init: Use root's home directory in single-user mode

[jilles]

Summary

When starting single-user mode, use the home directory from user root as current directory and for the HOME environment variable. If the directory does not exist, set HOME=/.

Also adjust /root/.profile to stop setting HOME, since it should always have been set now.

This is intended to keep shell startup files working in single-user mode after /.profile has been removed by a change like https://reviews.freebsd.org/D52161 .

Test Plan

• build and install world
• ensure /.profile does not exist
• boot in single-user mode
• verify HOME is /root and the prompt is root@:~ # as set by /root/.shrc

[kevans]

Yeah, this seems fine to me. I did wonder for a second if the /rescue variants of shells do or should assume setugid behavior and avoid loading dotfiles in case something in the profile or other bits finds a way to make the shell unusable, but I didn't really convince myself.

[ivy]

i was planning to do this myself at some point, thanks!

[jilles]

In D52527#1199611, @kevans wrote:

I did wonder for a second if the /rescue variants of shells do or should assume setugid behavior and avoid loading dotfiles in case something in the profile or other bits finds a way to make the shell unusable, but I didn't really convince myself.

The /bin/csh and /rescue/csh have different dotfiles and could therefore be used to recover from this kind of issue. Anyway, this was the case before this change as well.