[H-GEN] kernel info

Christopher Biggs chris at stallion.oz.au
Sun Sep 7 19:02:49 EDT 1997


Byron Ellacott <rodent at homer.humbug.org.au> moved upon the face of the 'Net and spake thusly:

> I want to make the kernel beep every second or so for a brief moment after
> it has been shut down, so I know when to turn the box off when it is
> headless.
> 

The kernel shuts down by killing all processes and calling the HLT
instruction.

This instruction halts the CPU *UNTIL AN INTERRUPT IS RECEIVED*, so
anything that causes an interrupt (network input, keypress, timer
interrupt) will wake the kernel up again.

There was a recent problem with IP-masquerading and firewalling:  as
certain filter processes were user jobs and not kernel tasks, and
since a halted box is essentially still running, after a firewall had
been halted, it was leaking unauthorized traffic.   The process that
would otherwise do the filtering was dead, but the kernel (in
particular the network drivers) was still operating.

Chris.

p.s. Oh yes, an answer to the question:  anything hanging off a timer
interrupt should therefore do the trick.

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