New skb extension for use by LSMs (skb "security blob")?

[Casey Schaufler]

On 8/22/2019 3:36 PM, David Miller wrote:
> From: Casey Schaufler <casey at schaufler-ca.com>
> Date: Thu, 22 Aug 2019 15:34:44 -0700
>
>> On 8/22/2019 3:28 PM, David Miller wrote:
>>> From: Casey Schaufler <casey at schaufler-ca.com>
>>> Date: Thu, 22 Aug 2019 14:59:37 -0700
>>>
>>>> Sure, you *can* do that, but it would be insane to do so.
>>> We look up the neighbour table entries on every single packet we
>>> transmit from the kernel in the same exact way.
>>>
>>> And it was exactly to get rid of a pointer in a data structure.
>> I very much expect that the lifecycle management issues would
>> be completely different, but I'll admit to having little understanding
>> of the details of the neighbour table.
> Neighbour table entries can live anywhere from essentially forever down
> to several microseconds.
>
> If your hash is good, and you use RCU locking on the read side, it's a
> single pointer dereference in cost.

The secmark is the data used by the netfilter system.
While it would be (Turing compatible, after all) possible,
we're talking multiple attributes with different lifecycles
being managed in a table (list, whatever) that may expand
explosively. Using a single ID to reference into a table that
could contain:
	secmark from iptables for SELinux
	secmark from iptables for AppArmor
	SELinux secid/context for the packet
	AppArmor secid/context for the packet
will be hairy. In the netfilter processing we may have to
allocate a new table entry. There's no way to identify that
the entry is no longer necessary, as there is no lifecycle
on a secmark. Is it possible to come up with something that
will limp along? Possibly. If there's a blob pointer, we know
how to do all this effectively.