[libvirt] [PATCH] remoteClientCloseFunc: Don't mangle connection object refcount

Michal Privoznik mprivozn at redhat.com
Thu Sep 3 10:31:46 UTC 2015

Well, in 8ad126e6 we tried to fix a memory corruption problem.
However, the fix was not as good as it could be. I mean, the
commit has one line more than it should. I've noticed this output
just recently:

  # ./run valgrind --leak-check=full --show-reachable=yes ./tools/virsh domblklist gentoo
  ==17019== Memcheck, a memory error detector
  ==17019== Copyright (C) 2002-2013, and GNU GPL'd, by Julian Seward et al.
  ==17019== Using Valgrind-3.10.1 and LibVEX; rerun with -h for copyright info
  ==17019== Command: /home/zippy/work/libvirt/libvirt.git/tools/.libs/virsh domblklist gentoo
  Target     Source
  fda        /var/lib/libvirt/images/fd.img
  vda        /var/lib/libvirt/images/gentoo.qcow2
  hdc        /home/zippy/tmp/install-amd64-minimal-20150402.iso

  ==17019== Thread 2:
  ==17019== Invalid read of size 4
  ==17019==    at 0x4EFF5B4: virObjectUnref (virobject.c:258)
  ==17019==    by 0x5038CFF: remoteClientCloseFunc (remote_driver.c:552)
  ==17019==    by 0x5069D57: virNetClientCloseLocked (virnetclient.c:685)
  ==17019==    by 0x506C848: virNetClientIncomingEvent (virnetclient.c:1852)
  ==17019==    by 0x5082136: virNetSocketEventHandle (virnetsocket.c:1913)
  ==17019==    by 0x4ECD64E: virEventPollDispatchHandles (vireventpoll.c:509)
  ==17019==    by 0x4ECDE02: virEventPollRunOnce (vireventpoll.c:658)
  ==17019==    by 0x4ECBF00: virEventRunDefaultImpl (virevent.c:308)
  ==17019==    by 0x130386: vshEventLoop (vsh.c:1864)
  ==17019==    by 0x4F1EB07: virThreadHelper (virthread.c:206)
  ==17019==    by 0xA8462D3: start_thread (in /lib64/libpthread-2.20.so)
  ==17019==    by 0xAB441FC: clone (in /lib64/libc-2.20.so)
  ==17019==  Address 0x139023f4 is 4 bytes inside a block of size 240 free'd
  ==17019==    at 0x4C2B1F0: free (in /usr/lib64/valgrind/vgpreload_memcheck-amd64-linux.so)
  ==17019==    by 0x4EA8949: virFree (viralloc.c:582)
  ==17019==    by 0x4EFF6D0: virObjectUnref (virobject.c:273)
  ==17019==    by 0x4FE74D6: virConnectClose (libvirt.c:1390)
  ==17019==    by 0x13342A: virshDeinit (virsh.c:406)
  ==17019==    by 0x134A37: main (virsh.c:950)

The problem is, when registering remoteClientCloseFunc(), it's
conn->closeCallback which is ref'd. But in the function itself
it's conn->closeCallback->conn what is unref'd. This is causing
imbalance in reference counting. Moreover, there's no need for
the remote driver to increase/decrease conn refcount since it's
not used anywhere. It's just merely passed to client registered
callback. And for that purpose it's correctly ref'd in
virConnectRegisterCloseCallback() and then unref'd in

Signed-off-by: Michal Privoznik <mprivozn at redhat.com>

Not only this is nice because it fixes a bug by removing some lines, it maybe
needs to be backported all the way down to v1.0.5 main branches.

 src/remote/remote_driver.c | 4 ----
 1 file changed, 4 deletions(-)

diff --git a/src/remote/remote_driver.c b/src/remote/remote_driver.c
index ec26ebe..aca00c0 100644
--- a/src/remote/remote_driver.c
+++ b/src/remote/remote_driver.c
@@ -546,10 +546,6 @@ remoteClientCloseFunc(virNetClientPtr client ATTRIBUTE_UNUSED,
         cbdata->freeCallback = NULL;
-    /* free the connection reference that comes along with the callback
-     * registration */
-    virObjectUnref(cbdata->conn);
 /* helper macro to ease extraction of arguments from the URI */

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