* [Qemu-devel] QEMU segfault: Booting an overlay with backing_file over NBD: nbd.c:nbd_receive_request():L756: read failed
@ 2015-01-29 16:25 Kashyap Chamarthy
2015-01-29 16:47 ` Richard W.M. Jones
2015-01-30 17:15 ` Kevin Wolf
0 siblings, 2 replies; 9+ messages in thread
From: Kashyap Chamarthy @ 2015-01-29 16:25 UTC (permalink / raw)
To: qemu-devel
A simple reproducer below.
Export a disk image over NBD (I realize port 10809 is default, thought
I'd explicitly mention anyhow):
$ qemu-nbd --f qcow2 -p10809 \
/var/lib/libvirt/images/cirros-0.3.3-x86_64-disk.img -t
Create an overlay with backing file exported via NBD:
$ qemu-img create -f qcow2 -F \
nbd -o backing_file=nbd://localhost overlay1.qcow2
Formatting 'overlay1.qcow2', fmt=qcow2 size=41126400 backing_file='nbd://localhost' backing_fmt='nbd' encryption=off cluster_size=65536 lazy_refcounts=off
Let's attempt to boot the overlay with a minimal QEMU:
$ 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
--
/kashyap
^ permalink raw reply [flat|nested] 9+ messages in thread
* Re: [Qemu-devel] QEMU segfault: Booting an overlay with backing_file over NBD: nbd.c:nbd_receive_request():L756: read failed
2015-01-29 16:25 [Qemu-devel] QEMU segfault: Booting an overlay with backing_file over NBD: nbd.c:nbd_receive_request():L756: read failed Kashyap Chamarthy
@ 2015-01-29 16:47 ` Richard W.M. Jones
2015-01-29 17:22 ` Kashyap Chamarthy
2015-01-30 17:15 ` Kevin Wolf
1 sibling, 1 reply; 9+ messages in thread
From: Richard W.M. Jones @ 2015-01-29 16:47 UTC (permalink / raw)
To: Kashyap Chamarthy; +Cc: qemu-devel
On Thu, Jan 29, 2015 at 05:25:09PM +0100, Kashyap Chamarthy wrote:
> A simple reproducer below.
>
> Export a disk image over NBD (I realize port 10809 is default, thought
> I'd explicitly mention anyhow):
>
> $ qemu-nbd --f qcow2 -p10809 \
> /var/lib/libvirt/images/cirros-0.3.3-x86_64-disk.img -t
>
>
> Create an overlay with backing file exported via NBD:
>
> $ qemu-img create -f qcow2 -F \
> nbd -o backing_file=nbd://localhost overlay1.qcow2
> Formatting 'overlay1.qcow2', fmt=qcow2 size=41126400 backing_file='nbd://localhost' backing_fmt='nbd' encryption=off cluster_size=65536 lazy_refcounts=off
>
>
> Let's attempt to boot the overlay with a minimal QEMU:
>
> $ 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
This is a "normal error" -- it just means the client dropped the
connection.
You really need to get the stack trace from that core dump to
debug this further.
Rich.
> 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
>
> --
> /kashyap
--
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
Read my programming and virtualization blog: http://rwmj.wordpress.com
Fedora Windows cross-compiler. Compile Windows programs, test, and
build Windows installers. Over 100 libraries supported.
http://fedoraproject.org/wiki/MinGW
^ permalink raw reply [flat|nested] 9+ messages in thread
* Re: [Qemu-devel] QEMU segfault: Booting an overlay with backing_file over NBD: nbd.c:nbd_receive_request():L756: read failed
2015-01-29 16:47 ` Richard W.M. Jones
@ 2015-01-29 17:22 ` Kashyap Chamarthy
2015-01-29 23:33 ` Kashyap Chamarthy
0 siblings, 1 reply; 9+ messages in thread
From: Kashyap Chamarthy @ 2015-01-29 17:22 UTC (permalink / raw)
To: Richard W.M. Jones; +Cc: qemu-devel
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:
> > A simple reproducer below.
> >
> > Export a disk image over NBD (I realize port 10809 is default, thought
> > I'd explicitly mention anyhow):
> >
> > $ qemu-nbd --f qcow2 -p10809 \
> > /var/lib/libvirt/images/cirros-0.3.3-x86_64-disk.img -t
> >
> >
> > Create an overlay with backing file exported via NBD:
> >
> > $ qemu-img create -f qcow2 -F \
> > nbd -o backing_file=nbd://localhost overlay1.qcow2
> > Formatting 'overlay1.qcow2', fmt=qcow2 size=41126400 backing_file='nbd://localhost' backing_fmt='nbd' encryption=off cluster_size=65536 lazy_refcounts=off
> >
> >
> > Let's attempt to boot the overlay with a minimal QEMU:
> >
> > $ 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
>
> This is a "normal error" -- it just means the client dropped the
> connection.
Yeah, deduced so.
> 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.
--
/kashyap
^ permalink raw reply [flat|nested] 9+ messages in thread
* Re: [Qemu-devel] QEMU segfault: Booting an overlay with backing_file over NBD: nbd.c:nbd_receive_request():L756: read failed
2015-01-29 17:22 ` Kashyap Chamarthy
@ 2015-01-29 23:33 ` Kashyap Chamarthy
0 siblings, 0 replies; 9+ messages in thread
From: Kashyap Chamarthy @ 2015-01-29 23:33 UTC (permalink / raw)
To: Richard W.M. Jones; +Cc: qemu-devel
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
^ permalink raw reply [flat|nested] 9+ messages in thread
* Re: [Qemu-devel] QEMU segfault: Booting an overlay with backing_file over NBD: nbd.c:nbd_receive_request():L756: read failed
2015-01-29 16:25 [Qemu-devel] QEMU segfault: Booting an overlay with backing_file over NBD: nbd.c:nbd_receive_request():L756: read failed Kashyap Chamarthy
2015-01-29 16:47 ` Richard W.M. Jones
@ 2015-01-30 17:15 ` Kevin Wolf
2015-01-30 18:41 ` Kashyap Chamarthy
1 sibling, 1 reply; 9+ messages in thread
From: Kevin Wolf @ 2015-01-30 17:15 UTC (permalink / raw)
To: Kashyap Chamarthy; +Cc: qemu-devel, stefanha
Am 29.01.2015 um 17:25 hat Kashyap Chamarthy geschrieben:
> A simple reproducer below.
>
> Export a disk image over NBD (I realize port 10809 is default, thought
> I'd explicitly mention anyhow):
>
> $ qemu-nbd --f qcow2 -p10809 \
> /var/lib/libvirt/images/cirros-0.3.3-x86_64-disk.img -t
>
>
> Create an overlay with backing file exported via NBD:
>
> $ qemu-img create -f qcow2 -F \
> nbd -o backing_file=nbd://localhost overlay1.qcow2
> Formatting 'overlay1.qcow2', fmt=qcow2 size=41126400 backing_file='nbd://localhost' backing_fmt='nbd' encryption=off cluster_size=65536 lazy_refcounts=off
>
>
> Let's attempt to boot the overlay with a minimal QEMU:
>
> $ 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?
Kevin
^ permalink raw reply [flat|nested] 9+ messages in thread
* Re: [Qemu-devel] QEMU segfault: Booting an overlay with backing_file over NBD: nbd.c:nbd_receive_request():L756: read failed
2015-01-30 17:15 ` Kevin Wolf
@ 2015-01-30 18:41 ` Kashyap Chamarthy
2015-01-30 19:32 ` Max Reitz
0 siblings, 1 reply; 9+ messages in thread
From: Kashyap Chamarthy @ 2015-01-30 18:41 UTC (permalink / raw)
To: Kevin Wolf; +Cc: qemu-devel, stefanha
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)
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.
--
/kashyap
^ permalink raw reply [flat|nested] 9+ messages in thread
* Re: [Qemu-devel] QEMU segfault: Booting an overlay with backing_file over NBD: nbd.c:nbd_receive_request():L756: read failed
2015-01-30 18:41 ` Kashyap Chamarthy
@ 2015-01-30 19:32 ` Max Reitz
2015-01-30 22:13 ` Kashyap Chamarthy
2015-02-02 8:14 ` Stefan Hajnoczi
0 siblings, 2 replies; 9+ messages in thread
From: Max Reitz @ 2015-01-30 19:32 UTC (permalink / raw)
To: Kashyap Chamarthy, Kevin Wolf; +Cc: qemu-devel, stefanha
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.
>
>
^ permalink raw reply [flat|nested] 9+ messages in thread
* Re: [Qemu-devel] QEMU segfault: Booting an overlay with backing_file over NBD: nbd.c:nbd_receive_request():L756: read failed
2015-01-30 19:32 ` Max Reitz
@ 2015-01-30 22:13 ` Kashyap Chamarthy
2015-02-02 8:14 ` Stefan Hajnoczi
1 sibling, 0 replies; 9+ messages in thread
From: Kashyap Chamarthy @ 2015-01-30 22:13 UTC (permalink / raw)
To: Max Reitz; +Cc: Kevin Wolf, qemu-devel, stefanha
On Fri, Jan 30, 2015 at 02:32:25PM -0500, Max Reitz wrote:
> 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:
[. . .]
> >>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).
Noted, thanks for this detail. However, the `qemu-img` binary from the
system (version noted earlier in the thread) honors the "-F nbd" option
just fine:
$ qemu-img create -f qcow2 -F nbd \
-o backing_file=nbd://localhost overlay3-of-f21vm.qcow2
Formatting 'overlay3-of-f21vm.qcow2', fmt=qcow2 size=42949672960 backing_file='nbd://localhost' backing_fmt='nbd' encryption=off cluster_size=65536 lazy_refcounts=off
Then, the segfault with the git-compiled `qemu-img` binary seems like
some kind of an incorrect "regression" (if you could call it so).
Anyhow, the `qemu-img` from git works correctly as per your reasoning:
$ /home/kashyapc/build/qemu/qemu-img create \
-f qcow2 -F raw -o backing_file=nbd://localhost
> 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).
Thank you for investigating.
--
/kashyap
^ permalink raw reply [flat|nested] 9+ messages in thread
* Re: [Qemu-devel] QEMU segfault: Booting an overlay with backing_file over NBD: nbd.c:nbd_receive_request():L756: read failed
2015-01-30 19:32 ` Max Reitz
2015-01-30 22:13 ` Kashyap Chamarthy
@ 2015-02-02 8:14 ` Stefan Hajnoczi
1 sibling, 0 replies; 9+ messages in thread
From: Stefan Hajnoczi @ 2015-02-02 8:14 UTC (permalink / raw)
To: Max Reitz; +Cc: Kevin Wolf, qemu-devel, Kashyap Chamarthy
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On Fri, Jan 30, 2015 at 02:32:25PM -0500, Max Reitz wrote:
> 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).
Sounds good.
On a related note I asked John Snow to look at QED and vvfat's
.bdrv_rebind() usage. I think we can get rid of that API after
propagating BlockDriverState *bs arguments to QED and vvfat functions.
Stefan
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^ permalink raw reply [flat|nested] 9+ messages in thread
end of thread, other threads:[~2015-02-02 8:14 UTC | newest]
Thread overview: 9+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2015-01-29 16:25 [Qemu-devel] QEMU segfault: Booting an overlay with backing_file over NBD: nbd.c:nbd_receive_request():L756: read failed Kashyap Chamarthy
2015-01-29 16:47 ` Richard W.M. Jones
2015-01-29 17:22 ` Kashyap Chamarthy
2015-01-29 23:33 ` Kashyap Chamarthy
2015-01-30 17:15 ` Kevin Wolf
2015-01-30 18:41 ` Kashyap Chamarthy
2015-01-30 19:32 ` Max Reitz
2015-01-30 22:13 ` Kashyap Chamarthy
2015-02-02 8:14 ` Stefan Hajnoczi
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