[PATCH] init: Add handshake to poweroff/reboot for signal handler setup

[Michael Conrad]

On 2/14/2018 12:53 PM, Deb McLemore wrote:
> The only reproduction we were able to perform injected via a BMC soft poweroff being triggered.
>
> This then called into kernel/reboot.c (orderly_poweroff where the schedule_work was performed) utilizing the
>
> usermodehelper during the run_cmd /sbin/poweroff.

I'd like to point out that there's still a race condition, since when 
init starts it might kick off the rest of your startup sequence before 
the idle reboot process gets a time slice to discover that init is alive 
and send the signal.

Would it maybe be better to have the main script that initializes your 
system check for the existence of the socket, and if found, stop 
everything and tell init to shutdown?

Also, why not have the reboot process look for something in the 
filesystem to indicate whether the system has started booting? Before 
init starts, there won't be anything in the filesystem, but that's not a 
problem because you can just wait until there is.

To make a complete proposal, why not:

reboot_helper:
   1) create abstract socket
   2) wait for any filesystem path that indicates init sequence has 
started (/proc maybe)
   3) send reboot signal to init

init_userspace.sh
   1) check for abstract socket, and if exists, send signal to init and 
then exit
   2) else run startup sequence

This will give you even better "instant boot sequence abort" performance 
than you would get with the design you're proposing, unless you have 
some insanely high polling rate in the reboot_helper that you think can 
actually deliver the signal before init has even forked for the first time.

-Mike