net: fix installing orphaned programs

When netdevice with offloaded BPF programs is destroyed
the programs are orphaned and removed from the program
IDA - their IDs get released (the programs may remain
accessible via existing open file descriptors and pinned
files). After IDs are released they are set to 0.

This confuses dev_change_xdp_fd() because it compares
the __dev_xdp_query() result where 0 means no program
with prog->aux->id where 0 means orphaned.

dev_change_xdp_fd() would have incorrectly returned success
even though it had not installed the program.

Since drivers already catch this case via bpf_offload_dev_match()
let them handle this case. The error message drivers produce in
this case ("program loaded for a different device") is in fact
correct as the orphaned program must had to be loaded for a
different device.

Fixes: c14a9f63 ("net: Don't call XDP_SETUP_PROG when nothing is changed")
Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/dev.c

@@ -8421,7 +8421,8 @@ int dev_change_xdp_fd(struct net_device *dev, struct netlink_ext_ack *extack,
 			return -EINVAL;
 		}
 
-		if (prog->aux->id == prog_id) {
+		/* prog->aux->id may be 0 for orphaned device-bound progs */
+		if (prog->aux->id && prog->aux->id == prog_id) {
 			bpf_prog_put(prog);
 			return 0;
 		}