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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