Support for older Linux kernels without getrandom

Change-Id: I1fb9f5b47a264a7617912f6f56f995f3c4c5e578
PiperOrigin-RevId: 216591484

pkg/rand/rand_linux.go

@@ -17,23 +17,46 @@
 package rand
 
 import (
+	"crypto/rand"
 	"io"
+	"sync"
 
 	"golang.org/x/sys/unix"
 )
 
 // reader implements an io.Reader that returns pseudorandom bytes.
-type reader struct{}
+type reader struct {
+	once         sync.Once
+	useGetrandom bool
+}
 
 // Read implements io.Reader.Read.
-func (reader) Read(p []byte) (int, error) {
+func (r *reader) Read(p []byte) (int, error) {
+	r.once.Do(func() {
+		_, err := unix.Getrandom(p, 0)
+		if err != unix.ENOSYS {
+			r.useGetrandom = true
+		}
+	})
+
+	if r.useGetrandom {
 		return unix.Getrandom(p, 0)
+	}
+	return rand.Read(p)
 }
 
 // Reader is the default reader.
-var Reader io.Reader = reader{}
+var Reader io.Reader = &reader{}
 
 // Read reads from the default reader.
 func Read(b []byte) (int, error) {
 	return io.ReadFull(Reader, b)
 }
+
+// Init can be called to make sure /dev/urandom is pre-opened on kernels that
+// do not support getrandom(2).
+func Init() error {
+	p := make([]byte, 1)
+	_, err := Read(p)
+	return err
+}

runsc/boot/BUILD

@@ -25,6 +25,7 @@ go_library(
         "//pkg/control/server",
         "//pkg/cpuid",
         "//pkg/log",
+        "//pkg/rand",
         "//pkg/sentry/arch",
         "//pkg/sentry/context",
         "//pkg/sentry/control",

runsc/boot/loader.go

@@ -17,7 +17,7 @@ package boot
 
 import (
 	"fmt"
-	"math/rand"
+	mrand "math/rand"
 	"os"
 	"os/signal"
 	"runtime"
@@ -30,6 +30,7 @@ import (
 	"gvisor.googlesource.com/gvisor/pkg/abi/linux"
 	"gvisor.googlesource.com/gvisor/pkg/cpuid"
 	"gvisor.googlesource.com/gvisor/pkg/log"
+	"gvisor.googlesource.com/gvisor/pkg/rand"
 	"gvisor.googlesource.com/gvisor/pkg/sentry/arch"
 	"gvisor.googlesource.com/gvisor/pkg/sentry/control"
 	"gvisor.googlesource.com/gvisor/pkg/sentry/fs/host"
@@ -133,7 +134,7 @@ type execProcess struct {
 
 func init() {
 	// Initialize the random number generator.
-	rand.Seed(gtime.Now().UnixNano())
+	mrand.Seed(gtime.Now().UnixNano())
 
 	// Register the global syscall table.
 	kernel.RegisterSyscallTable(slinux.AMD64)
@@ -167,9 +168,16 @@ type Args struct {
 // New initializes a new kernel loader configured by spec.
 // New also handles setting up a kernel for restoring a container.
 func New(args Args) (*Loader, error) {
+	// We initialize the rand package now to make sure /dev/urandom is pre-opened
+	// on kernels that do not support getrandom(2).
+	if err := rand.Init(); err != nil {
+		return nil, fmt.Errorf("error setting up rand: %v", err)
+	}
+
 	if err := usage.Init(); err != nil {
 		return nil, fmt.Errorf("error setting up memory usage: %v", err)
 	}
+
 	// Create kernel and platform.
 	p, err := createPlatform(args.Conf, args.DeviceFD)
 	if err != nil {