So while the patch is correct it leaves 5.6 and 5.7 with a bug in the
pidfd_getfd() implementation and that just doesn't seem right. I'm
wondering whether we should introduce:

void sock_update(struct file *file)
{
	struct socket *sock;
	int error;

	sock = sock_from_file(file, &error);
	if (sock) {
		sock_update_netprioidx(&sock->sk->sk_cgrp_data);
		sock_update_classid(&sock->sk->sk_cgrp_data);
	}
}

and switch pidfd_getfd() over to:

diff --git a/kernel/pid.c b/kernel/pid.c
index f1496b757162..c26bba822be3 100644
--- a/kernel/pid.c
+++ b/kernel/pid.c

@@ -642,10 +642,12 @@ static int pidfd_getfd(struct pid *pid, int fd)
        }

        ret = get_unused_fd_flags(O_CLOEXEC);
-       if (ret < 0)
+       if (ret < 0) {
                fput(file);
-       else
+       } else {
+               sock_update(file);
                fd_install(ret, file);
+       }

        return ret;
 }

first thing in the series and then all of the other patches on top of it
so that we can Cc stable for this and that can get it backported to 5.6,
5.7, and 5.8.

Alternatively, I can make this a separate bugfix patch series which I'll
send upstream soonish. Or we have specific patches just for 5.6, 5.7,
and 5.8. Thoughts?