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Date: Thu, 29 Jan 2015 18:22:22 +0100
From: Kashyap Chamarthy <kchamart@redhat.com>
Message-ID: <20150129172222.GC32706@tesla.redhat.com>
References: <20150129162509.GA32706@tesla.redhat.com>
	<20150129164723.GA3860@redhat.com>
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Subject: Re: [Qemu-devel] QEMU segfault: Booting an overlay with
 backing_file over NBD: nbd.c:nbd_receive_request():L756: read failed
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To: "Richard W.M. Jones" <rjones@redhat.com>
Cc: qemu-devel@nongnu.org

On Thu, Jan 29, 2015 at 04:47:23PM +0000, Richard W.M. Jones wrote:
> On Thu, Jan 29, 2015 at 05:25:09PM +0100, Kashyap Chamarthy wrote:
> > A simple reproducer below.
> > 
> > Export a disk image over NBD (I realize port 10809 is default, thought
> > I'd explicitly mention anyhow):
> > 
> >   $ qemu-nbd --f qcow2 -p10809 \
> >         /var/lib/libvirt/images/cirros-0.3.3-x86_64-disk.img -t
> > 
> > 
> > Create an overlay with backing file exported via NBD:
> > 
> >   $ qemu-img create -f qcow2 -F \
> >         nbd -o backing_file=nbd://localhost overlay1.qcow2
> >     Formatting 'overlay1.qcow2', fmt=qcow2 size=41126400 backing_file='nbd://localhost' backing_fmt='nbd' encryption=off cluster_size=65536 lazy_refcounts=off
> > 
> > 
> > Let's attempt to boot the overlay with a minimal QEMU:
> > 
> >   $ qemu-system-x86_64               \
> >      -nographic                      \
> >      -nodefconfig                    \
> >      -nodefaults                     \
> >      -m 2048                         \
> >      -device virtio-scsi-pci,id=scsi \
> >      -device virtio-serial-pci       \
> >      -serial stdio                   \
> >      -drive file=./overlay1.qcow2,format=qcow2,if=virtio,cache=writeback
> >   Segmentation fault (core dumped)
> > 
> > 
> > On the shell where `qemu-nbd` is running, I notice this
> > 
> >   nbd.c:nbd_receive_request():L756: read failed
> 
> This is a "normal error" -- it just means the client dropped the
> connection.

Yeah, deduced so.
 
> You really need to get the stack trace from that core dump to
> debug this further.

I don't see the core dump locally (ABRT or some such not configured),
will re-test this with `gdb` in a little while to get the traces.

-- 
/kashyap