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Date: Mon, 30 Sep 2019 16:51:03 -0700
From: Kees Cook <>
To: Muni Sekhar <>
Subject: Re: How to get the crash dump if system hangs?

On Thu, Sep 26, 2019 at 01:47:00AM +0530, Muni Sekhar wrote:
> I looked at the available tests with "cat
> /sys/kernel/debug/provoke-crash/DIRECT", from this I’d like to know
> which test causes system hang? I could not find any test case for
> deadlock, is any reason for this?

The various *LOCKUP tests will hang a CPU or task (though SPINLOCKUP
needs to be called twice). You could keep calling HARDLOCKUP until
you're out of CPUs, for example. :)

What kind of deadlock do you want to test?

> I’m having a Linux system, I’m seeing it gets hung during certain
> tests. When it hung, it does not even respond for SYSRQ button, only
> way to recover is power-button-only.  Does no response for SYSRQ
> button means kernel crashed?

That's an impressive hang! :(

> After reboot I looked at the kern.log and most of the times it has
> “^@^@^@^ ...“ line just before reboot. Can someone clarify me what the
> kernel log entry “^@^@^@^ ...“ means? I suspect kernel is crashed, but
> it does give any crashdump in kern.log.

That's a zero byte. I would suggest using something like pstore to
capture this in RAM instead of hoping it makes it to disk.

> Later I enabled the kernel crash dump(sudo apt install
> linux-crashdump) and rerun the test but still nothing copied to the
> disk(/var/crash/). I don’t have onboard serial port in my machine, so
> I tried get the crash dump via netconsole, but this method also does
> not able to catch the crash dump.
> Can someone help me how to debug in this scenario?
> And I'd like to know what other options available to get the crash
> dump? Can someone please clarify me on this?
> Also , does the crash dump fails if incase deadlock occurs?
> Any help will be greatly appreciated.

If you really need to hard-power your system to get it back, pstore may
only work if you're really quick and likely enable software ECC.

Kees Cook

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