Re: [Qemu-devel] QEMU segfault: Booting an overlay with backing_file over NBD: nbd.c:nbd_receive_request():L756: read failed

["Richard W.M. Jones" <rjones@redhat.com>]

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.

You really need to get the stack trace from that core dump to
debug this further.

Rich.

> Haven't investigated further with GDB, thought I'd bring it up here
> first.
> 
> 
> Versions
> --------
> 
>   $ rpm -q qemu; uname -r
>   qemu-2.1.2-7.fc21.x86_64
>   3.17.8-300.fc21.x86_64
> 
> -- 
> /kashyap

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