Re: [Qemu-devel] QEMU segfault: Booting an overlay with backing_file over NBD: nbd.c:nbd_receive_request():L756: read failed

[Max Reitz]

On 2015-01-30 at 13:41, Kashyap Chamarthy wrote:
> On Fri, Jan 30, 2015 at 06:15:21PM +0100, Kevin Wolf wrote:
> > Am 29.01.2015 um 17:25 hat Kashyap Chamarthy geschrieben:
> > >    $ 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?
> Just tested from git, and I can still reproduce it.
>
> That's the commit I'm at:
>
>    $ git describe
>    v2.2.0-682-g16017c4
>
>
> Run the NBD server, from git:
>
>    $ /home/kashyapc/build/qemu/qemu-nbd -f qcow2 \
>        -p10809 ./f21vm.qcow2 -t
>
>
> Create the overlay:
>
>    $ /home/kashyapc/build/qemu/qemu-img create \
>        -f qcow2 -F nbd -o backing_file=nbd://localhost overlay2-of-f21vm.qcow2
>    Segmentation fault (core dumped)

You want to use -F raw. The file format is raw, not nbd (nbd is the 
protocol over which the data is read, which is in format raw).

Anyway, -F nbd shouldn't result in a segfault. One way to prevent this 
is to check whether the backing file format specified (or any format 
given to qemu-img in general) is a real format or the name of a protocol 
driver and then error out if it's the latter; but that would be more of 
a hotfix.

Kevin, Stefan: The real problem is that block/nbd.c stores a 
BDRVNBDState object in bs->opaque and passes &BDRVNBDState.client (an 
NbdClientSession object) to the block/nbd-client.c functions. Those 
functions then receive the BDS pointer from client->bs. If an NBD BDS is 
a root BDS (as in this case), at some point a bdrv_swap() may happen 
(and it does happen here) which leads to ((BDRVNBDState 
*)bs->opaque)->client.bs != bs, and that's where the segfault comes from 
(bdrv_get_aio_context() returns NULL).

One way to fix this real problem is to remove the BDS pointer from the 
NbdClientSession and to always pass the BDS explicitly to the 
block/nbd-client.c functions; the other is to always update the BDS 
pointer in NbdClientSession in block/nbd.c. I'll try the former, and if 
it doesn't work, will do the latter (if you don't object).

Max

> Creating the overlay from the  git-compiled `qemu-img` binary fails.
>
> So, let's create the overlay using the `qemu-img` binary from the system
> (RPM version noted above) and boot the overlay from the just compiled
> QEMU x86_64 binary from git, still core dumps:
>
>    $ /home/kashyapc/build/qemu/x86_64-softmmu/qemu-system-x86_64 \
>        -nographic                      \
>        -nodefconfig                    \
>        -nodefaults                     \
>        -m 2048                         \
>        -device virtio-scsi-pci,id=scsi \
>        -device virtio-serial-pci       \
>        -serial stdio                   \
>        -drive file=./overlay2-f21vm.qcow2,format=qcow2,if=virtio,cache=writeback
>    Segmentation fault (core dumped)
>
>
> PS: I'm traveling, so I'll be a little slow to respond here, but can
> provide more debugging info from the coredump of `qemu-img` binary as I
> have access to a real computer.