Re: [Qemu-devel] [PATCH] vhost-user: return if no net clients found

[Chen Hanxiao]

At 2016-09-01 20:52:44, "Marc-André Lureau" <address@hidden> wrote:
>Hi
>
>On Thu, Sep 1, 2016 at 4:00 PM Chen Hanxiao <address@hidden> wrote:
>
>>
>> Hi, here is the backtrace:
>>
>> #0  net_vhost_user_event (opaque=0x7fc2f6893be0, event=5) at
>> net/vhost-user.c:196
>> #1  0x00007fc2f4ebfb2b in tcp_chr_disconnect (chr=0x7fc2f68cc400) at
>> qemu-char.c:2837
>> #2  0x00007fc2f4ebfba9 in tcp_chr_sync_read (chr=0x7fc2f68cc400,
>> buf=<optimized out>, len=<optimized out>) at qemu-char.c:2888
>> #3  0x00007fc2f4ec106d in qemu_chr_fe_read_all (s=0x7fc2f68cc400,
>> address@hidden "", address@hidden) at qemu-char.c:264
>> #4  0x00007fc2f4f9a43a in net_vhost_user_watch (chan=<optimized out>,
>> cond=<optimized out>, opaque=<optimized out>) at net/vhost-user.c:190
>> #5  0x00007fc2f321999a in g_main_context_dispatch () from
>> /lib64/libglib-2.0.so.0
>> #6  0x00007fc2f4fd8fe8 in glib_pollfds_poll () at main-loop.c:209
>> #7  os_host_main_loop_wait (timeout=<optimized out>) at main-loop.c:254
>> #8  main_loop_wait (nonblocking=<optimized out>) at main-loop.c:503
>> #9  0x00007fc2f4dd7b1e in main_loop () at vl.c:1818
>> #10 main (argc=<optimized out>, argv=<optimized out>, envp=<optimized
>> out>) at vl.c:4394
>>
>>
>> Program received signal SIGSEGV, Segmentation fault.
>> net_vhost_user_event (opaque=0x7fc2f6893be0, event=5) at
>> net/vhost-user.c:207
>> 207    trace_vhost_user_event(s->chr->label, event);
>>
>>
>thanks for the backtrace, that helps
>
>However, I fail to understand how that can happen, as there has to be at
>least one net_client to start qemu with vhost-user and that callback must
>have at least the first netclient still around because the opaque pointer
>is shared with the netclient struct. So it looks like something destroyed
>the netclient before the callback, and in this case, the opaque pointer is
>invalid, and things are going all wrong. But it can't be host-net-remove,
>since the net-client is not on a registered hub.

The call back give qemu_find_net_clients_except id == 'filename'.
But could not find a netclient match.
Then ncs[i] did not get a valid net client, then we will get a seg fault.

>
>Could you try to find a simple reproducer using qemu only?

I'll try.

Regards,
- Chen

>
>thanks
>
>-- 
>Marc-André Lureau