Re: [Qemu-devel] Crashing in tcp_close

[Stefan Hajnoczi <stefanha@gmail.com>]

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On Thu, Oct 20, 2016 at 10:53:50PM +0100, Brian Candler wrote:

CCing slirp maintainers to get attention on this bug

> I have some reproducible-ish segfaults in qemu 2.7.0 (built from source)
> running under ubuntu 16.04, on a quad-core i7 Mac Mini Server.
> 
> I can reproduce these problems on a different Mac Mini, and I also replaced
> the RAM on mine, so I'm sure it's not hardware related.
> 
> It's somewhat painful to reproduce (taking about 30 minutes each attempt,
> and using a lot of network bandwidth).
> 
> This is using packer (packer.io) to create a VM and then using ansible to do
> a whole load of package installation and provisioning inside that VM.
> packer starts qemu with a user-mode network interface.
> 
> If I part-build the VM, I can continue the build by restarting it under gdb
> and qemu directly at the command line, and get a backtrace. Here's the first
> one:
> 
> Thread 1 "qemu-system-x86" received signal SIGSEGV, Segmentation fault.
> 0x00007ffff6a1bb5b in _int_free (av=0x7ffff6d5fb20 <main_arena>,
> p=<optimised out>, have_lock=0) at malloc.c:4006
> 4006    malloc.c: No such file or directory.
> (gdb) bt
> #0  0x00007ffff6a1bb5b in _int_free (av=0x7ffff6d5fb20 <main_arena>,
> p=<optimised out>, have_lock=0)
>     at malloc.c:4006
> #1  0x00007ffff6a1fabc in __GI___libc_free (mem=<optimised out>) at
> malloc.c:2969
> #2  0x00005555559a6c0f in tcp_close (tp=tp@entry=0x555556621ed0) at
> slirp/tcp_subr.c:334
> #3  0x00005555559a6c8f in tcp_drop (tp=tp@entry=0x555556621ed0,
> err=<optimised out>) at slirp/tcp_subr.c:298
> #4  0x00005555559a816b in tcp_timers (timer=<optimised out>,
> tp=0x555556621ed0) at slirp/tcp_timer.c:179
> #5  tcp_slowtimo (slirp=slirp@entry=0x55555658ecf0) at slirp/tcp_timer.c:89
> #6  0x00005555559a0be8 in slirp_pollfds_poll (pollfds=0x555556531f20,
> select_error=select_error@entry=0)
>     at slirp/slirp.c:576
> #7  0x00005555559d4b0c in main_loop_wait (nonblocking=<optimised out>) at
> main-loop.c:508
> #8  0x000055555573fea1 in main_loop () at vl.c:1908
> #9  main (argc=<optimised out>, argv=<optimised out>, envp=<optimised out>)
> at vl.c:4604
> (gdb)
> 
> So:
> 
> * Is this of interest?

Yes.  Thank you for reporting it.

> * If so, what additional gdb output would you like me to provide?

I wonder if this connection has already been closed/freed before and the
timer fires shortly afterward.  That's just a guess based on the
backtrace.

> * If developers want to reproduce this, let me know and I can probably send
> the VM qcow2 file and/or packer source privately off-list [I need to check
> permission for that]
> 
> Thanks,
> 
> Brian Candler.
> 
> 

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