

[kolyshkin]

LGTM overall. I've checked the test fails before the fix, and changed it slightly so that if it fails we see what the stat output is (and simplified it a bit).

@test "runc run with tmpfs perms" {
        # shellcheck disable=SC2016
        update_config '.process.args = ["sh", "-c", "stat -c %a /tmp/test"]'
        update_config '.mounts += [{"destination": "/tmp/test", "type": "tmpfs", "source": "tmpfs", "options": ["mode=0444"]}]'

        # Directory is to be created by runc.
        runc run test_tmpfs
        [ "$status" -eq 0 ]
        [ "$output" = "444" ]

        # Run a 2nd time with the pre-existing directory.
        # Ref: https://github.com/opencontainers/runc/issues/3911
        runc run test_tmpfs
        [ "$status" -eq 0 ]
        [ "$output" = "444" ]

        # Existing directory, custom perms, no mode on the mount,
        # so it should use the directory's perms.
        update_config '.mounts[-1].options = []'
        chmod 0710 rootfs/tmp/test
        # shellcheck disable=SC2016
        runc run test_tmpfs
        [ "$status" -eq 0 ]
        [ "$output" = "710" ]

        # Add back the mode on the mount, and it should use that instead.
        # Just for fun, use different perms than was used earlier.
        # shellcheck disable=SC2016
        update_config '.mounts[-1].options = ["mode=0410"]'
        runc run test_tmpfs
        [ "$status" -eq 0 ]
        [ "$output" = "410" ]
}

[kolyshkin]

LGTM, thanks.

We should backport this to 1.1

[kolyshkin]

@opencontainers/runc-maintainers PTAL

[thaJeztah]

oh! thought I already reviewed this one, but I guess we only discussed it and I forgot.

LGTM, thanks!