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August 2010

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[PATCH] Enable coredumps to be captured from the appliance (RHBZ#619334).
by Richard W.M. Jones 02 Aug '10

02 Aug '10

https://bugzilla.redhat.com/show_bug.cgi?id=619334 This is a slightly unsatisfactory patch which allows coredumps to be captured when they occur inside the appliance. You can capture coredumps by doing: export LIBGUESTFS_COREDUMP=/sysroot/core.%t.%p.%e or equivalently: g.set_coredump ("/sysroot/core.%t.%p.%e") or variations thereof, see the manual page. When a coredump occurs, a core file will be dropped into the root directory of the currently mounted guest disk. If it is guestfsd which crashes, then the appliance will exit, but I have checked that the coredump is written and synched first. You can then copy this out of the guest disk using guestfish and debug using gdb: $ guestfish --ro -a guest_disk -m /dev/vg/lv_root ><fs> ll / ><fs> download /core.xxx.yyy.zzz core $ gdb daemon/guestfsd core http://fedoraproject.org/wiki/StackTraces#Obtaining_a_stack_trace_from_a_co… Clearly you need to have a writable guest disk mounted for this to work, and you have to not care too much about having large core files written to it. Ideally we would have liked to use a special capture disk, but it turns out that the kernel contains code which disallows using a /dev device (or any non-regular file) for coredumps: http://lxr.linux.no/linux+v2.6.34.1/fs/exec.c#L1930 We could do it using a coredump-to-pipe, but I think it would be better to just remove this limitation in the kernel. Another shortcoming is that a coredump isn't a stack trace, and to get a stack trace you really need a copy of the daemon around with debugging enabled. It is possible we could change the RPM packaging to ship this. Luckily the suspected bug in Augeas which we are chasing (RHBZ#613967) happens in aug_init or aug_save when the conditions above are satisfied and we should be able to get a useful coredump. However since I am unable to reproduce this bug at all, I cannot help with this, so it's over to Marek and Matt. Rich. -- Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones virt-p2v converts physical machines to virtual machines. Boot with a live CD or over the network (PXE) and turn machines into Xen guests. http://et.redhat.com/~rjones/virt-p2v

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