diff --git a/user/go.mod b/user/go.mod
index ad6231c..c494b18 100644
--- a/user/go.mod
+++ b/user/go.mod
@@ -1,6 +1,6 @@
 module github.com/moby/sys/user
 
-go 1.18
+go 1.24
 
 require golang.org/x/sys v0.1.0
 
diff --git a/user/idtools.go b/user/idtools.go
index 595b7a9..829e504 100644
--- a/user/idtools.go
+++ b/user/idtools.go
@@ -3,6 +3,21 @@ package user
 import (
 	"fmt"
 	"os"
+	"path/filepath"
+)
+
+// FS is the filesystem contract used by the Mkdir*AndChownFS helpers.
+type FS interface {
+	Stat(name string) (os.FileInfo, error)
+	Mkdir(name string, perm os.FileMode) error
+	MkdirAll(name string, perm os.FileMode) error
+	Chmod(name string, mode os.FileMode) error
+	Chown(name string, uid, gid int) error
+}
+
+var (
+	_ FS = &os.Root{}
+	_ FS = &hostFS{}
 )
 
 // MkdirOpt is a type for options to pass to Mkdir calls
@@ -23,12 +38,24 @@ func WithOnlyNew(o *mkdirOptions) {
 // function will still change ownership and permissions. If WithOnlyNew is passed as an
 // option, then only the newly created directories will have ownership and permissions changed.
 func MkdirAllAndChown(path string, mode os.FileMode, uid, gid int, opts ...MkdirOpt) error {
-	var options mkdirOptions
-	for _, opt := range opts {
-		opt(&options)
+	return MkdirAllAndChownFS(nil, path, mode, uid, gid, opts...)
+}
+
+// MkdirAllAndChownFS creates a directory (including any along the path) on the
+// provided filesystem and then modifies ownership to the requested uid/gid. If
+// fsys is nil, the host filesystem is used.
+func MkdirAllAndChownFS(fsys FS, path string, mode os.FileMode, uid, gid int, opts ...MkdirOpt) error {
+	if fsys == nil {
+		absPath, err := filepath.Abs(path)
+		if err != nil {
+			return err
+		}
+		fsys = &hostFS{}
+		path = absPath
 	}
 
-	return mkdirAs(path, mode, uid, gid, true, options.onlyNew)
+	options := mkdirOpts(opts)
+	return mkdirAs(fsys, path, mode, uid, gid, true, options.onlyNew)
 }
 
 // MkdirAndChown creates a directory and then modifies ownership to the requested uid/gid.
@@ -38,11 +65,32 @@ func MkdirAllAndChown(path string, mode os.FileMode, uid, gid int, opts ...Mkdir
 // Note that unlike os.Mkdir(), this function does not return IsExist error
 // in case path already exists.
 func MkdirAndChown(path string, mode os.FileMode, uid, gid int, opts ...MkdirOpt) error {
+	return MkdirAndChownFS(nil, path, mode, uid, gid, opts...)
+}
+
+// MkdirAndChownFS creates a directory on the provided filesystem and then
+// modifies ownership to the requested uid/gid. If fsys is nil, the host
+// filesystem is used.
+func MkdirAndChownFS(fsys FS, path string, mode os.FileMode, uid, gid int, opts ...MkdirOpt) error {
+	if fsys == nil {
+		absPath, err := filepath.Abs(path)
+		if err != nil {
+			return err
+		}
+		fsys = &hostFS{}
+		path = absPath
+	}
+
+	options := mkdirOpts(opts)
+	return mkdirAs(fsys, path, mode, uid, gid, false, options.onlyNew)
+}
+
+func mkdirOpts(opts []MkdirOpt) mkdirOptions {
 	var options mkdirOptions
 	for _, opt := range opts {
 		opt(&options)
 	}
-	return mkdirAs(path, mode, uid, gid, false, options.onlyNew)
+	return options
 }
 
 // getRootUIDGID retrieves the remapped root uid/gid pair from the set of maps.
@@ -139,3 +187,25 @@ func (i IdentityMapping) ToContainer(uid, gid int) (int, int, error) {
 func (i IdentityMapping) Empty() bool {
 	return len(i.UIDMaps) == 0 && len(i.GIDMaps) == 0
 }
+
+type hostFS struct{}
+
+func (*hostFS) Stat(name string) (os.FileInfo, error) {
+	return os.Stat(name)
+}
+
+func (*hostFS) Mkdir(name string, perm os.FileMode) error {
+	return os.Mkdir(name, perm)
+}
+
+func (*hostFS) MkdirAll(name string, perm os.FileMode) error {
+	return os.MkdirAll(name, perm)
+}
+
+func (*hostFS) Chmod(name string, mode os.FileMode) error {
+	return os.Chmod(name, mode)
+}
+
+func (*hostFS) Chown(name string, uid, gid int) error {
+	return os.Chown(name, uid, gid)
+}
diff --git a/user/idtools_unix.go b/user/idtools_unix.go
index 4e39d24..ea1dcbd 100644
--- a/user/idtools_unix.go
+++ b/user/idtools_unix.go
@@ -4,19 +4,15 @@ package user
 
 import (
 	"fmt"
+	"io/fs"
 	"os"
 	"path/filepath"
 	"strconv"
 	"syscall"
 )
 
-func mkdirAs(path string, mode os.FileMode, uid, gid int, mkAll, onlyNew bool) error {
-	path, err := filepath.Abs(path)
-	if err != nil {
-		return err
-	}
-
-	stat, err := os.Stat(path)
+func mkdirAs(fsys FS, path string, mode os.FileMode, uid, gid int, mkAll, onlyNew bool) error {
+	stat, err := fsys.Stat(path)
 	if err == nil {
 		if !stat.IsDir() {
 			return &os.PathError{Op: "mkdir", Path: path, Err: syscall.ENOTDIR}
@@ -26,7 +22,7 @@ func mkdirAs(path string, mode os.FileMode, uid, gid int, mkAll, onlyNew bool) e
 		}
 
 		// short-circuit -- we were called with an existing directory and chown was requested
-		return setPermissions(path, mode, uid, gid, stat)
+		return setPermissions(fsys, path, mode, uid, gid, stat)
 	}
 
 	// make an array containing the original path asked for, plus (for mkAll == true)
@@ -44,23 +40,23 @@ func mkdirAs(path string, mode os.FileMode, uid, gid int, mkAll, onlyNew bool) e
 		dirPath := path
 		for {
 			dirPath = filepath.Dir(dirPath)
-			if dirPath == "/" {
+			if dirPath == string(filepath.Separator) || dirPath == "." {
 				break
 			}
-			if _, err = os.Stat(dirPath); os.IsNotExist(err) {
+			if _, err = fsys.Stat(dirPath); os.IsNotExist(err) {
 				paths = append(paths, dirPath)
 			}
 		}
-		if err = os.MkdirAll(path, mode); err != nil {
+		if err = fsys.MkdirAll(path, mode); err != nil {
 			return err
 		}
-	} else if err = os.Mkdir(path, mode); err != nil {
+	} else if err = fsys.Mkdir(path, mode); err != nil {
 		return err
 	}
 	// even if it existed, we will chown the requested path + any subpaths that
 	// didn't exist when we called MkdirAll
 	for _, pathComponent := range paths {
-		if err = setPermissions(pathComponent, mode, uid, gid, nil); err != nil {
+		if err = setPermissions(fsys, pathComponent, mode, uid, gid, nil); err != nil {
 			return err
 		}
 	}
@@ -71,16 +67,16 @@ func mkdirAs(path string, mode os.FileMode, uid, gid int, mkAll, onlyNew bool) e
 // Normally a Chown is a no-op if uid/gid match, but in some cases this can still cause an error, e.g. if the
 // dir is on an NFS share, so don't call chown unless we absolutely must.
 // Likewise for setting permissions.
-func setPermissions(p string, mode os.FileMode, uid, gid int, stat os.FileInfo) error {
+func setPermissions(fsys FS, p string, mode os.FileMode, uid, gid int, stat fs.FileInfo) error {
 	if stat == nil {
 		var err error
-		stat, err = os.Stat(p)
+		stat, err = fsys.Stat(p)
 		if err != nil {
 			return err
 		}
 	}
 	if stat.Mode().Perm() != mode.Perm() {
-		if err := os.Chmod(p, mode.Perm()); err != nil {
+		if err := fsys.Chmod(p, mode.Perm()); err != nil {
 			return err
 		}
 	}
@@ -88,7 +84,7 @@ func setPermissions(p string, mode os.FileMode, uid, gid int, stat os.FileInfo)
 	if ssi.Uid == uint32(uid) && ssi.Gid == uint32(gid) {
 		return nil
 	}
-	return os.Chown(p, uid, gid)
+	return fsys.Chown(p, uid, gid)
 }
 
 // LoadIdentityMapping takes a requested username and
diff --git a/user/idtools_unix_test.go b/user/idtools_unix_test.go
index db7fc42..603ed42 100644
--- a/user/idtools_unix_test.go
+++ b/user/idtools_unix_test.go
@@ -5,6 +5,7 @@ package user
 import (
 	"errors"
 	"fmt"
+	"maps"
 	"os"
 	"path/filepath"
 	"testing"
@@ -292,9 +293,7 @@ func readTree(base, root string) (map[string]node, error) {
 			if err != nil {
 				return nil, err
 			}
-			for path, nodeinfo := range subtree {
-				tree[path] = nodeinfo
-			}
+			maps.Copy(tree, subtree)
 		}
 	}
 	return tree, nil
@@ -385,12 +384,59 @@ func TestMkdirIsNotDir(t *testing.T) {
 		t.Fatalf("Couldn't create temp dir: %v", err)
 	}
 
-	err = mkdirAs(file.Name(), 0o755, 0, 0, false, false)
+	fsys := FS(&hostFS{})
+
+	err = mkdirAs(fsys, file.Name(), 0o755, 0, 0, false, false)
 	if expected := "mkdir " + file.Name() + ": not a directory"; err.Error() != expected {
 		t.Fatalf("expected error: %v, got: %v", expected, err)
 	}
 }
 
+func TestMkdirAllAndChownFSNilUsesHostFS(t *testing.T) {
+	requiresRoot(t)
+
+	baseDir := t.TempDir()
+	path := filepath.Join(baseDir, "usr", "share")
+
+	if err := MkdirAllAndChownFS(nil, path, 0o755, 99, 99); err != nil {
+		t.Fatal(err)
+	}
+
+	s := &unix.Stat_t{}
+	if err := unix.Stat(path, s); err != nil {
+		t.Fatal(err)
+	}
+	if s.Uid != 99 || s.Gid != 99 {
+		t.Fatalf("expected ownership 99:99, got %d:%d", s.Uid, s.Gid)
+	}
+}
+
+func TestMkdirAllAndChownFSWithRoot(t *testing.T) {
+	requiresRoot(t)
+
+	baseDir := t.TempDir()
+	root, err := os.OpenRoot(baseDir)
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer root.Close()
+
+	if err := MkdirAllAndChownFS(root, "usr/share", 0o755, 123, 124); err != nil {
+		t.Fatal(err)
+	}
+
+	verifyTree, err := readTree(baseDir, "")
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	expected := map[string]node{
+		"usr":       {123, 124},
+		"usr/share": {123, 124},
+	}
+	compareTrees(t, expected, verifyTree)
+}
+
 func requiresRoot(t *testing.T) {
 	if os.Getuid() != 0 {
 		t.Skip("skipping test that requires root")
diff --git a/user/idtools_windows.go b/user/idtools_windows.go
index 9de730c..c8a7cca 100644
--- a/user/idtools_windows.go
+++ b/user/idtools_windows.go
@@ -8,6 +8,6 @@ import (
 // permissions aren't set through this path, the identity isn't utilized.
 // Ownership is handled elsewhere, but in the future could be support here
 // too.
-func mkdirAs(path string, _ os.FileMode, _, _ int, _, _ bool) error {
-	return os.MkdirAll(path, 0)
+func mkdirAs(fsys FS, path string, _ os.FileMode, _, _ int, _, _ bool) error {
+	return fsys.MkdirAll(path, 0)
 }
diff --git a/user/user.go b/user/user.go
index e2788a1..1b7e999 100644
--- a/user/user.go
+++ b/user/user.go
@@ -7,6 +7,7 @@ import (
 	"fmt"
 	"io"
 	"os"
+	"slices"
 	"strconv"
 	"strings"
 )
@@ -373,12 +374,7 @@ func GetExecUser(userSpec string, defaults *ExecUser, passwd, group io.Reader) (
 			// If the group argument isn't explicit, we'll just search for it.
 			if groupArg == "" {
 				// Check if user is a member of this group.
-				for _, u := range g.List {
-					if u == matchedUserName {
-						return true
-					}
-				}
-				return false
+				return slices.Contains(g.List, matchedUserName)
 			}
 
 			if gidErr == nil {