Re: [PATCH net-next 5/6] netlink: add mask validation

[Johannes Berg <johannes@sipsolutions.net>]

On Mon, 2020-10-05 at 21:28 +0200, Michal Kubecek wrote:

> > > +	if (value & ~(u64)pt->mask) {
> > > +		NL_SET_ERR_MSG_ATTR(extack, nla, "reserved bit set");
> > > +		return -EINVAL;
> > 
> > You had an export of the valid bits there in ethtool, using the cookie.
> > Just pointing out you lost it now. I'm not sure I like using the cookie,
> > that seems a bit strange, but we could easily define a different attr?
> 
> The idea behind the cookie was that if new userspace sends a request
> with multiple flags which may not be supported by an old kernel, getting
> only -EOPNOTSUPP (and badattr pointing to the flags) would not be very
> helpful as multiple iteration would be necessary to find out which flags
> are supported and which not.

Message crossing, I guess.

I completely agree. I just don't like using the (somewhat vague)
_cookie_ for that vs. adding a new explicit NLMSGERR_ATTR_SOMETHING :)

I would totally support doing that here in the general validation code,
but (again) don't really think NLMSGERR_ATTR_COOKIE is an appropriate
attribute for it.

johannes