[libvirt] Executing gdb/gcore as an on_crash action

[Daniel P. Berrange]

On Thu, Dec 21, 2017 at 11:22:48AM +0100, Sergio Lopez wrote:
> Hi,
> 
> Right now, the actions coredump-[destroy|restart] for on_crash work in
> a similar way as "virsh dump", requesting a dump of Guest's memory
> contents to QEMU.
> 
> I think it'd be nice to have the ability for executing gdb/gcore to
> produce a full core dump of QEMU. This would be especially useful for
> debugging hard/soft lockups on the Guest.
> 
> Of course, this should be implemented in a completely optional way,
> checking if the gdb binary is present, to avoid creating a direct
> dependency between libvirt and gdb.
> 
> I'm willing to work on a patch implementing this feature myself, but
> first I'd like to know if this sounds good to you. Also, if it does,
> do you think the behavior of coredump-[destroy|restart] should be
> changed to use gdb if available, falling back to qemuDomainCoreDump if
> it isn't, or just implement another action for on_crash?

I really don't like the idea of running GDB automatically in response to
anything that can be triggered by the guest. GDB is not robust wrt
parsing cores, so a suitably corrupted memory image of QEMU could be
used to abuse GDB. Further, few production deployments of libvirt & QEMU
will ever have GDB installed. Many organizations outright forbid install
of such tools on production machines.

A much simpler way is to just have an "abort" action, which would cause
libvirt to send SIG_ABRT to QEMU.  This should cause QEMU's default
SIG_ABRT handler to kick in which will make the kernel trigger a coredump.
If really wanted, you can then use abrt to catch this coredump.

Regards,
Daniel
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