This revision is good one independently of how the API to delete entries is going to look like. To be fair I'm not very happy with new function names, but can't quickly come with a good alternatives.

In D58356#1339476, @rscheff wrote:

• provide a way to check for the mere existance, and contents of an entry for a given IP address.

This is why callback now can be NULL? What's the practical use?

In D58356#1339602, @glebius wrote:

This is why callback now can be NULL? What's the practical use?

Userspace sysctl actually makes 5 calls:

1. NULL, 0 -> gets returned the buffer size, allocates that buffer
2. NULL, size -> gets returned the old value
3. newval, size -> sets the new value
4. NULL, 0 -> gets returned the (new) buffer size
5. NULL, newsize -> retrieves the values after setting

By allowing a NULL for callback, (and a NULL for the passed-on argument), I can check for the mere existance of an entry (return 0 / ENOENT), without touching the expiry timer.
With callback NULL but the pass-one argument a pointer to hc_entries, I can additionally get the actual values of that entry if it exists.

So, in the custom sysctl handler can statically return sizeof(struct hc_entry) in (1) and (4).
In (2) and (5), the current values can be retrieved (NULL, &(struct hc_entry))
This allow then the userspace tool to modify the values from (2) as necessary (leave some unmodified, change others...) before writing the value back. And also validate the set values finally match what got set.

The sysctl utility (userspace) already has a few special handlers for some S,xxx struct / CTLTYPE_OPAQUE/CTLTYPE_STRUCT binary blob exchanges - but i think those are so far mostly read-only.

Likely I can also change from a (void *) arg to a (struct hc_entry *)arg, with proper typechecking - and no typecasting.

See the companion D58358 over the next couple days when i get to completing this properly. Ultimately this facility will allow proper unit testing / priming of the tcp hostcache and thus start the tcp stack in various states not reachable by purgeall of the tcp hostcache.