pkg/specgen: Add support for Linux emulation on FreeBSD

This is limited to images that don't depend on complex cgroup or capability
setups but does cover enough functionality to be useful.

[NO NEW TESTS NEEDED]

Signed-off-by: Doug Rabson <dfr@rabson.org>

pkg/specgen/generate/oci_freebsd.go

@@ -4,6 +4,7 @@ package generate
 
 import (
 	"context"
+	"fmt"
 	"strings"
 
 	"github.com/containers/common/libimage"
@@ -17,7 +18,11 @@ import (
 
 // SpecGenToOCI returns the base configuration for the container.
 func SpecGenToOCI(ctx context.Context, s *specgen.SpecGenerator, rt *libpod.Runtime, rtc *config.Config, newImage *libimage.Image, mounts []spec.Mount, pod *libpod.Pod, finalCmd []string, compatibleOptions *libpod.InfraInherit) (*spec.Spec, error) {
-	g, err := generate.New("freebsd")
+	if s.ImageOS != "freebsd" && s.ImageOS != "linux" {
+		return nil, fmt.Errorf("unsupported image OS: %s", s.ImageOS)
+	}
+
+	g, err := generate.New(s.ImageOS)
 	if err != nil {
 		return nil, err
 	}
@@ -49,6 +54,51 @@ func SpecGenToOCI(ctx context.Context, s *specgen.SpecGenerator, rt *libpod.Runt
 		return nil, err
 	}
 
+	// Linux emulatioon
+	if s.ImageOS == "linux" {
+		var mounts []spec.Mount
+		for _, m := range configSpec.Mounts {
+			switch m.Destination {
+			case "/proc":
+				m.Type = "linprocfs"
+				m.Options = []string{"nodev"}
+				mounts = append(mounts, m)
+				continue
+			case "/sys":
+				m.Type = "linsysfs"
+				m.Options = []string{"nodev"}
+				mounts = append(mounts, m)
+				continue
+			case "/dev", "/dev/pts", "/dev/shm", "/dev/mqueue":
+				continue
+			}
+		}
+		mounts = append(mounts,
+			spec.Mount{
+				Destination: "/dev",
+				Type:        "devfs",
+				Source:      "devfs",
+				Options: []string{
+					"ruleset=4",
+					"rule=path shm unhide mode 1777",
+				},
+			},
+			spec.Mount{
+				Destination: "/dev/fd",
+				Type:        "fdescfs",
+				Source:      "fdesc",
+				Options:     []string{},
+			},
+			spec.Mount{
+				Destination: "/dev/shm",
+				Type:        "tmpfs",
+				Source:      "shm",
+				Options:     []string{"notmpcopyup"},
+			},
+		)
+		configSpec.Mounts = mounts
+	}
+
 	// BIND MOUNTS
 	configSpec.Mounts = SupersedeUserMounts(mounts, configSpec.Mounts)
 	// Process mounts to ensure correct options