manager: use pre-seeded fastrandn instead of math.rand

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

2 files changed, 5 insertions, 12 deletions

diff --git a/main.go b/main.go
index 9a8a8607..3fad1a24 100644
--- a/main.go
+++ b/main.go
@@ -6,18 +6,15 @@
 package main
 
 import (
-	"crypto/rand"
 	"debug/pe"
 	"errors"
 	"fmt"
 	"io"
 	"log"
-	unsafeRand "math/rand"
 	"os"
 	"strconv"
 	"strings"
 	"time"
-	"unsafe"
 
 	"golang.org/x/sys/windows"
 
@@ -153,19 +150,12 @@ func pipeFromHandleArgument(handleStr string) (*os.File, error) {
 	return os.NewFile(uintptr(handleInt), "pipe"), nil
 }
 
-func seedUnsafeRng() {
-	var seed int64
-	rand.Read(unsafe.Slice((*byte)(unsafe.Pointer(&seed)), unsafe.Sizeof(seed)))
-	unsafeRand.Seed(seed)
-}
-
 func main() {
 	if windows.SetDllDirectory("") != nil || windows.SetDefaultDllDirectories(windows.LOAD_LIBRARY_SEARCH_SYSTEM32) != nil {
 		panic("failed to restrict dll search path")
 	}
 
 	setLogFile()
-	seedUnsafeRng()
 	checkForWow64()
 
 	if len(os.Args) <= 1 {
diff --git a/manager/updatestate.go b/manager/updatestate.go
index 5ea0f661..cb65e7dd 100644
--- a/manager/updatestate.go
+++ b/manager/updatestate.go
@@ -7,14 +7,17 @@ package manager
 
 import (
 	"log"
-	unsafeRand "math/rand"
 	"time"
+	_ "unsafe"
 
 	"golang.zx2c4.com/wireguard/windows/services"
 	"golang.zx2c4.com/wireguard/windows/updater"
 	"golang.zx2c4.com/wireguard/windows/version"
 )
 
+//go:linkname fastrandn runtime.fastrandn
+func fastrandn(n uint32) uint32
+
 type UpdateState uint32
 
 const (
@@ -26,7 +29,7 @@ const (
 var updateState = UpdateStateUnknown
 
 func jitterSleep(min, max time.Duration) {
-	time.Sleep(min + time.Duration(unsafeRand.Int63n(int64(max-min+1))))
+	time.Sleep(min + time.Millisecond*time.Duration(fastrandn(uint32((max-min+1)/time.Millisecond))))
 }
 
 func checkForUpdates() {