Leave nlmsg_pid at 0 for netlink multicast

Multicast messages from the kernel have nlmsg_pid=0.
Unicast messages from the kernel have nlmsg_pid as the socket's pid.
Multicast messages from userland have whatever nlmsg_pid set by the app.

PiperOrigin-RevId: 836804950

Author: shailend-g

pkg/sentry/socket/netlink/socket.go

@@ -414,7 +414,8 @@ func (s *Socket) HandleInterfaceChangeEvent(ctx context.Context, idx int32, i in
 		panic(fmt.Sprintf("Non-ROUTE netlink socket (protocol %d) cannot handle interface events", s.Protocol()))
 	}
 
-	ms := nlmsg.NewMessageSet(s.GetPortID(), 0)
+	// Multicast messages from the kernel leave hdr->nlmsg_pid at 0.
+	ms := nlmsg.NewMessageSet(0 /*portID*/, 0 /*seq*/)
 	routeProtocol.AddNewLinkMessage(ms, idx, i)
 	s.SendResponse(ctx, ms)
 }
@@ -426,7 +427,8 @@ func (s *Socket) HandleInterfaceDeleteEvent(ctx context.Context, idx int32, i in
 		panic(fmt.Sprintf("Non-ROUTE netlink socket (protocol %d) cannot handle interface events", s.Protocol()))
 	}
 
-	ms := nlmsg.NewMessageSet(s.GetPortID(), 0)
+	// Multicast messages from the kernel leave hdr->nlmsg_pid at 0.
+	ms := nlmsg.NewMessageSet(0 /*portID*/, 0 /*seq*/)
 	routeProtocol.AddDelLinkMessage(ms, idx, i)
 	s.SendResponse(ctx, ms)
 }

test/syscalls/linux/socket_netlink_route.cc

@@ -1958,6 +1958,7 @@ TEST(NetlinkRouteTest, LinkMulticastGroupBasic) {
           if (msg->ifi_index != tc.link.index) {
             return;
           }
+          EXPECT_EQ(hdr->nlmsg_pid, 0);
           CheckLinkMsg(hdr, tc.link);
           got_msg = true;
         },
@@ -2040,6 +2041,7 @@ TEST(NetlinkRouteTest, LinkMulticastGroupNamespaced) {
               reinterpret_cast<const struct ifinfomsg*>(NLMSG_DATA(hdr));
           if (hdr->nlmsg_type != RTM_DELLINK) return;
           if (msg->ifi_index != inner_veth_idx) return;
+          EXPECT_EQ(hdr->nlmsg_pid, 0);
           got_msg = true;
         },
         /*expect_nlmsgerr=*/false));
@@ -2065,6 +2067,7 @@ TEST(NetlinkRouteTest, LinkMulticastGroupNamespaced) {
           char ifname[IF_NAMESIZE];
           EXPECT_NE(if_indextoname(msg->ifi_index, ifname), nullptr);
           EXPECT_STREQ(ifname, "veth2");
+          EXPECT_EQ(hdr->nlmsg_pid, 0);
           got_msg = true;
         },
         /*expect_nlmsgerr=*/false));
@@ -2232,6 +2235,8 @@ TEST(NetlinkRouteTest, LinkMulticastGroupUserToUserSend) {
         const struct nlmsgerr* msg =
             reinterpret_cast<const struct nlmsgerr*>(NLMSG_DATA(hdr));
         ASSERT_EQ(msg->error, 0);
+        EXPECT_EQ(hdr->nlmsg_pid,
+                  ASSERT_NO_ERRNO_AND_VALUE(NetlinkPortID(nlsk_send.get())));
         got_msg = true;
       },
       /*expect_nlmsgerr=*/true));