> (assuming that (a) /var was mounted R-W at some point
Not a good assumption if the initial r/w mount is the one that fails...
Re:Fixing Getrandom()
by throw0101a on Thursday October 03, 2019 @10:36AM
(#21147477)
If the initial /var (or /) is broken, then one has bigger problems and probably has to go in manually to fix things.
But for >99% of cases where the system comes up cleanly, and runs cleanly for a period of time, you'll have a bunch of seed files ready to go for the next boot. This configuration optimizes for the common case.
FreeBSD runs a regular cron job so that there are multiple files. If /var goes R-O, you still have a bunch of files from when it wasn't and they are not the same as on initial boot (assuming that (a) /var was mounted R-W at some point, and (b) cron managed to run as well).
Also, on shutdown there is an attempt to write 4096B to both /entropy and /boot/entropy:
* https://svnweb.freebsd.org/base/head/libexec/rc/rc.d/random?...
Further, all of these various seed files are not the only sources of entropy (timers, RDRAND, etc).