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From: Michal Svoboda <pht@spatium.org>
To: qemu-devel@nongnu.org
Subject: [Qemu-devel] [Bug 1054180] [NEW] DNS activity in slirp (user networking) mode	quickly depletes file descriptors and crashes qemu
Date: Fri, 21 Sep 2012 16:13:23 -0000	[thread overview]
Message-ID: <20120921161324.22633.34633.malonedeb@chaenomeles.canonical.com> (raw)
In-Reply-To: 20120921161324.22633.34633.malonedeb@chaenomeles.canonical.com

Public bug reported:

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.

** Affects: qemu
     Importance: Undecided
         Status: New

-- 
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https://bugs.launchpad.net/bugs/1054180

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

Status in QEMU:
  New

Bug description:
  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.

To manage notifications about this bug go to:
https://bugs.launchpad.net/qemu/+bug/1054180/+subscriptions

       reply	other threads:[~2012-09-21 16:20 UTC|newest]

Thread overview: 4+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2012-09-21 16:13 Michal Svoboda [this message]
2017-08-21  7:47 ` [Qemu-devel] [Bug 1054180] Re: DNS activity in slirp (user networking) mode quickly depletes file descriptors and crashes qemu Thomas Huth
2017-08-21  7:48 ` Thomas Huth
2017-10-21  4:17 ` Launchpad Bug Tracker

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