[Qemu-devel] [Bug 1671876] Re: qemu 2.7.0 segfaults in qemu_co_queue_run_restart()

[Mohammed Gamal <m.gamal005@gmail.com>]

The VMs were running with the following arguments
---------------------------------------------------------------------
-m 1024,slots=255,maxmem=256G -M pc-i440fx-2.7 -enable-kvm -nodefconfig -nodefaults -rtc base=utc -netdev tap,ifname=n020133f0895e,id=hostnet6,vhost=on,vhostforce=on,vnet_hdr=off,script=no,downscript=no -device virtio-net-pci,netdev=hostnet6,id=net6,mac=02:01:33:f0:89:5e,bus=pci.0,addr=0x6 -chardev pty,id=charserial0 -device isa-serial,chardev=charserial0,id=serial0 -usb -device usb-tablet,id=input0 -vnc 0.0.0.0:94 -vga qxl -cpu Haswell,+vmx -smp 6,sockets=32,cores=1,maxcpus=64,threads=2 -drive file=/dev/md10,if=none,id=drive-virtio-disk5,format=raw,snapshot=off,aio=native,cache=none -device virtio-blk-pci,bus=pci.0,addr=0x5,drive=drive-virtio-disk5,num-queues=3,id=virtio-disk5,bootindex=1 -S
---------------------------------------------------------------------

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https://bugs.launchpad.net/bugs/1671876

Title:
  qemu 2.7.0 segfaults in qemu_co_queue_run_restart()

Status in QEMU:
  New

Bug description:
  Hi,

  I've been experiencing frequent segfaults lately with qemu 2.7.0
  running Ubuntu 16.04 guests. The crash usually happens in
  qemu_co_queue_run_restart(). I haven't seen this so far with any other
  guests or distros.

  Here is one back trace I obtained from one of the crashing VMs.

  --------------------------------------------------------------------------
  (gdb) bt
  #0  qemu_co_queue_run_restart (co=0x7fba8ff05aa0) at /build/pb-qemu-pssKUp/pb-qemu-2.7.0/util/qemu-coroutine-lock.c:59
  #1  0x000055c1656f39a9 in qemu_coroutine_enter (co=0x7fba8ff05aa0) at /build/pb-qemu-pssKUp/pb-qemu-2.7.0/util/qemu-coroutine.c:119
  #2  0x000055c1656f3e74 in qemu_co_queue_run_restart (co=0x7fba8dd20430) at /build/pb-qemu-pssKUp/pb-qemu-2.7.0/util/qemu-coroutine-lock.c:60
  #3  0x000055c1656f39a9 in qemu_coroutine_enter (co=0x7fba8dd20430) at /build/pb-qemu-pssKUp/pb-qemu-2.7.0/util/qemu-coroutine.c:119
  #4  0x000055c1656f3e74 in qemu_co_queue_run_restart (co=0x7fba8dd14ea0) at /build/pb-qemu-pssKUp/pb-qemu-2.7.0/util/qemu-coroutine-lock.c:60
  #5  0x000055c1656f39a9 in qemu_coroutine_enter (co=0x7fba8dd14ea0) at /build/pb-qemu-pssKUp/pb-qemu-2.7.0/util/qemu-coroutine.c:119
  #6  0x000055c1656f3e74 in qemu_co_queue_run_restart (co=0x7fba80c11dc0) at /build/pb-qemu-pssKUp/pb-qemu-2.7.0/util/qemu-coroutine-lock.c:60
  #7  0x000055c1656f39a9 in qemu_coroutine_enter (co=0x7fba80c11dc0) at /build/pb-qemu-pssKUp/pb-qemu-2.7.0/util/qemu-coroutine.c:119
  #8  0x000055c1656f3e74 in qemu_co_queue_run_restart (co=0x7fba8dd0bd70) at /build/pb-qemu-pssKUp/pb-qemu-2.7.0/util/qemu-coroutine-lock.c:60
  #9  0x000055c1656f39a9 in qemu_coroutine_enter (co=0x7fba8dd0bd70) at /build/pb-qemu-pssKUp/pb-qemu-2.7.0/util/qemu-coroutine.c:119
  #10 0x000055c1656f3fa0 in qemu_co_enter_next (queue=queue@entry=0x55c1669e75e0) at /build/pb-qemu-pssKUp/pb-qemu-2.7.0/util/qemu-coroutine-lock.c:106
  #11 0x000055c165692060 in timer_cb (blk=0x55c1669e7590, is_write=<optimized out>) at /build/pb-qemu-pssKUp/pb-qemu-2.7.0/block/throttle-groups.c:400
  #12 0x000055c16564f615 in timerlist_run_timers (timer_list=0x55c166a53e80) at /build/pb-qemu-pssKUp/pb-qemu-2.7.0/qemu-timer.c:528
  #13 0x000055c16564f679 in timerlistgroup_run_timers (tlg=tlg@entry=0x55c167c81cf8) at /build/pb-qemu-pssKUp/pb-qemu-2.7.0/qemu-timer.c:564
  #14 0x000055c16564ff47 in aio_dispatch (ctx=ctx@entry=0x55c167c81bb0) at /build/pb-qemu-pssKUp/pb-qemu-2.7.0/aio-posix.c:357
  #15 0x000055c1656500e8 in aio_poll (ctx=0x55c167c81bb0, blocking=<optimized out>) at /build/pb-qemu-pssKUp/pb-qemu-2.7.0/aio-posix.c:479
  #16 0x000055c1654b1c79 in iothread_run (opaque=0x55c167c81960) at /build/pb-qemu-pssKUp/pb-qemu-2.7.0/iothread.c:46
  #17 0x00007fbc4b64f0a4 in allocate_stack (stack=<synthetic pointer>, pdp=<synthetic pointer>, attr=0x0) at allocatestack.c:416
  #18 __pthread_create_2_1 (newthread=<error reading variable: Cannot access memory at address 0xffffffffffffff48>, attr=<error reading variable: Cannot access memory at address 0xffffffffffffff40>,
      start_routine=<error reading variable: Cannot access memory at address 0xffffffffffffff58>, arg=<error reading variable: Cannot access memory at address 0xffffffffffffff50>) at pthread_create.c:539
  Backtrace stopped: Cannot access memory at address 0x8
  --------------------------------------------------------------------------

  The code that crashes is this
  --------------------------------------------------------------------------
  void qemu_co_queue_run_restart(Coroutine *co)
  {
      Coroutine *next;

      trace_qemu_co_queue_run_restart(co);
      while ((next = QSIMPLEQ_FIRST(&co->co_queue_wakeup))) {
          QSIMPLEQ_REMOVE_HEAD(&co->co_queue_wakeup, co_queue_next);       <--- Crash occurs here this time
          qemu_coroutine_enter(next);
      }
  }
  --------------------------------------------------------------------------

  Expanding the macro QSIMPLEQ_REMOVE_HEAD gives us
  --------------------------------------------------------------------------
  #define QSIMPLEQ_REMOVE_HEAD(head, field) do {                          \
      if (((head)->sqh_first = (head)->sqh_first->field.sqe_next) == NULL)\
          (head)->sqh_last = &(head)->sqh_first;                          \
  } while (/*CONSTCOND*/0)
  --------------------------------------------------------------------------

  which corrsponds to
  --------------------------------------------------------------------------
  if (((&co->co_queue_wakeup)->sqh_first = (&co->co_queue_wakeup)->sqh_first->co_queue_next.sqe_next) == NULL)\
          (&co->co_queue_wakeup)->sqh_last = &(&co->co_queue_wakeup)->sqh_first;
  --------------------------------------------------------------------------

  Debugging the list we see
  --------------------------------------------------------------------------
  (gdb) print *(&co->co_queue_wakeup->sqh_first)
  $6 = (struct Coroutine *) 0x1000
  (gdb) print *(&co->co_queue_wakeup->sqh_first->co_queue_next)
  Cannot access memory at address 0x1030
  --------------------------------------------------------------------------

  So the data in co->co_queue_wakeup->sqh_first is corrupted and
  represents an invalid address. Any idea why is that?

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