update(falco_rules): exclude systemd helpers from Read sensitive file…

[mayanksekhar]

/kind bug

/area rules

/area maturity-stable

What this PR does / why we need it:

The Read sensitive file untrusted rule (maturity_stable) fires false positives on modern systemd hosts when systemd's own short-lived helper processes read account/PAM files during normal operation.

The rule already attempts to exclude systemd, but those exclusions match on proc.name, which is unusable for these helpers: it is a bare file-descriptor number (e.g. 16) for systemd-executor, and comm-truncated to 15 chars (systemd-userwor) for systemd-userwork. The existing name-based exclusions therefore never match, and the rule alerts.

This adds a systemd_sensitive_file_readers macro that matches on proc.exepath instead. proc.exepath is kernel-resolved and cannot be influenced by argv[0], and each entry is anchored to its expected systemd parent so the exclusion stays tight rather than blanket-trusting /usr/lib/systemd/*. Two helpers are covered:

• systemd-executor — unit (re)exec deserialize handoff
• systemd-userwork — userdb (User/Group Record Lookup) query worker

Per the repo's versioning guidelines this is a backward-compatible patch change (adding an exception to an existing rule; no required_engine_version change).

Which issue(s) this PR fixes:

Fixes #291
Related: falcosecurity/falco#3480

Special notes for your reviewer:

The systemd-userwork variant was reproduced locally on systemd 259 / Falco 0.44.1 and verified by deterministic capture replay: replaying the same .scap against the rule produces 2 Read sensitive file untrusted alerts before this change and 0 after, while a genuine untrusted read (cat /etc/shadow) still fires.

I could not reproduce the systemd-executor variant on my systemd version, but it is documented with full field dumps in #291 (thanks to @marwinski, who identified that proc.name matching was the blocker) and in falcosecurity/falco#3480.

If you'd prefer to land this incrementally, I'm happy to scope it down to systemd-userwork only — the case I reproduced directly — and track systemd-executor separately.

Does this PR introduce a user-facing change?:

update(falco_rules): exclude legitimate systemd helper processes (systemd-executor, systemd-userwork) from false positives in `Read sensitive file untrusted`

[poiana]

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[poiana]

Welcome @mayanksekhar! It looks like this is your first PR to falcosecurity/rules 🎉