Linux containers on FreeBSD

This allows running Linux containers on FreeBSD and modifies the
mounts so that they represent the linux emulated filesystems, as per:
https://wiki.freebsd.org/LinuxJails

Co-authored-by: Gijs Peskens <gijs@peskens.net>, Samuel Karp <samuelkarp@users.noreply.github.com>
Signed-off-by: Artem Khramov <akhramov@pm.me>

oci/spec_opts_freebsd.go

@@ -0,0 +1,50 @@
+/*
+   Copyright The containerd Authors.
+
+   Licensed under the Apache License, Version 2.0 (the "License");
+   you may not use this file except in compliance with the License.
+   You may obtain a copy of the License at
+
+       http://www.apache.org/licenses/LICENSE-2.0
+
+   Unless required by applicable law or agreed to in writing, software
+   distributed under the License is distributed on an "AS IS" BASIS,
+   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+   See the License for the specific language governing permissions and
+   limitations under the License.
+*/
+
+package oci
+
+import (
+	specs "github.com/opencontainers/runtime-spec/specs-go"
+)
+
+// appendOSMounts modifies the mount spec to mount emulated Linux filesystems on FreeBSD,
+// as per: https://wiki.freebsd.org/LinuxJails
+func appendOSMounts(s *Spec, os string) error {
+	// No-op for FreeBSD containers
+	if os != "linux" {
+		return nil
+	}
+	/* The nosuid noexec options are for consistency with Linux mounts: on FreeBSD it is
+	   by default impossible to execute anything from these filesystems.
+	*/
+	var mounts = []specs.Mount{
+		{
+			Destination: "/proc",
+			Type:        "linprocfs",
+			Source:      "linprocfs",
+			Options:     []string{"nosuid", "noexec"},
+		},
+		{
+			Destination: "/sys",
+			Type:        "linsysfs",
+			Source:      "linsysfs",
+			Options:     []string{"nosuid", "noexec", "nodev"},
+		},
+	}
+
+	s.Mounts = append(mounts, s.Mounts...)
+	return nil
+}