diff --git a/osbase/build/fwprune/main.go b/osbase/build/fwprune/main.go
new file mode 100644
index 0000000..0bedcfd
--- /dev/null
+++ b/osbase/build/fwprune/main.go
@@ -0,0 +1,218 @@
+// fwprune is a buildsystem utility that filters linux-firmware repository
+// contents to include only files required by the built-in kernel modules,
+// that are specified in modules.builtin.modinfo.
+// (see: https://www.kernel.org/doc/Documentation/kbuild/kbuild.txt)
+package main
+
+import (
+	"debug/elf"
+	"flag"
+	"io/fs"
+	"log"
+	"os"
+	"path"
+	"path/filepath"
+	"regexp"
+	"sort"
+	"strings"
+
+	"google.golang.org/protobuf/encoding/prototext"
+	"google.golang.org/protobuf/proto"
+
+	"source.monogon.dev/osbase/build/fsspec"
+	"source.monogon.dev/osbase/kmod"
+)
+
+// linkRegexp parses the Link: lines in the WHENCE file. This does not have
+// an official grammar, the regexp has been written in an approximation of
+// the original parsing algorithm at @linux-firmware//:copy_firmware.sh.
+var linkRegexp = regexp.MustCompile(`(?m:^Link:\s*([^\s]+)\s+->\s+([^\s]+)\s*$)`)
+
+var (
+	modinfoPath      = flag.String("modinfo", "", "Path to the modules.builtin.modinfo file built with the kernel")
+	modulesPath      = flag.String("modules", "", "Path to the directory containing the dynamically loaded kernel modules (.ko files)")
+	firmwareListPath = flag.String("firmware-file-list", "", "Path to a file containing a newline-separated list of paths to firmware files")
+	whenceFilePath   = flag.String("firmware-whence", "", "Path to the linux-firmware WHENCE file containing aliases for firmware files")
+	outMetaPath      = flag.String("out-meta", "", "Path where the resulting module metadata protobuf file should be created")
+	outFSSpecPath    = flag.String("out-fsspec", "", "Path where the resulting fsspec should be created")
+)
+
+func main() {
+	flag.Parse()
+	if *modinfoPath == "" || *modulesPath == "" || *firmwareListPath == "" ||
+		*whenceFilePath == "" || *outMetaPath == "" || *outFSSpecPath == "" {
+		log.Fatal("all flags are required and need to be provided")
+	}
+
+	allFirmwareData, err := os.ReadFile(*firmwareListPath)
+	if err != nil {
+		log.Fatalf("Failed to read firmware source list: %v", err)
+	}
+	allFirmwarePaths := strings.Split(string(allFirmwareData), "\n")
+
+	// Create a look-up table of all possible suffixes to their full paths as
+	// this is much faster at O(n) than calling strings.HasSuffix for every
+	// possible combination which is O(n^2).
+	// For example a build output at out/a/b/c.bin will be entered into
+	// the suffix LUT as build as out/a/b/c.bin, a/b/c.bin, b/c.bin and c.bin.
+	// If the firmware then requests b/c.bin, the output path is contained in
+	// the suffix LUT.
+	suffixLUT := make(map[string]string)
+	for _, firmwarePath := range allFirmwarePaths {
+		pathParts := strings.Split(firmwarePath, string(os.PathSeparator))
+		for i := range pathParts {
+			suffixLUT[path.Join(pathParts[i:]...)] = firmwarePath
+		}
+	}
+
+	// The linux-firmware repo contains a WHENCE file which contains (among
+	// other information) aliases for firmware which should be symlinked.
+	// Open this file and create a map of aliases in it.
+	linkMap := make(map[string]string)
+	metadata, err := os.ReadFile(*whenceFilePath)
+	if err != nil {
+		log.Fatalf("Failed to read metadata file: %v", err)
+	}
+	linksRaw := linkRegexp.FindAllStringSubmatch(string(metadata), -1)
+	for _, link := range linksRaw {
+		// For links we know the exact path referenced by kernel drives so
+		// a suffix LUT is unnecessary.
+		linkMap[link[1]] = link[2]
+	}
+
+	// Collect module metadata (modinfo) from both built-in modules via the
+	// kbuild-generated metadata file as well as from the loadable modules by
+	// walking them.
+	var files []*fsspec.File
+	var symlinks []*fsspec.SymbolicLink
+
+	mi, err := os.Open(*modinfoPath)
+	if err != nil {
+		log.Fatalf("While reading modinfo: %v", err)
+	}
+	modMeta, err := kmod.GetBuiltinModulesInfo(mi)
+	if err != nil {
+		log.Fatalf("Failed to read modules modinfo data: %v", err)
+	}
+
+	err = filepath.WalkDir(*modulesPath, func(p string, d fs.DirEntry, err error) error {
+		if err != nil {
+			log.Fatal(err)
+		}
+		if d.IsDir() {
+			return nil
+		}
+		mod, err := elf.Open(p)
+		if err != nil {
+			log.Fatal(err)
+		}
+		defer mod.Close()
+		out, err := kmod.GetModuleInfo(mod)
+		if err != nil {
+			log.Fatal(err)
+		}
+		relPath, err := filepath.Rel(*modulesPath, p)
+		if err != nil {
+			return err
+		}
+		// Add path information for MakeMetaFromModuleInfo.
+		out["path"] = []string{relPath}
+		modMeta = append(modMeta, out)
+		files = append(files, &fsspec.File{
+			Path:       path.Join("/lib/modules", relPath),
+			SourcePath: filepath.Join(*modulesPath, relPath),
+			Mode:       0555,
+		})
+		return nil
+	})
+	if err != nil {
+		log.Fatalf("Error walking modules: %v", err)
+	}
+
+	// Generate loading metadata from all known modules.
+	meta, err := kmod.MakeMetaFromModuleInfo(modMeta)
+	if err != nil {
+		log.Fatal(err)
+	}
+	metaRaw, err := proto.Marshal(meta)
+	if err != nil {
+		log.Fatal(err)
+	}
+	if err := os.WriteFile(*outMetaPath, metaRaw, 0640); err != nil {
+		log.Fatal(err)
+	}
+	files = append(files, &fsspec.File{
+		Path:       "/lib/modules/meta.pb",
+		SourcePath: *outMetaPath,
+		Mode:       0444,
+	})
+
+	// Create set of all firmware paths required by modules
+	fwset := make(map[string]bool)
+	for _, m := range modMeta {
+		if len(m["path"]) == 0 && len(m.Firmware()) > 0 {
+			log.Fatalf("Module %v is built-in, but requires firmware. Linux does not support this in all configurations.", m.Name())
+		}
+		for _, fw := range m.Firmware() {
+			fwset[fw] = true
+		}
+	}
+
+	// Convert set to list and sort for determinism
+	fwp := make([]string, 0, len(fwset))
+	for p := range fwset {
+		fwp = append(fwp, p)
+	}
+	sort.Strings(fwp)
+
+	// This function is called for every requested firmware file and adds and
+	// resolves symlinks until it finds the target file and adds that too.
+	populatedPaths := make(map[string]bool)
+	var chaseReference func(string)
+	chaseReference = func(p string) {
+		if populatedPaths[p] {
+			// Bail if path is already populated. Because of the DAG-like
+			// property of links in filesystems everything transitively pointed
+			// to by anything at this path has already been included.
+			return
+		}
+		placedPath := path.Join("/lib/firmware", p)
+		if linkTarget := linkMap[p]; linkTarget != "" {
+			symlinks = append(symlinks, &fsspec.SymbolicLink{
+				Path:       placedPath,
+				TargetPath: linkTarget,
+			})
+			populatedPaths[p] = true
+			// Symlinks are relative to their place, resolve them to be relative
+			// to the firmware root directory.
+			chaseReference(path.Join(path.Dir(p), linkTarget))
+			return
+		}
+		sourcePath := suffixLUT[p]
+		if sourcePath == "" {
+			// This should not be fatal as sometimes linux-firmware cannot
+			// ship all firmware usable by the kernel for mostly legal reasons.
+			log.Printf("WARNING: Requested firmware %q not found", p)
+			return
+		}
+		files = append(files, &fsspec.File{
+			Path:       path.Join("/lib/firmware", p),
+			Mode:       0444,
+			SourcePath: sourcePath,
+		})
+		populatedPaths[p] = true
+	}
+
+	for _, p := range fwp {
+		chaseReference(p)
+	}
+	// Format output in a both human- and machine-readable form
+	marshalOpts := prototext.MarshalOptions{Multiline: true, Indent: "  "}
+	fsspecRaw, err := marshalOpts.Marshal(&fsspec.FSSpec{File: files, SymbolicLink: symlinks})
+	if err != nil {
+		log.Fatalf("failed to marshal fsspec: %v", err)
+	}
+	if err := os.WriteFile(*outFSSpecPath, fsspecRaw, 0644); err != nil {
+		log.Fatalf("failed writing output: %v", err)
+	}
+}