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

[Kashyap Chamarthy <kchamart@redhat.com>]

On Fri, Jan 30, 2015 at 06:15:21PM +0100, Kevin Wolf wrote:
> Am 29.01.2015 um 17:25 hat Kashyap Chamarthy geschrieben:

> >   $ 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
> 
> Copying Stefan because he's the master of AIO contexts and it is
> bs->aio_context that becomes NULL. I couldn't see anything obvious.
>
> 
> In the meantime, could you retest on git master?

Just tested from git, and I can still reproduce it.

That's the commit I'm at:

  $ git describe 
  v2.2.0-682-g16017c4


Run the NBD server, from git:

  $ /home/kashyapc/build/qemu/qemu-nbd -f qcow2 \
      -p10809 ./f21vm.qcow2 -t


Create the overlay:

  $ /home/kashyapc/build/qemu/qemu-img create \
      -f qcow2 -F nbd -o backing_file=nbd://localhost overlay2-of-f21vm.qcow2
  Segmentation fault (core dumped)

Creating the overlay from the  git-compiled `qemu-img` binary fails.

So, let's create the overlay using the `qemu-img` binary from the system
(RPM version noted above) and boot the overlay from the just compiled
QEMU x86_64 binary from git, still core dumps:

  $ /home/kashyapc/build/qemu/x86_64-softmmu/qemu-system-x86_64 \
      -nographic                      \
      -nodefconfig                    \
      -nodefaults                     \
      -m 2048                         \
      -device virtio-scsi-pci,id=scsi \
      -device virtio-serial-pci       \
      -serial stdio                   \
      -drive file=./overlay2-f21vm.qcow2,format=qcow2,if=virtio,cache=writeback
  Segmentation fault (core dumped)


PS: I'm traveling, so I'll be a little slow to respond here, but can
provide more debugging info from the coredump of `qemu-img` binary as I
have access to a real computer.


-- 
/kashyap