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Re: [Qemu-devel] [Bug 1793791] [NEW] Crash with nbd_reply_chunk_iter_rec

From: Eric Blake
Subject: Re: [Qemu-devel] [Bug 1793791] [NEW] Crash with nbd_reply_chunk_iter_receive: Assertion `chunk->flags & NBD_REPLY_FLAG_DONE' failed
Date: Fri, 21 Sep 2018 17:36:28 -0500
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.0

On 9/21/18 11:42 AM, Matthew Schumacher wrote:
Public bug reported:

Qemu version on both sides: 2.12.1
Host A Linux: 4.9.76
Host B Linux: 4.14.67

While calling from Host A:
virsh migrate virtualmachine qemu+ssh://hostB/system --live --undefinesource 
--persistent --verbose --copy-storage-all

Can you show the qemu command line generated by libvirt?

I get a qemu crash with:

2018-09-21 16:12:23.073+0000: 14428: info : virObjectUnref:350 : OBJECT_UNREF: 
qemu-system-x86_64: block/nbd-client.c:606: nbd_reply_chunk_iter_receive: Assertion 
`chunk->flags & NBD_REPLY_FLAG_DONE' failed.

That assertion is the client complaining that it things the server has sent invalid data.

2018-09-21 16:12:41.230+0000: shutting down, reason=crashed
2018-09-21 16:12:52.900+0000: shutting down, reason=failed

It doesn't do it every time, but most of the time.

Hmm - you're running under libvirt, so it's harder to inject command line options, but I'd be interested in figuring out how to enable trace output of nbd_* on both the server and the destination during the block migration portion of 'virsh migrate', to see if the tail of that trace when reproducing the failure gives any more insights into which side is breaking the protocol. I'm also trying to inspect the code in nbd/server.c to see if the server can ever actually send a NBD_REPLY_TYPE_NONE packet without setting the NBD_REPLY_FLAG_DONE bit, which is what the assertion is complaining about.

Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org

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