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Date: Thu, 29 Jan 2015 17:25:09 +0100
From: Kashyap Chamarthy <kchamart@redhat.com>
Message-ID: <20150129162509.GA32706@tesla.redhat.com>
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Subject: [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: qemu-devel@nongnu.org

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


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