get backtrace of KVM VM from host

Mark Wielaard
Tue May 22 14:05:00 GMT 2018


On Mon, 2018-05-21 at 10:26 +0200, Justin Cinkelj wrote:
> Is it possible to get stack backtrace into KVM VM from the host side?
> So 
> if I run './stack -p PID' (stack from elfutilfs 
>, I get backtrace of some process. I 
> would like to do the same for VM. I can assume VM will run only a kernel 
> (a unikernel, like OSv or IncludeOS), so most/all debug symbols will be 
> there in a single file, and at least IncludeOS doesnt load any code 
> beside its own kernel.
> I did notice KVM_GET_REGS and KVM_SET_MEMORY_REGION, and at least for 
> > trivial examples (like this 
> provides enough information to track which code was loaded into VM, 
> observe current stack content and registers. I can only guess much more 
> work is required to get similar result with qemu-kvm. Hence I'm asking 
> if this is already implemented.

Providing the registers and memory view inside the KVM VM would be the
first step. elfutils would also need to know the memory/ELF process
layout. For a normal process that would come from e.g. /proc/pid/maps.
Using such a layout eu-stack would then be able to find the unwind
tables and symbols associated with a particular address.

I believe qemu already has an gdb stub that gdb can use to get at the
registers, memory and process layout. Maybe you could adapt that
provide the information needed.



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