Date: Thu, 3 Oct 2019 22:18:48 +0530 From: Muni Sekhar <munisekharrms@...il.com> To: Kees Cook <keescook@...omium.org> Cc: kernel-hardening@...ts.openwall.com Subject: Re: How to get the crash dump if system hangs? On Tue, Oct 1, 2019 at 5:21 AM Kees Cook <keescook@...omium.org> wrote: > > 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 looking for a test where crash dump fails. > > > 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. Thanks a lot for letting me know about pstore, will try this option. It will be helpful if you can share some pointers on 'how to enable software ECC'? > > -- > Kees Cook -- Thanks, Sekhar
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