Re: Bug with Windows tap network when running qemu-system-ppc with Mac OS 9 guest

[BALATON Zoltan]

On Fri, 26 Mar 2021, Howard Spoelstra wrote:
> On Fri, Mar 26, 2021 at 2:50 AM Bin Meng <bmeng.cn@gmail.com> wrote:
> > Hi Howard,
> >
> > On Fri, Mar 26, 2021 at 1:35 AM Peter Maydell <peter.maydell@linaro.org> wrote:
> > > (adding the relevant people to the cc list)
> > >
> > > On Thu, 25 Mar 2021 at 17:26, Howard Spoelstra <hsp.cat7@gmail.com> wrote:
> > > > Hi,
> > > >
> > > > When running qemu-system-ppc with Mac OS guest, the guest crashes when
> > > > using a tap network connection. Openvpn 2.4.9-I601-win10 is installed
> > > > with TAP-Windows 9.24.2. A tap connection called TapQemu is bridged
> > > > with the default ethernet connection. It gets activated when I start
> > > > qemu.
> > > >
> > > > To reproduce, compile qemu-system-ppc from current source and run:
> > > >
> > > > qemu-system-ppc.exe ^
> > > > -L pc-bios ^
> > > > -M mac99 ^
> > > > -m 128 ^
> > > > -sdl -serial stdio ^
> > > > -boot c ^
> > > > -drive file=C:\Mac-disks\9.2.img,format=raw,media=disk ^
> > > > -device sungem,netdev=network01 -netdev tap,ifname=TapQemu,id=network01
> > > >
> > > > I bisected to the commit below. Thanks for looking into this.
> >
> > Thanks for reporting.
> >
> > Can you please provide some further information:
> >
> > 1. Does "-net user" work on Windows?
> > 2. If running QEMU under Linux, does "-net tap" or "-net user" work?
> >
> > Regards,
> > Bin
>
> Hello Bin,
>
> Thanks for getting back to me. I forgot to mention that reverting the
> above patch restores functionality. And that other applications using
> the same tap device work correctly.
> In answer to your questions:
>
> 1. Yes, slirp works on Windows 10 with this setup.
> 2. Yes, in Linux both tap and slirp work.
>
> My Windows build is done with a fully up to date msys2 installation.
>
> I tried to debug in Windows:
> (gdb) run
> Starting program: c:\qemu-master-msys2\qemu-system-ppc.exe -L pc-bios
> -M mac99 -m 128 -sdl -serial stdio -boot c -drive
> "file=C:\Mac-disks\9.2-usb-pci-ddk.img,format=raw,media=disk" -device
> "sungem,netdev=network01" -netdev "tap,ifname=TapQemu,id=network01" -S
> [New Thread 13304.0x1f00]
> [New Thread 13304.0x2f84]
> [New Thread 13304.0x3524]
> [New Thread 13304.0x2b8c]
> [New Thread 13304.0x368c]
> [New Thread 13304.0x3668]
> [New Thread 13304.0xf4c]
> [New Thread 13304.0x49c]
> [New Thread 13304.0x1d4c]
> [New Thread 13304.0x7fc]
> [Thread 13304.0x7fc exited with code 0]
> [New Thread 13304.0x357c]
> [New Thread 13304.0x7c0]
> [New Thread 13304.0x3564]
> [New Thread 13304.0x26f4]
> [New Thread 13304.0x2f68]
>
> Program received signal SIGSEGV, Segmentation fault.
> 0x00007ffb9edea991 in ?? () from c:\qemu-master-msys2\libglib-2.0-0.dll
> (gdb) bt
> #0  0x00007ffb9edea991 in ?? () from c:\qemu-master-msys2\libglib-2.0-0.dll
> #1  0x000800000480bf50 in ?? ()
> Backtrace stopped: previous frame inner to this frame (corrupt stack?)
> (gdb)
>
> Even before I could attach to the process.

If you run QEMU under gdb you don't have to attach to it but to get a 
meaningful backtrace you should configure and compile QEMU with 
--enable-debug (this will make it run slower so not recommended normally 
but for debugging that would be needed). If the stack is really corrupted 
then you may not get a useful backtrace or it may be a problem with gdb on 
Windows. I've found that gdb on Windows works for simple things but could 
give bad results for more complex stuff. WinDbg may be better but it's 
harder to use (needs some registry change I think to enable core dumps 
then you could open and analyze core dumps with it or it should be able 
to run command directly but I don't know how that works).

Another idea: maybe you could check other threads in gdb. Not sure if that 
would reveal anything but may worth a try. I think the commands you need 
are "info threads" and "apply all bt" or something similar.

Regards,
BALATON Zoltan