The Design and Implementation of the Anykernel and Rump Kernels

211
Multiplicity
Multiplicity means it is possible to run an arbitrary number of kernels and is an
important feature especially for networking tests. It allows running tests on for
example routing code by simulating a network with n arbitrarily connected hosts.
• PR kern/43548 describes a kernel panic which happens under certain configurations
when the ip_forward() routing routine sends an ICMP error for
a suitably crafted packet. The test case in tests/net/icmp/t_forward.c
forks two rump kernels: one is a router and the other one sends the triggering
packet. Both rump kernels are configured accordingly. When the triggering
party has finished configuration, it immediately sends the triggering packet.
Since the test setup uses shmif, we do not have to wait for the router to be
configured — the router will receive the packet from the shmif bus as soon
as it is configured.
The test monitors the status of the process containing the router. If the bug
is triggered, the result is a kernel panic. This panic causes the process to
dump core, the test case to notice it, and the test to fail.
• The Common Address Redundancy Protocol (CARP) [86] protocol allows
several hosts to handle traffic to the same IP address. A common usage
scenario is a hot spare for a router: if a router goes down, a preconfigured
hot spare will take over. The CARP protocol allows the hot spare unit to
detect the failure via timeout and assume control of the IP address of the
service.
Atestintests/net/carp tests the in-kernel hot spare functionality with
a straightforward approach. First, two rump kernels are initialized and a
common IP address is configured for them using CARP. After verifying that
the master unit responds to ping, it is killed by sending SIGKILL. The test