[Bug 1867519] Re: qemu 4.2 segfaults on VF detach

[Christian Ehrhardt ]

At the breaking function we have:

29      void notifier_remove(Notifier *notifier)
30      {
31          QLIST_REMOVE(notifier, node);
32      }


(gdb) p notifier
$1 = (Notifier *) 0x55d2f40c5078
(gdb) p *notifier
$2 = {notify = 0x0, node = {le_next = 0x0, le_prev = 0x0}}

And since QLIST_REMOVE is defined as:
140 #define QLIST_REMOVE(elm, field) do {                                   \   
     
141         if ((elm)->field.le_next != NULL)                               \   
     
142                 (elm)->field.le_next->field.le_prev =                   \   
     
143                     (elm)->field.le_prev;                               \   
     
144         *(elm)->field.le_prev = (elm)->field.le_next;                   \   
     
145 } while (/*CONSTCOND*/0)

(gdb) p (notifier)->node.le_next
$5 = (struct Notifier *) 0x0
(gdb) p &(notifier->node)
$11 = (struct {...} *) 0x55d2f40c5080

There actually is a != NULL check, might it have changed on the fly.
I need to look at it more thoroughly, but it should be enough to recognize a 
known issue.

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https://bugs.launchpad.net/bugs/1867519

Title:
  qemu 4.2 segfaults on VF detach

Status in QEMU:
  Fix Committed
Status in qemu package in Ubuntu:
  Confirmed

Bug description:
  After updating Ubuntu 20.04 to the Beta version, we get the following
  error and the virtual machines stucks when detaching PCI devices using
  virsh command:

  Error:
  error: Failed to detach device from /tmp/vf_interface_attached.xml
  error: internal error: End of file from qemu monitor

  steps to reproduce:
   1. create a VM over Ubuntu 20.04 (5.4.0-14-generic)
   2. attach PCI device to this VM (Mellanox VF for example)
   3. try to detaching  the PCI device using virsh command:
     a. create a pci interface xml file:
          
        <hostdev mode='subsystem' type='pci' managed='yes'>
        <driver name='vfio'/>
        <source>
        <address type='pci' domain='0x0000' bus='0x11' slot='0x00' 
function='0x2' />
        </source>
        </hostdev>
      
     b.  #virsh detach-device <VM-Doman-name> <pci interface xml file>


  - Ubuntu release:
    Description:    Ubuntu Focal Fossa (development branch)
    Release:        20.04

  - Package ver:
    libvirt0:
    Installed: 6.0.0-0ubuntu3
    Candidate: 6.0.0-0ubuntu5
    Version table:
       6.0.0-0ubuntu5 500
          500 http://il.archive.ubuntu.com/ubuntu focal/main amd64 Packages
   *** 6.0.0-0ubuntu3 100
          100 /var/lib/dpkg/status

  - What you expected to happen: 
    PCI device detached without any errors.

  - What happened instead:
    getting the errors above and he VM stuck

  additional info:
  after downgrading the libvirt0 package and all the dependent packages to 5.4 
the previous, version, seems that the issue disappeared

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