Fix possible crash with high client activity

[DL6ER]

What does this implement/fix?

Fix possible race-collision leading to a theoretical out-of-bounds read

Related issue or feature (if applicable): N/A

Pull request in docs with documentation (if applicable): N/A

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[yubiuser]

The race would be that counters->clients could be increased while the counter for clientID is still running?

[DL6ER]

We declare an object of size counters->clients here:

FTL/src/database/network-table.c

Lines 1288 to 1291 in 1a11413

	lock_shm();
	const int clients = counters->clients;
	unlock_shm();
	enum arp_status client_status[clients];

and later on use it in a loop that is running from 0 to counters->clients. The race is that counters->clients may have increased meanwhile. This is prevented by passing the counters->clients we have used before and use this as upper limit for the loop. It may mean we are missing a few clients, but we will pick them up shortly after when the code is run again.

[DL6ER]

Unfortunately, we never got feedback from the affected users. As they were target active before we suggested this bugfix, I can only assume it fixed the issue for them. Anyway, it's a meaningful change and we can use it to push a v5.x FTL patch release.

[yubiuser]

Mhh...

nanopi@nanopi:~$ pihole checkout ftl fix/many_clients
  Please note that changing branches severely alters your Pi-hole subsystems
  Features that work on the master branch, may not on a development branch
  This feature is NOT supported unless a Pi-hole developer explicitly asks!
  Have you read and understood this? [y/N] y

  [✗] Requested branch "fix/many_clients" is not available
  [i] Available branches for FTL are:
      - development
      - development-v6
      - fix/delete_dhcp_lease
      - fix/many_clients
      - group_dependabot

[pralor-bot]

This pull request has been mentioned on Pi-hole Userspace. There might be relevant details there:

https://discourse.pi-hole.net/t/pi-hole-ftl-v5-24-and-core-v5-17-3-released/67486/1