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Date: Fri, 30 Jan 2015 18:15:21 +0100
From: Kevin Wolf <kwolf@redhat.com>
Message-ID: <20150130171521.GD24537@noname.redhat.com>
References: <20150129162509.GA32706@tesla.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: Kashyap Chamarthy <kchamart@redhat.com>
Cc: qemu-devel@nongnu.org, stefanha@redhat.com

Am 29.01.2015 um 17:25 hat Kashyap Chamarthy geschrieben:
> 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

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?

Kevin