Red Hat Bugzilla – Bug 664783
race condition in radvd pidfile creation
Last modified: 2016-07-21 09:13:17 EDT
radvd uses daemonize() to daemonize itself, then creates its pidfile afterwards. This means that the process that exec'ed radvd will get back control before the pidfile has been created and/or populated, which could lead to a race condition. This creates a problem for libvirt, which attempts to run an instance of radvd and determine its pid (for later termination) by immediately reading the pidfile. The pidfile isn't yet created when libvirtd gets back control after waitpid'ing for the radvd process, so it fails to learn the pid. Instead of using daemonize() and creating the pidfile from the child process, radvd should do the fork manually, then create the pidfile in the parent, prior to exiting.
Actually it's been pointed out to me that the pidfile is *created* by the parent process, but written to by the child, after daemonize is called. So the process that exec's radvd will always find the file, but it may not yet have anything in it.
This request was evaluated by Red Hat Product Management for inclusion in the current release of Red Hat Enterprise Linux. Because the affected component is not scheduled to be updated in the current release, Red Hat is unfortunately unable to address this request at this time. Red Hat invites you to ask your support representative to propose this request, if appropriate and relevant, in the next release of Red Hat Enterprise Linux. If you would like it considered as an exception in the current release, please ask your support representative.
Reproducer (unreliable as for all races): $ radvd; kill -n 0 $(</var/run/radvd.pid)
Actually radvd in RHEL-6 stores PID into /var/run/radvd/radvd.pid.
Upstream has fixed this issue by using libdaemon with the following patch set: commit 1cda2ef33f505be34e690d1b7a1e5eac632819af Author: Petr Písař <ppisar@redhat.com> Date: Wed Apr 11 13:52:24 2012 +0200 Use pkg-config to discover libdaemon commit 4ff814023cf62e59a6048a2cc57c62e0664d3c56 Author: Petr Písař <ppisar@redhat.com> Date: Thu Apr 5 15:24:22 2012 +0200 Use libdaemon for daemonization The libc daemon(3) function suffers from race bewtween exiting parent and saving PID into a file. Using libdaemon library one can avoid this race and can simplify PID file manipulation. The only difference against older implementation is, the PID file will be inspected, created, and removed only if daemonization is requested.
Created attachment 577068 [details] Reproducer Use positional arguments to select executable, timeout, and PID file location.
Created attachment 577285 [details] Upsteam patch ported to 1.6
Notice for testers: radvd-1.6 requires enabled global (all device) IPv6 forwarding and a valid configuration file. One can use veth interface pair instead of real ethernet to not jam real network.
This request was evaluated by Red Hat Product Management for inclusion in the current release of Red Hat Enterprise Linux. Because the affected component is not scheduled to be updated in the current release, Red Hat is unable to address this request at this time. Red Hat invites you to ask your support representative to propose this request, if appropriate, in the next release of Red Hat Enterprise Linux.
Red Hat Enterprise Linux version 6 is entering the Production 2 phase of its lifetime and this bug doesn't meet the criteria for it, i.e. only high severity issues will be fixed. Please see https://access.redhat.com/support/policy/updates/errata/ for further information. This issue is fixed in Red Hat Enterprise Linux version 7.