src/internal/fuzz/worker.go

// Copyright 2020 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

package fuzz

import (
	"encoding/json"
	"errors"
	"fmt"
	"io"
	"io/ioutil"
	"os"
	"os/exec"
	"runtime"
	"sync"
	"time"
)

const (
	// workerFuzzDuration is the amount of time a worker can spend testing random
	// variations of an input given by the coordinator.
	workerFuzzDuration = 100 * time.Millisecond

	// workerTimeoutDuration is the amount of time a worker can go without
	// responding to the coordinator before being stopped.
	workerTimeoutDuration = 1 * time.Second
)

// worker manages a worker process running a test binary. The worker object
// exists only in the coordinator (the process started by 'go test -fuzz').
// workerClient is used by the coordinator to send RPCs to the worker process,
// which handles them with workerServer.
type worker struct {
	dir     string   // working directory, same as package directory
	binPath string   // path to test executable
	args    []string // arguments for test executable
	env     []string // environment for test executable

	coordinator *coordinator

	mem *sharedMem // shared memory with worker; persists across processes.

	cmd     *exec.Cmd     // current worker process
	client  *workerClient // used to communicate with worker process
	waitErr error         // last error returned by wait, set before termC is closed.
	termC   chan struct{} // closed by wait when worker process terminates
}

// cleanup releases persistent resources associated with the worker.
func (w *worker) cleanup() error {
	if w.mem == nil {
		return nil
	}
	err := w.mem.Close()
	w.mem = nil
	return err
}

// runFuzzing runs the test binary to perform fuzzing.
//
// This function loops until w.coordinator.doneC is closed or some
// fatal error is encountered. It receives inputs from w.coordinator.inputC,
// then passes those on to the worker process.
func (w *worker) runFuzzing() error {
	// Start the process.
	if err := w.start(); err != nil {
		// We couldn't start the worker process. We can't do anything, and it's
		// likely that other workers can't either, so give up.
		close(w.coordinator.doneC)
		return err
	}

	inputC := w.coordinator.inputC // set to nil when processing input
	fuzzC := make(chan struct{})   // sent when we finish processing an input.

	// Main event loop.
	for {
		select {
		case <-w.coordinator.doneC:
			// All workers were told to stop.
			return w.stop()

		case <-w.termC:
			// Worker process terminated unexpectedly, so inform the coordinator
			// that a crash occurred.
			b := w.mem.value() // These are the bytes that caused the crash.
			resB := make([]byte, len(b))
			copy(resB, b)
			resp := fuzzResponse{Value: resB, Err: "fuzzing process crashed unexpectedly"}
			w.coordinator.interestingC <- resp

			// TODO(jayconrod,katiehockman): if -keepfuzzing, restart worker.
			err := w.stop()
			if err == nil {
				err = fmt.Errorf("worker exited unexpectedly")
			}
			return err

		case input := <-inputC:
			// Received input from coordinator.
			inputC = nil // block new inputs until we finish with this one.
			go func() {
				args := fuzzArgs{Duration: workerFuzzDuration}
				resp, err := w.client.fuzz(input.b, args)
				if err != nil {
					// TODO(jayconrod): if we get an error here, something failed between
					// main and the call to testing.F.Fuzz. The error here won't
					// be useful. Collect stderr, clean it up, and return that.
					// TODO(jayconrod): we can get EPIPE if w.stop is called concurrently
					// and it kills the worker process. Suppress this message in
					// that case.
					// TODO(jayconrod): what happens if testing.F.Fuzz is never called?
					// TODO(jayconrod): time out if the test process hangs.
					fmt.Fprintf(os.Stderr, "communicating with worker: %v\n", err)
				} else {
					// TODO(jayconrod, katiehockman): Right now, this will just
					// send an empty fuzzResponse{} if nothing interesting came
					// up. Probably want to only pass to interestingC if fuzzing
					// found something interesting.

					// Inform the coordinator that fuzzing found something
					// interesting (ie. a crash or new coverage).
					w.coordinator.interestingC <- resp

					if resp.Err == "" {
						// Only unblock to allow more fuzzing to occur if
						// everything was successful with the last fuzzing
						// attempt.
						fuzzC <- struct{}{}
					}
				}
				// TODO(jayconrod,katiehockman): gather statistics.
			}()

		case <-fuzzC:
			// Worker finished fuzzing and nothing new happened.
			inputC = w.coordinator.inputC // unblock new inputs
		}
	}
}

// start runs a new worker process.
//
// If the process couldn't be started, start returns an error. Start won't
// return later termination errors from the process if they occur.
//
// If the process starts successfully, start returns nil. stop must be called
// once later to clean up, even if the process terminates on its own.
//
// When the process terminates, w.waitErr is set to the error (if any), and
// w.termC is closed.
func (w *worker) start() (err error) {
	if w.cmd != nil {
		panic("worker already started")
	}
	w.waitErr = nil
	w.termC = nil

	cmd := exec.Command(w.binPath, w.args...)
	cmd.Dir = w.dir
	cmd.Env = w.env
	// TODO(jayconrod): set stdout and stderr to nil or buffer. A large number
	// of workers may be very noisy, but for now, this output is useful for
	// debugging.
	cmd.Stdout = os.Stdout
	cmd.Stderr = os.Stderr

	// TODO(jayconrod): set up shared memory between the coordinator and worker to
	// transfer values and coverage data. If the worker crashes, we need to be
	// able to find the value that caused the crash.

	// Create the "fuzz_in" and "fuzz_out" pipes so we can communicate with
	// the worker. We don't use stdin and stdout, since the test binary may
	// do something else with those.
	//
	// Each pipe has a reader and a writer. The coordinator writes to fuzzInW
	// and reads from fuzzOutR. The worker inherits fuzzInR and fuzzOutW.
	// The coordinator closes fuzzInR and fuzzOutW after starting the worker,
	// since we have no further need of them.
	fuzzInR, fuzzInW, err := os.Pipe()
	if err != nil {
		return err
	}
	defer fuzzInR.Close()
	fuzzOutR, fuzzOutW, err := os.Pipe()
	if err != nil {
		fuzzInW.Close()
		return err
	}
	defer fuzzOutW.Close()
	setWorkerComm(cmd, workerComm{fuzzIn: fuzzInR, fuzzOut: fuzzOutW, mem: w.mem})

	// Start the worker process.
	if err := cmd.Start(); err != nil {
		fuzzInW.Close()
		fuzzOutR.Close()
		return err
	}

	// Worker started successfully.
	// After this, w.client owns fuzzInW and fuzzOutR, so w.client.Close must be
	// called later by stop.
	w.cmd = cmd
	w.termC = make(chan struct{})
	w.client = newWorkerClient(workerComm{fuzzIn: fuzzInW, fuzzOut: fuzzOutR, mem: w.mem})

	go func() {
		w.waitErr = w.cmd.Wait()
		close(w.termC)
	}()

	return nil
}

// stop tells the worker process to exit by closing w.client, then blocks until
// it terminates. If the worker doesn't terminate after a short time, stop
// signals it with os.Interrupt (where supported), then os.Kill.
//
// stop returns the error the process terminated with, if any (same as
// w.waitErr).
//
// stop must be called once after start returns successfully, even if the
// worker process terminates unexpectedly.
func (w *worker) stop() error {
	if w.termC == nil {
		panic("worker was not started successfully")
	}
	select {
	case <-w.termC:
		// Worker already terminated, perhaps unexpectedly.
		if w.client == nil {
			panic("worker already stopped")
		}
		w.client.Close()
		w.cmd = nil
		w.client = nil
		return w.waitErr
	default:
		// Worker still running.
	}

	// Tell the worker to stop by closing fuzz_in. It won't actually stop until it
	// finishes with earlier calls.
	closeC := make(chan struct{})
	go func() {
		w.client.Close()
		close(closeC)
	}()

	sig := os.Interrupt
	if runtime.GOOS == "windows" {
		// Per https://golang.org/pkg/os/#Signal, “Interrupt is not implemented on
		// Windows; using it with os.Process.Signal will return an error.”
		// Fall back to Kill instead.
		sig = os.Kill
	}

	t := time.NewTimer(workerTimeoutDuration)
	for {
		select {
		case <-w.termC:
			// Worker terminated.
			t.Stop()
			<-closeC
			w.cmd = nil
			w.client = nil
			return w.waitErr

		case <-t.C:
			// Timer fired before worker terminated.
			switch sig {
			case os.Interrupt:
				// Try to stop the worker with SIGINT and wait a little longer.
				w.cmd.Process.Signal(sig)
				sig = os.Kill
				t.Reset(workerTimeoutDuration)

			case os.Kill:
				// Try to stop the worker with SIGKILL and keep waiting.
				w.cmd.Process.Signal(sig)
				sig = nil
				t.Reset(workerTimeoutDuration)

			case nil:
				// Still waiting. Print a message to let the user know why.
				fmt.Fprintf(os.Stderr, "go: waiting for fuzz worker to terminate...\n")
			}
		}
	}
}

// RunFuzzWorker is called in a worker process to communicate with the
// coordinator process in order to fuzz random inputs. RunFuzzWorker loops
// until the coordinator tells it to stop.
//
// fn is a wrapper on the fuzz function. It may return an error to indicate
// a given input "crashed". The coordinator will also record a crasher if
// the function times out or terminates the process.
//
// RunFuzzWorker returns an error if it could not communicate with the
// coordinator process.
func RunFuzzWorker(fn func([]byte) error) error {
	comm, err := getWorkerComm()
	if err != nil {
		return err
	}
	srv := &workerServer{workerComm: comm, fuzzFn: fn}
	return srv.serve()
}

// call is serialized and sent from the coordinator on fuzz_in. It acts as
// a minimalist RPC mechanism. Exactly one of its fields must be set to indicate
// which method to call.
type call struct {
	Fuzz *fuzzArgs
}

type fuzzArgs struct {
	Duration time.Duration
}

type fuzzResponse struct {
	Value []byte // The bytes that yielded the response.
	Err   string // The error if the bytes resulted in a crash, nil otherwise.
}

// workerComm holds pipes and shared memory used for communication
// between the coordinator process (client) and a worker process (server).
type workerComm struct {
	fuzzIn, fuzzOut *os.File
	mem             *sharedMem
}

// workerServer is a minimalist RPC server, run by fuzz worker processes.
// It allows the coordinator process (using workerClient) to call methods in a
// worker process. This system allows the coordinator to run multiple worker
// processes in parallel and to collect inputs that caused crashes from shared
// memory after a worker process terminates unexpectedly.
type workerServer struct {
	workerComm

	// fuzzFn runs the worker's fuzz function on the given input and returns
	// an error if it finds a crasher (the process may also exit or crash).
	fuzzFn func([]byte) error
}

// serve reads serialized RPC messages on fuzzIn. When serve receives a message,
// it calls the corresponding method, then sends the serialized result back
// on fuzzOut.
//
// serve handles RPC calls synchronously; it will not attempt to read a message
// until the previous call has finished.
//
// serve returns errors that occurred when communicating over pipes. serve
// does not return errors from method calls; those are passed through serialized
// responses.
func (ws *workerServer) serve() error {
	enc := json.NewEncoder(ws.fuzzOut)
	dec := json.NewDecoder(ws.fuzzIn)
	for {
		var c call
		if err := dec.Decode(&c); err == io.EOF {
			return nil
		} else if err != nil {
			return err
		}

		var resp interface{}
		switch {
		case c.Fuzz != nil:
			value := ws.mem.value()
			resp = ws.fuzz(value, *c.Fuzz)
		default:
			return errors.New("no arguments provided for any call")
		}

		if err := enc.Encode(resp); err != nil {
			return err
		}
	}
}

// fuzz runs the test function on random variations of a given input value for
// a given amount of time. fuzz returns early if it finds an input that crashes
// the fuzz function or an input that expands coverage.
func (ws *workerServer) fuzz(value []byte, args fuzzArgs) fuzzResponse {
	t := time.NewTimer(args.Duration)
	for {
		select {
		case <-t.C:
			return fuzzResponse{}
		default:
			b := mutate(value)
			ws.mem.setValue(b) // Write the value to memory so it can be recovered it if the process dies
			if err := ws.fuzzFn(b); err != nil {
				return fuzzResponse{Value: b, Err: err.Error()}
			}
			// TODO(jayconrod,katiehockman): return early if coverage is expanded
			// by returning a fuzzResponse with the Value set but a nil Err.
		}
	}
}

// workerClient is a minimalist RPC client. The coordinator process uses a
// workerClient to call methods in each worker process (handled by
// workerServer).
type workerClient struct {
	workerComm

	mu  sync.Mutex
	enc *json.Encoder
	dec *json.Decoder
}

func newWorkerClient(comm workerComm) *workerClient {
	return &workerClient{
		workerComm: comm,
		enc:        json.NewEncoder(comm.fuzzIn),
		dec:        json.NewDecoder(comm.fuzzOut),
	}
}

// Close shuts down the connection to the RPC server (the worker process) by
// closing fuzz_in. Close drains fuzz_out (avoiding a SIGPIPE in the worker),
// and closes it after the worker process closes the other end.
func (wc *workerClient) Close() error {
	wc.mu.Lock()
	defer wc.mu.Unlock()

	// Close fuzzIn. This signals to the server that there are no more calls,
	// and it should exit.
	if err := wc.fuzzIn.Close(); err != nil {
		wc.fuzzOut.Close()
		return err
	}

	// Drain fuzzOut and close it. When the server exits, the kernel will close
	// its end of fuzzOut, and we'll get EOF.
	if _, err := io.Copy(ioutil.Discard, wc.fuzzOut); err != nil {
		wc.fuzzOut.Close()
		return err
	}
	return wc.fuzzOut.Close()
}

// fuzz tells the worker to call the fuzz method. See workerServer.fuzz.
func (wc *workerClient) fuzz(value []byte, args fuzzArgs) (fuzzResponse, error) {
	wc.mu.Lock()
	defer wc.mu.Unlock()

	wc.mem.setValue(value)
	c := call{Fuzz: &args}
	if err := wc.enc.Encode(c); err != nil {
		return fuzzResponse{}, err
	}
	var resp fuzzResponse
	if err := wc.dec.Decode(&resp); err != nil {
		return fuzzResponse{}, err
	}
	return resp, nil
}