diff --git a/osbase/build/mkcpio/main.go b/osbase/build/mkcpio/main.go
new file mode 100644
index 0000000..4886c3c
--- /dev/null
+++ b/osbase/build/mkcpio/main.go
@@ -0,0 +1,218 @@
+package main
+
+import (
+	"flag"
+	"io"
+	"log"
+	"os"
+	"path"
+	"sort"
+	"strings"
+
+	"github.com/cavaliergopher/cpio"
+	"github.com/klauspost/compress/zstd"
+	"golang.org/x/sys/unix"
+
+	"source.monogon.dev/osbase/build/fsspec"
+)
+
+var (
+	outPath = flag.String("out", "", "Output file path")
+)
+
+type placeEnum int
+
+const (
+	// placeNone implies that currently nothing is placed at that path.
+	// Can be overridden by everything.
+	placeNone placeEnum = 0
+	// placeDirImplicit means that there is currently a implied directory
+	// at the given path. It can be overridden by (and only by) an explicit
+	// directory.
+	placeDirImplicit placeEnum = 1
+	// placeDirExplicit means that there is an explicit (i.e. specified by
+	// the FSSpec) directory at the given path. Nothing else can override
+	// this.
+	placeDirExplicit placeEnum = 2
+	// placeNonDir means that there is a file-type resource (i.e a file, symlink
+	// or special_file) at the given path. Nothing else can override this.
+	placeNonDir placeEnum = 3
+)
+
+// place represents the state a given canonical path is in during metadata
+// construction. Its zero value is { State: placeNone, Inode: nil }.
+type place struct {
+	State placeEnum
+	// Inode contains one of the types inside an FSSpec (e.g. *fsspec.File)
+	Inode interface{}
+}
+
+// Usage: -out <out-path.cpio.zst> fsspec-path...
+func main() {
+	flag.Parse()
+	outFile, err := os.Create(*outPath)
+	if err != nil {
+		log.Fatalf("Failed to open CPIO output file: %v", err)
+	}
+	defer outFile.Close()
+	compressedOut, err := zstd.NewWriter(outFile)
+	if err != nil {
+		log.Fatalf("While initializing zstd writer: %v", err)
+	}
+	defer compressedOut.Close()
+	cpioWriter := cpio.NewWriter(compressedOut)
+	defer cpioWriter.Close()
+
+	spec, err := fsspec.ReadMergeSpecs(flag.Args())
+	if err != nil {
+		log.Fatalf("failed to load specs: %v", err)
+	}
+
+	// Map of paths to metadata for validation & implicit directory injection
+	places := make(map[string]place)
+
+	// The idea behind this machinery is that we try to place all files and
+	// directories into a map while creating the required parent directories
+	// on-the-fly as implicit directories. Overriding an implicit directory
+	// with an explicit one is allowed thus the actual order in which this
+	// structure is created does not matter. All non-directories cannot be
+	// overridden anyways so their insertion order does not matter.
+	// This also has the job of validating the FSSpec structure, ensuring that
+	// there are no duplicate paths and that there is nothing placed below a
+	// non-directory.
+	var placeInode func(p string, isDir bool, inode interface{})
+	placeInode = func(p string, isDir bool, inode interface{}) {
+		cleanPath := path.Clean(p)
+		if !isDir {
+			if places[cleanPath].State != placeNone {
+				log.Fatalf("Invalid FSSpec: Duplicate Inode at %q", cleanPath)
+			}
+			places[cleanPath] = place{
+				State: placeNonDir,
+				Inode: inode,
+			}
+		} else {
+			switch places[cleanPath].State {
+			case placeNone:
+				if inode != nil {
+					places[cleanPath] = place{
+						State: placeDirExplicit,
+						Inode: inode,
+					}
+				} else {
+					places[cleanPath] = place{
+						State: placeDirImplicit,
+						Inode: &fsspec.Directory{Path: cleanPath, Mode: 0555},
+					}
+				}
+			case placeDirImplicit:
+				if inode != nil {
+					places[cleanPath] = place{
+						State: placeDirExplicit,
+						Inode: inode,
+					}
+				}
+			case placeDirExplicit:
+				if inode != nil {
+					log.Fatalf("Invalid FSSpec: Conflicting explicit directories at %v", cleanPath)
+				}
+			case placeNonDir:
+				log.Fatalf("Invalid FSSpec: Trying to place inode below non-directory at #{cleanPath}")
+			default:
+				panic("unhandled placeEnum value")
+			}
+		}
+		parentPath, _ := path.Split(p)
+		parentPath = path.Clean(parentPath)
+		if parentPath == "/" || parentPath == p {
+			return
+		}
+		placeInode(parentPath, true, nil)
+	}
+	for _, d := range spec.Directory {
+		placeInode(d.Path, true, d)
+	}
+	for _, f := range spec.File {
+		placeInode(f.Path, false, f)
+	}
+	for _, s := range spec.SymbolicLink {
+		placeInode(s.Path, false, s)
+	}
+	for _, s := range spec.SpecialFile {
+		placeInode(s.Path, false, s)
+	}
+
+	var writeOrder []string
+	for path := range places {
+		writeOrder = append(writeOrder, path)
+	}
+	// Sorting a list of normalized paths representing a tree gives us Depth-
+	// first search (DFS) order which is the correct order for writing archives.
+	// This also makes the output reproducible.
+	sort.Strings(writeOrder)
+
+	for _, path := range writeOrder {
+		place := places[path]
+		switch i := place.Inode.(type) {
+		case *fsspec.File:
+			inF, err := os.Open(i.SourcePath)
+			if err != nil {
+				log.Fatalf("Failed to open source path for file %q: %v", i.Path, err)
+			}
+			inFStat, err := inF.Stat()
+			if err != nil {
+				log.Fatalf("Failed to stat source path for file %q: %v", i.Path, err)
+			}
+			if err := cpioWriter.WriteHeader(&cpio.Header{
+				Mode: cpio.FileMode(i.Mode),
+				Name: strings.TrimPrefix(i.Path, "/"),
+				Size: inFStat.Size(),
+			}); err != nil {
+				log.Fatalf("Failed to write cpio header for file %q: %v", i.Path, err)
+			}
+			if n, err := io.Copy(cpioWriter, inF); err != nil || n != inFStat.Size() {
+				log.Fatalf("Failed to copy file %q into cpio: %v", i.SourcePath, err)
+			}
+			inF.Close()
+		case *fsspec.Directory:
+			if err := cpioWriter.WriteHeader(&cpio.Header{
+				Mode: cpio.FileMode(i.Mode) | cpio.TypeDir,
+				Name: strings.TrimPrefix(i.Path, "/"),
+			}); err != nil {
+				log.Fatalf("Failed to write cpio header for directory %q: %v", i.Path, err)
+			}
+		case *fsspec.SymbolicLink:
+			if err := cpioWriter.WriteHeader(&cpio.Header{
+				// Symlinks are 0777 by definition (from man 7 symlink on Linux)
+				Mode: 0777 | cpio.TypeSymlink,
+				Name: strings.TrimPrefix(i.Path, "/"),
+				Size: int64(len(i.TargetPath)),
+			}); err != nil {
+				log.Fatalf("Failed to write cpio header for symlink %q: %v", i.Path, err)
+			}
+			if _, err := cpioWriter.Write([]byte(i.TargetPath)); err != nil {
+				log.Fatalf("Failed to write cpio symlink %q: %v", i.Path, err)
+			}
+		case *fsspec.SpecialFile:
+			mode := cpio.FileMode(i.Mode)
+			switch i.Type {
+			case fsspec.SpecialFile_CHARACTER_DEV:
+				mode |= cpio.TypeChar
+			case fsspec.SpecialFile_BLOCK_DEV:
+				mode |= cpio.TypeBlock
+			case fsspec.SpecialFile_FIFO:
+				mode |= cpio.TypeFifo
+			}
+
+			if err := cpioWriter.WriteHeader(&cpio.Header{
+				Mode:     mode,
+				Name:     strings.TrimPrefix(i.Path, "/"),
+				DeviceID: int(unix.Mkdev(i.Major, i.Minor)),
+			}); err != nil {
+				log.Fatalf("Failed to write CPIO header for special file %q: %v", i.Path, err)
+			}
+		default:
+			panic("inode type not handled")
+		}
+	}
+}