unix: avoid "just past the end" pointers in UnixRights

Same as CL 201617 did for package syscall.

Caught with -d=checkptr

Updates golang/go#22218

Change-Id: I8208f8e6d9bd62376bf9e0458dc18956daabd785
Reviewed-on: https://go-review.googlesource.com/c/sys/+/201937
Run-TryBot: Tobias Klauser <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>
Reviewed-by: Elias Naur <[email protected]>

Author: tklauser

unix/sockcmsg_linux.go

@@ -17,7 +17,7 @@ func UnixCredentials(ucred *Ucred) []byte {
 	h.Level = SOL_SOCKET
 	h.Type = SCM_CREDENTIALS
 	h.SetLen(CmsgLen(SizeofUcred))
-	*((*Ucred)(cmsgData(h))) = *ucred
+	*(*Ucred)(h.data(0)) = *ucred
 	return b
 }
 

unix/sockcmsg_unix.go

@@ -50,8 +50,8 @@ func CmsgSpace(datalen int) int {
 	return cmsgAlignOf(SizeofCmsghdr) + cmsgAlignOf(datalen)
 }
 
-func cmsgData(h *Cmsghdr) unsafe.Pointer {
-	return unsafe.Pointer(uintptr(unsafe.Pointer(h)) + uintptr(cmsgAlignOf(SizeofCmsghdr)))
+func (h *Cmsghdr) data(offset uintptr) unsafe.Pointer {
+	return unsafe.Pointer(uintptr(unsafe.Pointer(h)) + uintptr(cmsgAlignOf(SizeofCmsghdr)) + offset)
 }
 
 // SocketControlMessage represents a socket control message.
@@ -94,10 +94,8 @@ func UnixRights(fds ...int) []byte {
 	h.Level = SOL_SOCKET
 	h.Type = SCM_RIGHTS
 	h.SetLen(CmsgLen(datalen))
-	data := cmsgData(h)
-	for _, fd := range fds {
-		*(*int32)(data) = int32(fd)
-		data = unsafe.Pointer(uintptr(data) + 4)
+	for i, fd := range fds {
+		*(*int32)(h.data(4 * uintptr(i))) = int32(fd)
 	}
 	return b
 }