save-entropy(8), rc.d/random: Set nodump flag

[cem]

Summary

Tag saved entropy files as "nodump," to signal that the files should not be
backed up by dump(8) or other automated backup software that honors the file
flag.

Do not produce an error if the target file resides on a filesystem that does
not support file flags (e.g., msdos /boot).

Test Plan

The Fortuna chapter points out that saved entropy should not be backed up or
snapshotted. We don't have a lot of control over the latter as a VM guest, but
we can at least attempt to set file flags that signal "don't backup."

This is a weak signal since a lot of software might not be aware of the nodump
file flag, and some commonly used filesystems do not support it. But we might
as well set it for those that do honor it.

Testing:

testvm# /bin/ls -lo /var/db/entropy
total 32
-r--------  1 operator  operator  nodump 4096 May 22 13:55 saved-entropy.1
-r--------  1 operator  operator  -      4096 May 22 13:44 saved-entropy.2
...

(i.e., periodic save-entropy(8) has rolled the first file and will eventually set nodump on all files as rotation occurs).

testvm# /bin/ls -lo /boot/entropy
-rw-------  1 root  wheel  - 4096 May 22 11:08 /boot/entropy
testvm# reboot
...
Feeding entropy: .
...
testvm# /bin/ls -lo /boot/entropy
-rw-------  1 root  wheel  nodump 4096 May 22 13:56 /boot/entropy

rc.d/random updates flag.

[delphij]

Looks good to me overall, but please remove the warn (see comment inline for reasoning).

libexec/rc/rc.d/random

31 ↗

(On Diff #57710)

Please remove the warn here.

The old code you have get rid of (which is a good thing) used warn because dd's stderr output was redirected to /dev/null, with the version in save_dev_random, dd would emit the accurate error message, when applicable, and it's much better than a vague (unwriteable or full fs?) message.

[cem]

libexec/rc/rc.d/random

31 ↗

(On Diff #57710)

Works for me, will do! Thanks

[cem]

Drop redundant warn, thanks delphij

[markm]

Good move!