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 Thu, Jan 29, 2015 at 06:22:22PM +0100, Kashyap Chamarthy wrote:
> On Thu, Jan 29, 2015 at 04:47:23PM +0000, Richard W.M. Jones wrote:
> > On Thu, Jan 29, 2015 at 05:25:09PM +0100, Kashyap Chamarthy wrote:

[. . .]

> > > On the shell where `qemu-nbd` is running, I notice this
> > > 
> > >   nbd.c:nbd_receive_request():L756: read failed
> > 
>  
> > You really need to get the stack trace from that core dump to
> > debug this further.
> 
> I don't see the core dump locally (ABRT or some such not configured),
> will re-test this with `gdb` in a little while to get the traces.

Okay, now I had the coredump, and ran GDB ('bt full') with it[1] with
qemu-debuginfo files installed. Not sure if it has inough info, as I
didn't install the ~ 3.6 GB of other missing debuginfo RPMs (this is my
primary laptop, I can replicate this test on a different machine with
the missing debuginfo files if needed).

Next, I tried the external wrapper technique you once wrote about[1], to
run QEMU under `gdbserver`, here's the result:

  $ gdb
  . . .

  (gdb) file /usr/bin/qemu-system-x86_64 
  Reading symbols from /usr/bin/qemu-system-x86_64...Reading symbols from /usr/lib/debug/usr/bin/qemu-system-x86_64.debug...done.
  done.
  (gdb) target remote tcp::1234
  Remote debugging using tcp::1234
  Reading symbols from /lib64/ld-linux-x86-64.so.2...Reading symbols from /usr/lib/debug/usr/lib64/ld-2.20.so.debug...done.
  done.
  Loaded symbols for /lib64/ld-linux-x86-64.so.2
  0x00007ffff7ddbcf0 in _start () from /lib64/ld-linux-x86-64.so.2
  (gdb) cont
  Continuing.
  
  Program received signal SIGSEGV, Segmentation fault.
  0x0000555555872159 in aio_set_fd_handler (ctx=ctx@entry=0x0, fd=8, io_read=io_read@entry=0x5555558a5470 <nbd_reply_ready>, io_write=io_write@entry=0x5555558a4d80 <nbd_restart_write>, 
      opaque=opaque@entry=0x5555562605e0) at aio-posix.c:50

Does this help?


Precise steps of what I did:

(1) Use the below wrapper script with `gdbserver`
----------------------------------------------------------------
$ cat /export/qemu-wrapper.sh 
#!/bin/bash -

if ! echo "$@" | grep -sqE -- '-help|-version|-device \?' ; then
      gdbserver="gdbserver :1234"
      fi

      exec $gdbserver /usr/bin/qemu-system-x86_64 "$@"
----------------------------------------------------------------

(2) On shell #1, export the backing file over QEMU NBD:

    $ qemu-nbd -f qcow2 -p10809 \
        /var/lib/libvirt/images/f21vm.qcow2 -t


(3) On shell #3, Invoke the QEMU wrapper script:

    $ ./qemu-wrapper.sh             \
    -nographic                      \
    -nodefconfig                    \
    -nodefaults                     \
    -m 2048                         \
    -device virtio-scsi-pci,id=scsi \
    -device virtio-serial-pci       \
    -serial stdio                   \
    -drive file=./overlay1-f21vm.qcow2,format=qcow2,if=virtio,cache=writeback
    Process /usr/bin/qemu-system-x86_64 created; pid = 1944
    Listening on port 1234


(4) On shell #3, run GDB (output is above).


[1] https://kashyapc.fedorapeople.org/virt/qemu-nbd-test/stack-traces-from-coredump.txt

-- 
/kashyap