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Date: Thu, 26 Sep 2019 01:47:00 +0530
From: Muni Sekhar <>
Cc: Kees Cook <>
Subject: How to get the crash dump if system hangs?

Hi All,

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?

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?

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.

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.


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