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Message-ID: <201909301645.5FA44A4@keescook>
Date: Mon, 30 Sep 2019 16:51:03 -0700
From: Kees Cook <keescook@...omium.org>
To: Muni Sekhar <munisekharrms@...il.com>
Cc: kernel-hardening@...ts.openwall.com
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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