internal/netlink: generalise implementation from container

This is useful for uevent implementation.

Signed-off-by: Ophestra <cat@gensokyo.uk>

internal/netlink/rtnl.go

@@ -0,0 +1,132 @@
+package netlink
+
+import (
+	"syscall"
+	"unsafe"
+)
+
+// RouteConn represents a NETLINK_ROUTE socket.
+type RouteConn struct{ *conn }
+
+// DialRoute returns the address of a newly connected [RouteConn].
+func DialRoute() (*RouteConn, error) {
+	c, err := dial(syscall.NETLINK_ROUTE)
+	if err != nil {
+		return nil, err
+	}
+	return &RouteConn{c}, nil
+}
+
+// rtnlConsume consumes a message from rtnetlink.
+func rtnlConsume(msg *syscall.NetlinkMessage) error {
+	switch msg.Header.Type {
+	case syscall.NLMSG_DONE:
+		return Complete{}
+
+	case syscall.NLMSG_ERROR:
+		if e := As[syscall.NlMsgerr](msg.Data); e != nil {
+			if e.Error == 0 {
+				return Complete{}
+			}
+			return syscall.Errno(-e.Error)
+		}
+		return syscall.EBADE
+
+	default:
+		return nil
+	}
+}
+
+// InAddr is equivalent to struct in_addr.
+type InAddr [4]byte
+
+// RtAttrMsg holds syscall.RtAttr alongside its payload.
+type RtAttrMsg[D any] struct {
+	syscall.RtAttr
+	Data D
+}
+
+// populate populates the Len field of the embedded syscall.RtAttr.
+func (attr *RtAttrMsg[M]) populate() {
+	attr.Len = syscall.SizeofRtAttr + uint16(unsafe.Sizeof(attr.Data))
+}
+
+// writeIfAddrmsg writes an ifaddrmsg structure to conn.
+func (c *RouteConn) writeIfAddrmsg(
+	typ, flags uint16,
+	msg *syscall.IfAddrmsg,
+	attrs ...RtAttrMsg[InAddr],
+) bool {
+	c.typ, c.flags = typ, syscall.NLM_F_REQUEST|syscall.NLM_F_ACK|flags
+	if !add(c.conn, msg) {
+		return false
+	}
+	for _, attr := range attrs {
+		attr.populate()
+		if !add(c.conn, &attr) {
+			return false
+		}
+	}
+	return true
+}
+
+// SendIfAddrmsg sends an ifaddrmsg structure to rtnetlink.
+func (c *RouteConn) SendIfAddrmsg(
+	typ, flags uint16,
+	msg *syscall.IfAddrmsg,
+	attrs ...RtAttrMsg[InAddr],
+) error {
+	if !c.writeIfAddrmsg(typ, flags, msg, attrs...) {
+		return syscall.ENOMEM
+	}
+	return c.Roundtrip(rtnlConsume)
+}
+
+// writeNewaddrLo writes a RTM_NEWADDR message for the loopback address.
+func (c *RouteConn) writeNewaddrLo(lo uint32) bool {
+	return c.writeIfAddrmsg(
+		syscall.RTM_NEWADDR,
+		syscall.NLM_F_CREATE|syscall.NLM_F_EXCL,
+		&syscall.IfAddrmsg{
+			Family:    syscall.AF_INET,
+			Prefixlen: 8,
+			Flags:     syscall.IFA_F_PERMANENT,
+			Scope:     syscall.RT_SCOPE_HOST,
+			Index:     lo,
+		},
+		RtAttrMsg[InAddr]{syscall.RtAttr{
+			Type: syscall.IFA_LOCAL,
+		}, InAddr{127, 0, 0, 1}},
+		RtAttrMsg[InAddr]{syscall.RtAttr{
+			Type: syscall.IFA_ADDRESS,
+		}, InAddr{127, 0, 0, 1}},
+	)
+}
+
+// SendNewaddrLo sends a RTM_NEWADDR message for the loopback address to the kernel.
+func (c *RouteConn) SendNewaddrLo(lo uint32) error {
+	if !c.writeNewaddrLo(lo) {
+		return syscall.ENOMEM
+	}
+	return c.Roundtrip(rtnlConsume)
+}
+
+// writeIfInfomsg writes an ifinfomsg structure to conn.
+func (c *RouteConn) writeIfInfomsg(
+	typ, flags uint16,
+	msg *syscall.IfInfomsg,
+) bool {
+	c.typ, c.flags = typ, syscall.NLM_F_REQUEST|syscall.NLM_F_ACK|flags
+	return add(c.conn, msg)
+}
+
+// SendIfInfomsg sends an ifinfomsg structure to rtnetlink.
+func (c *RouteConn) SendIfInfomsg(
+	typ, flags uint16,
+	msg *syscall.IfInfomsg,
+) error {
+	if !c.writeIfInfomsg(typ, flags, msg) {
+		return syscall.ENOMEM
+	}
+	return c.Roundtrip(rtnlConsume)
+}