libexec/save-entropy/save-entropy.sh should probably fsync the directory it modifies as well as the new file it creates to ensure newly saved entropy makes it to disk in the event of a crash. (The entire point of save-entropy is to make saved-entropy RNG seeding crash-resistant — otherwise we could just rely on rc.d/random's shutdown component.)
A commit references this bug: Author: cem Date: Sun Mar 31 04:57:51 UTC 2019 New revision: 345744 URL: https://svnweb.freebsd.org/changeset/base/345744 Log: random(4): Attempt to persist entropy promptly The goal of saving entropy in Fortuna is two-fold: (1) to provide early availability of the random device (unblocking) on next boot; and (2), to have known, high-quality entropy available for that initial seed. We know it is high quality because it's output taken from Fortuna. The FS&K paper makes it clear that Fortuna unblocks when enough bits have been input that the output //may// be safely seeded. But they emphasize that the quality of various entropy sources is unknown, and a saved entropy file is essential for both availability and ensuring initial unpredictability. In FreeBSD we persist entropy using two mechanisms: 1. The /etc/rc.d/random shutdown() function, which is used for ordinary shutdowns and reboots; and, 2. A cron job that runs every dozen minutes or so to persist new entropy, in case the system suffers from power loss or a crash (bypassing the ordinary shutdown path). Filesystems are free to cache dirty data indefinitely, with arbitrary flush policy. Fsync must be used to ensure the data is persisted, especially for the cron job save-entropy, whose entire goal is power loss and crash safe entropy persistence. Ordinary shutdown may not need the fsync because unmount should flush out the dirty entropy file shortly afterwards. But it is always possible power loss or crash occurs during the short window after rc.d/random shutdown runs and before the filesystem is unmounted, so the additional fsync there seems harmless. PR: 230876 Reviewed by: delphij, markj, markm Approved by: secteam (delphij) Differential Revision: https://reviews.freebsd.org/D19742 Changes: head/libexec/rc/rc.d/random head/libexec/save-entropy/save-entropy.sh