From: Guillaume Nault <redacted>
Date: Wed, 23 May 2018 15:57:08 +0200

I'd rather add
+	if (cmd == PPPIOCDETACH) {
+		err = -EINVAL;
+		goto out;
+	}

Making PPPIOCDETACH unknown to ppp_generic means that the ioctl would
be handled by the underlying channel when pf->kind == CHANNEL (see the
chan->ops->ioctl() call further down). That shouldn't be a problem per
se, but even though PPPIOCDETACH is unsupported, I feel that it should
remain a ppp_generic thing. I don't really want its value to be reused
for other purposes in the future or have different behaviour depending
on the underlying channel.

Also PPPIOCDETACH can already fail with -EINVAL. Therefore, if ever
there really were programs out there using this call, they'd already
have to handle this case. Unconditionally returning -EINVAL would
further minimise possibilities for breakage.

I agree.