helper: implementation of helper.Helper using bwrap

Signed-off-by: Ophestra Umiker <cat@ophivana.moe>

helper/bwrap.go

@@ -0,0 +1,144 @@
+package helper
+
+import (
+	"errors"
+	"io"
+	"os/exec"
+	"strconv"
+	"sync"
+
+	"git.ophivana.moe/cat/fortify/helper/bwrap"
+)
+
+// BubblewrapName is the file name or path to bubblewrap.
+var BubblewrapName = "bwrap"
+
+type bubblewrap struct {
+	// bwrap child file name
+	name string
+
+	// bwrap pipes
+	p *pipes
+	// sealed bwrap config
+	config *bwrap.Config
+	// returns an array of arguments passed directly
+	// to the child process spawned by bwrap
+	argF func(argsFD, statFD int) []string
+
+	// pipes received by the child
+	// nil if no pipes are required
+	cp *pipes
+
+	lock sync.RWMutex
+	*exec.Cmd
+}
+
+func (b *bubblewrap) StartNotify(ready chan error) error {
+	b.lock.Lock()
+	defer b.lock.Unlock()
+
+	if ready != nil && b.cp == nil {
+		panic("attempted to start with status monitoring on a bwrap child initialised without pipes")
+	}
+
+	// Check for doubled Start calls before we defer failure cleanup. If the prior
+	// call to Start succeeded, we don't want to spuriously close its pipes.
+	if b.Cmd.Process != nil {
+		return errors.New("exec: already started")
+	}
+
+	// prepare bwrap pipe and args
+	if argsFD, _, err := b.p.prepareCmd(b.Cmd); err != nil {
+		return err
+	} else {
+		b.Cmd.Args = append(b.Cmd.Args, "--args", strconv.Itoa(argsFD), "--", b.name)
+	}
+
+	// prepare child args and pipes if enabled
+	if b.cp != nil {
+		b.cp.ready = ready
+		if argsFD, statFD, err := b.cp.prepareCmd(b.Cmd); err != nil {
+			return err
+		} else {
+			b.Cmd.Args = append(b.Cmd.Args, b.argF(argsFD, statFD)...)
+		}
+	} else {
+		b.Cmd.Args = append(b.Cmd.Args, b.argF(-1, -1)...)
+	}
+
+	if ready != nil {
+		b.Cmd.Env = append(b.Cmd.Env, FortifyHelper+"=1", FortifyStatus+"=1")
+	} else if b.cp != nil {
+		b.Cmd.Env = append(b.Cmd.Env, FortifyHelper+"=1", FortifyStatus+"=0")
+	} else {
+		b.Cmd.Env = append(b.Cmd.Env, FortifyHelper+"=1", FortifyStatus+"=-1")
+	}
+
+	if err := b.Cmd.Start(); err != nil {
+		return err
+	}
+
+	// write bwrap args first
+	if err := b.p.readyWriteArgs(); err != nil {
+		return err
+	}
+
+	// write child args if enabled
+	if b.cp != nil {
+		if err := b.cp.readyWriteArgs(); err != nil {
+			return err
+		}
+	}
+
+	return nil
+}
+
+func (b *bubblewrap) Close() error {
+	if b.cp == nil {
+		panic("attempted to close bwrap child initialised without pipes")
+	}
+
+	return b.cp.closeStatus()
+}
+
+func (b *bubblewrap) Start() error {
+	return b.StartNotify(nil)
+}
+
+func (b *bubblewrap) Unwrap() *exec.Cmd {
+	return b.Cmd
+}
+
+// MustNewBwrap initialises a new Bwrap instance with wt as the null-terminated argument writer.
+// If wt is nil, the child process spawned by bwrap will not get an argument pipe.
+// Function argF returns an array of arguments passed directly to the child process.
+func MustNewBwrap(conf *bwrap.Config, wt io.WriterTo, name string, argF func(argsFD, statFD int) []string) Helper {
+	b, err := NewBwrap(conf, wt, name, argF)
+	if err != nil {
+		panic(err.Error())
+	} else {
+		return b
+	}
+}
+
+// NewBwrap initialises a new Bwrap instance with wt as the null-terminated argument writer.
+// If wt is nil, the child process spawned by bwrap will not get an argument pipe.
+// Function argF returns an array of arguments passed directly to the child process.
+func NewBwrap(conf *bwrap.Config, wt io.WriterTo, name string, argF func(argsFD, statFD int) []string) (Helper, error) {
+	b := new(bubblewrap)
+
+	if args, err := NewCheckedArgs(conf.Args()); err != nil {
+		return nil, err
+	} else {
+		b.p = &pipes{args: args}
+	}
+
+	b.argF = argF
+	b.name = name
+	if wt != nil {
+		b.cp = &pipes{args: wt}
+	}
+	b.Cmd = execCommand(BubblewrapName)
+
+	return b, nil
+}

helper/bwrap_test.go

@@ -0,0 +1,112 @@
+package helper_test
+
+import (
+	"errors"
+	"fmt"
+	"os"
+	"strings"
+	"testing"
+
+	"git.ophivana.moe/cat/fortify/helper"
+	"git.ophivana.moe/cat/fortify/helper/bwrap"
+)
+
+func TestBwrap(t *testing.T) {
+	sc := &bwrap.Config{
+		Unshare:       nil,
+		Net:           true,
+		UserNS:        false,
+		Hostname:      "localhost",
+		Chdir:         "/nonexistent",
+		Clearenv:      true,
+		NewSession:    true,
+		DieWithParent: true,
+		AsInit:        true,
+	}
+
+	t.Run("nonexistent bwrap name", func(t *testing.T) {
+		bubblewrapName := helper.BubblewrapName
+		helper.BubblewrapName = "/nonexistent"
+		t.Cleanup(func() {
+			helper.BubblewrapName = bubblewrapName
+		})
+
+		h := helper.MustNewBwrap(sc, argsWt, "fortify", argF)
+
+		if err := h.Start(); !errors.Is(err, os.ErrNotExist) {
+			t.Errorf("Start() error = %v, wantErr %v",
+				err, os.ErrNotExist)
+		}
+	})
+
+	t.Run("valid new helper nil check", func(t *testing.T) {
+		if got := helper.MustNewBwrap(sc, argsWt, "fortify", argF); got == nil {
+			t.Errorf("MustNewBwrap(%#v, %#v, %#v) got nil",
+				sc, argsWt, "fortify")
+			return
+		}
+	})
+
+	t.Run("invalid bwrap config new helper panic", func(t *testing.T) {
+		defer func() {
+			wantPanic := "argument contains null character"
+			if r := recover(); r != wantPanic {
+				t.Errorf("MustNewBwrap: panic = %q, want %q",
+					r, wantPanic)
+			}
+		}()
+
+		helper.MustNewBwrap(&bwrap.Config{Hostname: "\x00"}, nil, "fortify", argF)
+	})
+
+	t.Run("start notify without pipes panic", func(t *testing.T) {
+		defer func() {
+			wantPanic := "attempted to start with status monitoring on a bwrap child initialised without pipes"
+			if r := recover(); r != wantPanic {
+				t.Errorf("StartNotify: panic = %q, want %q",
+					r, wantPanic)
+			}
+		}()
+
+		panic(fmt.Sprintf("unreachable: %v",
+			helper.MustNewBwrap(sc, nil, "fortify", argF).StartNotify(make(chan error))))
+	})
+
+	t.Run("start without pipes", func(t *testing.T) {
+		helper.InternalReplaceExecCommand(t)
+
+		h := helper.MustNewBwrap(sc, nil, "crash-test-dummy", argFChecked)
+		cmd := h.Unwrap()
+
+		stdout, stderr := new(strings.Builder), new(strings.Builder)
+		cmd.Stdout, cmd.Stderr = stdout, stderr
+
+		t.Run("close without pipes panic", func(t *testing.T) {
+			defer func() {
+				wantPanic := "attempted to close bwrap child initialised without pipes"
+				if r := recover(); r != wantPanic {
+					t.Errorf("Close: panic = %q, want %q",
+						r, wantPanic)
+				}
+			}()
+
+			panic(fmt.Sprintf("unreachable: %v",
+				h.Close()))
+		})
+
+		if err := h.Start(); err != nil {
+			t.Errorf("Start() error = %v",
+				err)
+			return
+		}
+
+		if err := h.Wait(); err != nil {
+			t.Errorf("Wait() err = %v stderr = %s",
+				err, stderr)
+		}
+	})
+
+	t.Run("implementation compliance", func(t *testing.T) {
+		testHelper(t, func() helper.Helper { return helper.MustNewBwrap(sc, argsWt, "crash-test-dummy", argF) })
+	})
+}

helper/helper_test.go

@@ -23,11 +23,15 @@ var (
 	argsWt      = helper.MustNewCheckedArgs(wantArgs)
 )
 
-func argF(argsFD int, statFD int) []string {
+func argF(argsFD, statFD int) []string {
 	if argsFD == -1 {
 		panic("invalid args fd")
 	}
 
+	return argFChecked(argsFD, statFD)
+}
+
+func argFChecked(argsFD, statFD int) []string {
 	if statFD == -1 {
 		return []string{"--args", strconv.Itoa(argsFD)}
 	} else {

helper/stub.go

@@ -6,15 +6,19 @@ import (
 	"os"
 	"os/exec"
 	"strconv"
+	"strings"
 	"syscall"
 	"testing"
+
+	"git.ophivana.moe/cat/fortify/helper/bwrap"
 )
 
 // InternalChildStub is an internal function but exported because it is cross-package;
 // it is part of the implementation of the helper stub.
 func InternalChildStub() {
 	// this test mocks the helper process
-	if os.Getenv(FortifyHelper) != "1" {
+	if os.Getenv(FortifyHelper) != "1" ||
+		os.Getenv(FortifyStatus) == "-1" { // this indicates the stub is being invoked as a bwrap child without pipes
 		return
 	}
 
@@ -22,6 +26,35 @@ func InternalChildStub() {
 	statFD := flag.Int("fd", -1, "")
 	_ = flag.CommandLine.Parse(os.Args[4:])
 
+	switch os.Args[3] {
+	case "bwrap":
+		bwrapStub(argsFD, statFD)
+	default:
+		genericStub(argsFD, statFD)
+	}
+
+	os.Exit(0)
+}
+
+// InternalReplaceExecCommand is an internal function but exported because it is cross-package;
+// it is part of the implementation of the helper stub.
+func InternalReplaceExecCommand(t *testing.T) {
+	t.Cleanup(func() {
+		execCommand = exec.Command
+	})
+
+	// replace execCommand to have the resulting *exec.Cmd launch TestHelperChildStub
+	execCommand = func(name string, arg ...string) *exec.Cmd {
+		// pass through nonexistent path
+		if name == "/nonexistent" && len(arg) == 0 {
+			return exec.Command(name)
+		}
+
+		return exec.Command(os.Args[0], append([]string{"-test.run=TestHelperChildStub", "--", name}, arg...)...)
+	}
+}
+
+func genericStub(argsFD, statFD *int) {
 	// simulate args pipe behaviour
 	func() {
 		if *argsFD == -1 {
@@ -89,20 +122,53 @@ func InternalChildStub() {
 	}
 }
 
-// InternalReplaceExecCommand is an internal function but exported because it is cross-package;
+func bwrapStub(argsFD, statFD *int) {
-// it is part of the implementation of the helper stub.
+	// the bwrap launcher does not ever launch with sync fd
-func InternalReplaceExecCommand(t *testing.T) {
+	if *statFD != -1 {
-	t.Cleanup(func() {
+		panic("attempted to launch bwrap with status monitoring")
-		execCommand = exec.Command
-	})
-
-	// replace execCommand to have the resulting *exec.Cmd launch TestHelperChildStub
-	execCommand = func(name string, arg ...string) *exec.Cmd {
-		// pass through nonexistent path
-		if name == "/nonexistent" && len(arg) == 0 {
-			return exec.Command(name)
 	}
 
-		return exec.Command(os.Args[0], append([]string{"-test.run=TestHelperChildStub", "--", name}, arg...)...)
+	// test args pipe behaviour
+	func() {
+		if *argsFD == -1 {
+			panic("attempted to start bwrap without passing args pipe fd")
+		}
+
+		f := os.NewFile(uintptr(*argsFD), "|0")
+		if f == nil {
+			panic("attempted to start helper without args pipe")
+		}
+
+		got, want := new(strings.Builder), new(strings.Builder)
+
+		if _, err := io.Copy(got, f); err != nil {
+			panic("cannot read args: " + err.Error())
+		}
+
+		// hardcoded bwrap configuration used by test
+		if _, err := MustNewCheckedArgs((&bwrap.Config{
+			Unshare:       nil,
+			Net:           true,
+			UserNS:        false,
+			Hostname:      "localhost",
+			Chdir:         "/nonexistent",
+			Clearenv:      true,
+			NewSession:    true,
+			DieWithParent: true,
+			AsInit:        true,
+		}).Args()).WriteTo(want); err != nil {
+			panic("cannot read want: " + err.Error())
+		}
+
+		if got.String() != want.String() {
+			panic("bad bwrap args\ngot: " + got.String() + "\nwant: " + want.String())
+		}
+	}()
+
+	if err := syscall.Exec(
+		os.Args[0],
+		append([]string{os.Args[0], "-test.run=TestHelperChildStub", "--"}, flag.CommandLine.Args()...),
+		os.Environ()); err != nil {
+		panic("cannot start general stub: " + err.Error())
 	}
 }