Bug 1868271 - from time to time virsh net-dhcp-leases <net> does not show all leases [rhel-8.2.1.z]
Summary: from time to time virsh net-dhcp-leases <net> does not show all leases [rhel-...
Alias: None
Product: Red Hat Enterprise Linux Advanced Virtualization
Classification: Red Hat
Component: libvirt
Version: 8.2
Hardware: Unspecified
OS: Unspecified
Target Milestone: rc
: 8.3
Assignee: Michal Privoznik
QA Contact: yalzhang@redhat.com
Depends On: 1840307
TreeView+ depends on / blocked
Reported: 2020-08-12 07:57 UTC by Oneata Mircea Teodor
Modified: 2020-11-02 06:56 UTC (History)
18 users (show)

Fixed In Version: libvirt-6.0.0-25.2.el8
Doc Type: Bug Fix
Doc Text:
Cause: When libvirt starts a NATed virtual network (that is with dnsmasq), it configures dnsmasq so that so called leases helper is ran on DHCP traffic. This helper binary updates a JSON file corresponding to the network, e.g. on new lease it adds IP address into the file, on lease timeout it removes it. The file is then used by 'virsh net-dhcp-leases' to report assigned IP addresses. However, to avoid two helpers writing the same file at once (which will certainly result in corrupted JSON) a PID file was used - the leases helper will try to acquire it (note that only one process can lock a file = pid file at once). Consequence: Due to a bug in implementation, the acquisition of the PID file was a single attempt without any wait only => if two dnsmasq-s from two different network decided to run the binary at once, the first process will acquire the PID file and starts updating a JSON file, while the other will fail to acquire it and exits immediately without updating the JSON file => the assigned IP address is not recorded and thus not reported later in virsh (or corresponding C API). Fix: The fix consists of making the PID file acquisition wait until the other process releases it. In code it's passing true instead of false to a function. Result: In the end, since the PID file acquisition waits, lease helpers will mutually exclude with each other, and wait for the data to be updated in the JSON file and thus virsh reports IP addresses correctly, without missing one.
Clone Of: 1840307
Last Closed: 2020-09-01 09:38:55 UTC
Type: Bug
Target Upstream Version:

Attachments (Terms of Use)

System ID Priority Status Summary Last Updated
Red Hat Product Errata RHSA-2020:3586 None None None 2020-09-01 09:39:45 UTC

Comment 7 yalzhang@redhat.com 2020-08-22 12:51:58 UTC
Reproduce the failure on libvirt-6.0.0-25.module+el8.2.1+7154+47ffd890.x86_64 with the test steps in https://bugzilla.redhat.com/show_bug.cgi?id=1840307#c9
And test on libvirt-6.0.0-25.2.module+el8.2.1+7722+a9e38cf3.x86_64 with the same steps, run 20 times, and no failures, the result is as expected.

Comment 9 errata-xmlrpc 2020-09-01 09:38:55 UTC
Since the problem described in this bug report should be
resolved in a recent advisory, it has been closed with a
resolution of ERRATA.

For information on the advisory (Important: virt:8.2 and virt-devel:8.2 security and bug fix update), and where to find the updated
files, follow the link below.

If the solution does not work for you, open a new bug report.


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