internal/netlink: expose multicast groups

This also gets rid of the cached pid value for port since that prevents multiple sockets from being open at once.

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

internal/netlink/netlink.go

@@ -5,33 +5,29 @@ import (
 	"context"
 	"fmt"
 	"os"
-	"sync"
 	"syscall"
 	"time"
 	"unsafe"
 )
 
-// AF_NETLINK socket is never shared
-var (
-	nlPid     uint32
-	nlPidOnce sync.Once
-)
-
-// getpid returns a cached pid value.
-func getpid() uint32 {
-	nlPidOnce.Do(func() { nlPid = uint32(os.Getpid()) })
-	return nlPid
-}
-
 // net/netlink/af_netlink.c
 const maxRecvmsgLen = 32768
 
+const (
+	// stateOpen denotes an open conn.
+	stateOpen uint32 = 1 << iota
+)
+
 // A conn represents resources associated to a netlink socket.
 type conn struct {
 	// AF_NETLINK socket.
 	f *os.File
 	// For using runtime polling via f.
 	raw syscall.RawConn
+	// Port ID assigned by the kernel.
+	port uint32
+	// Internal connection status.
+	state uint32
 	// Kernel module or netlink group to communicate with.
 	family int
 	// Message sequence number.
@@ -49,7 +45,7 @@ type conn struct {
 }
 
 // dial returns the address of a newly connected conn of specified family.
-func dial(family int) (*conn, error) {
+func dial(family int, groups uint32) (*conn, error) {
 	var c conn
 	if fd, err := syscall.Socket(
 		syscall.AF_NETLINK,
@@ -59,17 +55,32 @@ func dial(family int) (*conn, error) {
 		return nil, os.NewSyscallError("socket", err)
 	} else if err = syscall.Bind(fd, &syscall.SockaddrNetlink{
 		Family: syscall.AF_NETLINK,
-		Pid:    getpid(),
+		Groups: groups,
 	}); err != nil {
 		_ = syscall.Close(fd)
 		return nil, os.NewSyscallError("bind", err)
 	} else {
+		var addr syscall.Sockaddr
+		if addr, err = syscall.Getsockname(fd); err != nil {
+			_ = syscall.Close(fd)
+			return nil, os.NewSyscallError("getsockname", err)
+		}
+		switch a := addr.(type) {
+		case *syscall.SockaddrNetlink:
+			c.port = a.Pid
+
+		default: // unreachable
+			_ = syscall.Close(fd)
+			return nil, syscall.ENOTRECOVERABLE
+		}
+
 		c.family = family
 		c.f = os.NewFile(uintptr(fd), "netlink")
 		if c.raw, err = c.f.SyscallConn(); err != nil {
 			_ = c.f.Close()
 			return nil, err
 		}
+		c.state |= stateOpen
 	}
 
 	c.pos = syscall.NLMSG_HDRLEN
@@ -78,14 +89,14 @@ func dial(family int) (*conn, error) {
 }
 
 // ok returns whether conn is still open.
-func (c *conn) ok() bool { return c.family >= 0 }
+func (c *conn) ok() bool { return c.state&stateOpen != 0 }
 
 // Close closes the underlying socket.
 func (c *conn) Close() error {
 	if !c.ok() {
 		return syscall.EINVAL
 	}
-	c.family = -1
+	c.state &= ^stateOpen
 	return c.f.Close()
 }
 
@@ -231,7 +242,7 @@ func (c *conn) pending() []byte {
 		Type:  c.typ,
 		Flags: c.flags,
 		Seq:   c.seq,
-		Pid:   getpid(),
+		Pid:   c.port,
 	}
 	return buf
 }
@@ -266,8 +277,8 @@ func (c *conn) receive(ctx context.Context, f HandlerFunc, flags int) error {
 
 		for i := range resp {
 			header := &resp[i].Header
-			if header.Seq != c.seq || header.Pid != getpid() {
+			if header.Seq != c.seq || header.Pid != c.port {
-				return &InconsistentError{*header, c.seq, getpid()}
+				return &InconsistentError{*header, c.seq, c.port}
 			}
 		}
 		if err = f(resp); err != nil {

internal/netlink/netlink_test.go

@@ -5,8 +5,6 @@ import (
 	"testing"
 )
 
-func init() { nlPidOnce.Do(func() {}); nlPid = 1 }
-
 type payloadTestCase struct {
 	name string
 	f    func(c *conn)
@@ -21,8 +19,9 @@ func checkPayload(t *testing.T, testCases []payloadTestCase) {
 	for _, tc := range testCases {
 		t.Run(tc.name, func(t *testing.T) {
 			t.Parallel()
+			t.Helper()
 
-			c := conn{pos: syscall.NLMSG_HDRLEN}
+			c := conn{port: 1, pos: syscall.NLMSG_HDRLEN}
 			tc.f(&c)
 			if got := c.pending(); string(got) != string(tc.want) {
 				t.Errorf("pending: %#v, want %#v", got, tc.want)

internal/netlink/rtnl.go

@@ -11,7 +11,7 @@ type RouteConn struct{ *conn }
 
 // DialRoute returns the address of a newly connected [RouteConn].
 func DialRoute() (*RouteConn, error) {
-	c, err := dial(syscall.NETLINK_ROUTE)
+	c, err := dial(syscall.NETLINK_ROUTE, 0)
 	if err != nil {
 		return nil, err
 	}