dnsdist: Fix BPFFilter::addRangeRule

[rgacogne]

Short description

Reported by Prasanna Dabi (thanks!):

"The eBPF DDoS mitigation implementation in dnsdist contains a critical logic error that prevents new range-based block rules from being applied. When the BPFFilter::addRangeRule() function is called to block a subnet, it first checks the eBPF map to determine if the rule already exists. If the subnet is not currently in the map, the bpf_lookup_elem call returns -1. In this failure state, the local CounterAndActionValue value struct remains in its default, zeroed-out state, where the action field is automatically set to BPFFilter::MatchAction::Pass.

The conditional check intended to skip redundant rules contains a logic typo: it evaluates value.action == BPFFilter::MatchAction::Pass instead of comparing the requested action parameter.Because the default state of the unpopulated struct is always Pass, the condition (res == -1 && value.action == BPFFilter::MatchAction::Pass) evaluates to true for every new rule attempt.This causes the daemon to throw a std::runtime_error and reject the mitigation."

Checklist

I have:

• read the CONTRIBUTING.md document
• read and accepted the Developer Certificate of Origin document, including the AI Policy, and added a "Signed-off-by" to my commits
• compiled this code
• tested this code
• included documentation (including possible behaviour changes)
• documented the code
• added or modified regression test(s)
• added or modified unit test(s)