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

[Denys Vlasenko]

On Wed, Feb 14, 2018 at 4:29 AM, Deb McLemore <debmc at linux.vnet.ibm.com> wrote:
> On 02/13/2018 08:32 PM, Laurent Bercot wrote:
>>> Even when process=1 is started, it still leaves a window when the
>>> signal handler setup has not been completed.
>>
>>  Yes, but you can still use kill(pid, 0) to check whether init is
>> ready to receive signals: doublefork a zombie and repeatedly kill it
>> with signal 0. When you get -1 ESRCH, it means init has reapt the
>> zombie, so it's in its reaping loop, and at that point you know it
>> has installed its signal handlers.
>>
>>  See http://lists.busybox.net/pipermail/busybox/2017-October/085888.html
>>
>>  That's arguably uglier than using abstract sockets, but it can
>> be done without modifying the init code at all.

> From the original thread, we tried the suggestion, but when the do_execve is done from kernel_init
>
> for Busybox /sbin/init (to start PID=1) all process zombies are flushed (the usermode helper that kicks
>
> Busybox /sbin/poweroff started by PID=2, before PID=1 is do_execve'd from kernel init).  So when the line is crossed
>
> when PID=1 begins all zombies and pre-work are flushed, so nothing is carried over into the life of PID=1
> to synchronize with or detect when the signals are properly setup.

You can detect this: before init is started, kill(1,0) should be giving ESRCH.
Thus: first wait for kill(1,0) to return 0.
Then, do a child reaping test. Or just sleep(1). :]