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

[Bin Meng]

On Fri, Mar 26, 2021 at 4:49 PM Jason Wang <jasowang@redhat.com> wrote:
>
>
> 在 2021/3/26 下午4:21, BALATON Zoltan 写道:
> > 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
> >
>
> It looks to me the patch tires to recycle a temporary buffer to tap thread.
>
> Please try to attached fix to see it if works.

Yep, good catch, thanks! This patch looks correct to me.

Regards,
Bin