tun: fix checksum test failures on non-4KiB page sizes

When generating page-aligned random bytes, random data started at the
beginning of the buffer that will be chopped off. When the page size
differs, the start of the returned slice is different than expected for
the expected checksums, causing the tests to fail.

tun/checksum_test.go

@@ -21,26 +21,33 @@ type archChecksumDetails struct {
 	f         func([]byte, uint16) uint16
 }
 
-func deterministicRandomBytes(seed int64, length int) []byte {
+func fillRandomBuffer(seed int64, buf []byte) {
 	rng := rand.New(rand.NewSource(seed))
-	buf := make([]byte, length)
 	n, err := rng.Read(buf)
 	if err != nil {
 		panic(err)
 	}
-	if n != length {
+	if n != len(buf) {
 		panic("incomplete random buffer")
 	}
+}
+
+func deterministicRandomBytes(seed int64, length int) []byte {
+	buf := make([]byte, length)
+	fillRandomBuffer(seed, buf)
 	return buf
 }
 
 func getPageAlignedRandomBytes(seed int64, length int) []byte {
 	alignment := syscall.Getpagesize()
-	buf := deterministicRandomBytes(seed, length+(alignment-1))
+	buf := make([]byte, length+(alignment-1))
 	bufPtr := uintptr(unsafe.Pointer(&buf[0]))
 	alignedBufPtr := (bufPtr + uintptr(alignment-1)) & ^uintptr(alignment-1)
 	alignedStart := int(alignedBufPtr - bufPtr)
-	return buf[alignedStart:]
+
+	buf = buf[alignedStart : alignedStart+length]
+	fillRandomBuffer(seed, buf)
+	return buf
 }
 
 func TestChecksum(t *testing.T) {