unix: fix trimmed socket opt string in GetsockoptString

Fixes golang/go#63217

Change-Id: Ia1b0ae88b7e7a7c611067a0b30e454b66f45ce51
Reviewed-on: https://go-review.googlesource.com/c/sys/+/530897
Reviewed-by: Mauri de Souza Meneguzzo <mauri870@gmail.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Auto-Submit: Tobias Klauser <tobias.klauser@gmail.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Heschi Kreinick <heschi@google.com>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>

unix/syscall_bsd.go

@@ -316,7 +316,7 @@ func GetsockoptString(fd, level, opt int) (string, error) {
 	if err != nil {
 		return "", err
 	}
-	return string(buf[:vallen-1]), nil
+	return ByteSliceToString(buf[:vallen]), nil
 }
 
 //sys	recvfrom(fd int, p []byte, flags int, from *RawSockaddrAny, fromlen *_Socklen) (n int, err error)

unix/syscall_linux.go

@@ -1310,7 +1310,7 @@ func GetsockoptString(fd, level, opt int) (string, error) {
 			return "", err
 		}
 	}
-	return string(buf[:vallen-1]), nil
+	return ByteSliceToString(buf[:vallen]), nil
 }
 
 func GetsockoptTpacketStats(fd, level, opt int) (*TpacketStats, error) {

unix/syscall_solaris.go

@@ -158,7 +158,7 @@ func GetsockoptString(fd, level, opt int) (string, error) {
 	if err != nil {
 		return "", err
 	}
-	return string(buf[:vallen-1]), nil
+	return ByteSliceToString(buf[:vallen]), nil
 }
 
 const ImplementsGetwd = true

unix/syscall_zos_s390x.go

@@ -1104,7 +1104,7 @@ func GetsockoptString(fd, level, opt int) (string, error) {
 		return "", err
 	}
 
-	return string(buf[:vallen-1]), nil
+	return ByteSliceToString(buf[:vallen]), nil
 }
 
 func Recvmsg(fd int, p, oob []byte, flags int) (n, oobn int, recvflags int, from Sockaddr, err error) {