DNS activity in slirp (user networking) mode quickly depletes file descriptors and crashes qemu

[Michal Svoboda]

Hi, we have encountered quite some trouble with filedescriptor depletion of the qemu process. We have figured out that it can be demonstrated easily by doing a lot of DNS queries inside the VM -- in our real world scenario this is caused by running centos network install with a fast mirror.

This situation is further problematic because qemu can't handle fd depletion very well:
1) if ulimit is 1024 then qemu hangs in infinite loop whenever it tries to open the 1025th fd
2) setting ulimit >1024 does not help that much because qemu uses select and max. fd set size is 1024 per default => qemu crashes because of buffer overflow in select()
3) setting ulimit > 1024 AND recompiling with large enough fd set size AND disabling gcc's fortify source seems to work, but that's really just a hot-fix

The problem can be replicated quite easily by running something like

while :; do echo >/dev/udp/10.0.2.3/53; done

inside a Linux VM -- crash comes very soon.

This problem is present in current qemu (1.2.0) and in earlier as well.

[Thomas Huth (th-huth)]

Triaging old bug tickets ... can you still reproduce this problem with the latest version of QEMU (currently v2.9 or a release candidate of 2.10)?

[Thomas Huth (th-huth)]

Also could you please provide the exact command line that you use to start QEMU?

[Launchpad Janitor (janitor)]

[Expired for QEMU because there has been no activity for 60 days.]