ceph: add a new "run_uml.sh" script to manage running a UML client

This script is used to automate most of what's required to run a
User-Mode Linux (UML) instance.  This is mainly of interest for
ceph client developers who might benefit from the debugger access
that UML affords.  It was written for ceph development but isn't
really dependent on ceph.  It basically makes a few assumptions and
follows some conventions, and in doing so is able to encapsulate
most of the "tricky parts" of setting up to run a UML instance.

Signed-off-by: Alex Elder <elder@dreamhost.com>

diff --git a/src/script/run_uml.sh b/src/script/run_uml.sh
new file mode 100755 (executable)
index 0000000..c1a08fc
--- /dev/null
+++ b/src/script/run_uml.sh
@@ -0,0 +1,214 @@
+#!/bin/bash -norc
+
+# Magic startup script for a UML instance.  As long as unique
+# instances are started, more than one of them can be concurrently
+# in use on a single system.  All their network interfaces are
+# bridged together onto the virtual bridge "virbr0" which is
+# supplied by the "libvirt" package.
+#
+# Note that a DHCP server is started for that interface.  It's
+# configured in this file:
+#      /etc/libvirt/qemu/networks/default.xml
+# Unfortunately what I see there serves all possible DHCP addresses,
+# so stealing them like we do here isn't really kosher.  To fix
+# it, that configuration should change to serve a smaller subset
+# of the available address range.
+#
+# Each instance uses its own tun/tap device, created using the
+# "tunctl" command.  The assigned tap device will correspond with
+# the guest id (a small integer representing the instance), i.e.,
+# guest id 1 uses tap1, etc.  The tap device is attached to the
+# virtual bridge, which will have its own subnet associated with it.
+# The guest side of that interface will have the same subnet as the
+# bridge interface, with the bottom bits representing (normally) 100
+# more than the guest id.  So for subnet 192.168.122.0/24, guest
+# id 1 will use ip 192.168.122.101, guest id 2 will use ip
+# 192.168.122.102, and so on.  Because these interfaces are bridged,
+# they can all communicate with each other.
+
+# You will want to override this by setting and exporting the
+# "CEPH_TOP" environment variable to be the directory that contains
+# the "ceph-client" source tree.
+CEPH_TOP="${CEPH_TOP:-/home/elder/ceph}"
+
+# You may want to change this too, if you want guest UML instances
+# to have a diffeerent IP address range.  The guest IP will be based
+# on this plus GUEST_ID (defined below).
+GUEST_IP_OFFSET="${GUEST_IP_OFFSET:-100}"
+
+# This will be what the guest host calls itself.
+GUEST_HOSTNAME="uml-${GUEST_ID}"
+
+# This is the path to the boot disk image used by UML.
+DISK_IMAGE_A="${CEPH_TOP}/ceph-client/uml.${GUEST_ID}"
+
+# Hostid 1 uses tun/tap device tap1, hostid 2 uses tap2, etc.
+TAP_ID="${GUEST_ID}"
+
+#############################
+
+if [ $# -gt 1 ]; then
+       echo "" >&2
+       echo "Usage: $(basename $0) [guest_id]" >&2
+       echo "" >&2
+       echo "    guest_id is a small integer (default 1)" >&2
+       echo "    (each UML instance needs a distinct guest_id)" >&2
+       echo "" >&2
+       exit 1
+elif [ $# -eq 1 ]; then
+       GUEST_ID="$1"
+else
+       GUEST_ID=1
+fi
+
+if [ ! -f "${DISK_IMAGE_A}" ]; then
+       echo "root disk image not found (or not a file)" >&2
+       exit 2
+fi
+
+# This is just used to mount an image temporarily
+TMP_MNT="/tmp/m$$"
+
+# This is the tap device used for this UML instance
+TAP="tap${TAP_ID}"
+
+# Where to put a config file generated for this tap device
+TAP_IFUPDOWN_CONFIG="/tmp/interface-${TAP}"
+
+# Compute the HOST_IP and BROADCAST address values to use,
+# and assign shell variables with those names to their values.
+# Also compute BITS, which is the network prefix length used.
+# The NETMASK is then computed using that BITS value.
+eval $(
+ip addr show virbr0 | awk '
+/inet/ {
+       split($2, a, "/")
+       printf("HOST_IP=%s\n", a[1]);
+       printf("BROADCAST=%s\n", $4);
+       printf("BITS=%s\n", a[2]);
+       exit(0);
+}')
+
+# Use bc to avoid 32-bit wrap when computing netmask
+eval $(
+echo -n "NETMASK="
+bc <<! | fmt | sed 's/ /./g'
+m = 2 ^ 32 - 2 ^ (32 - ${BITS})
+for (p = 24; p >= 0; p = p - 8)
+    m / (2 ^ p) % 256
+!
+)
+
+# Now use the netmask and the host IP to compute the subnet address
+# and from that the guest IP address to use.
+eval $(
+awk '
+function from_quad(addr,  a, val, i) {
+       if (split(addr, a, ".") != 4)
+               exit(1);        # address not in dotted quad format
+       val = 0;
+       for (i = 1; i <= 4; i++)
+               val = val * 256 + a[i];
+       return val;
+}
+function to_quad(val,  addr, i) {
+       addr = "";
+       for (i = 1; i <= 4; i++) {
+               addr = sprintf("%u%s%s", val % 256, i > 1 ? "." : "", addr);
+               val = int(val / 256);
+       }
+       if ((val + 0) != 0)
+               exit(1);        # value provided exceeded 32 bits
+       return addr;
+}
+BEGIN {
+       host_ip = from_quad("'${HOST_IP}'");
+       netmask = from_quad("'${NETMASK}'");
+       guest_net_ip = '${GUEST_IP_OFFSET}' + '${GUEST_ID}';
+       if (and(netmask, guest_net_ip))
+               exit(1);        # address too big for subnet
+       subnet = and(host_ip, netmask);
+       guest_ip = or(subnet, guest_net_ip);
+       if (guest_ip == host_ip)
+               exit(1);        # computed guest ip matches host ip
+
+       printf("SUBNET=%s\n", to_quad(subnet));
+       printf("GUEST_IP=%s\n", to_quad(guest_ip));
+}
+' < /dev/null
+)
+
+############## OK, we now know all our network parameters...
+
+# There is a series of things that need to be done as superuser,
+# so group them all into one big (and sort of nested!) sudo request.
+sudo -s <<EnD_Of_sUdO
+# Mount the boot disk for the UML and set up some configuration
+# files there.
+mkdir -p "${TMP_MNT}"
+mount -o loop "${DISK_IMAGE_A}" "${TMP_MNT}"
+
+# Arrange for loopback and eth0 to load automatically,
+# and for eth0 to have our desired network parameters.
+cat > "${TMP_MNT}/etc/network/interfaces" <<!
+# Used by ifup(8) and ifdown(8). See the interfaces(5) manpage or
+# /usr/share/doc/ifupdown/examples for more information.
+auto lo
+iface lo inet loopback
+auto eth0
+# iface eth0 inet dhcp
+iface eth0 inet static
+        address ${GUEST_IP}
+        netmask ${NETMASK}
+        broadcast ${BROADCAST}
+        gateway ${HOST_IP}
+!
+
+# Have the guest start with an appropriate host name.
+# Also record an entry for it in its own hosts file.
+echo "${GUEST_HOSTNAME}" > "${TMP_MNT}/etc/hostname"
+echo "${GUEST_IP}      ${GUEST_HOSTNAME}" >> "${TMP_MNT}/etc/hosts"
+
+# The host will serve as the name server also
+cat > "${TMP_MNT}/etc/resolv.conf" <<!
+nameserver ${HOST_IP}
+!
+
+# OK, done tweaking the boot image.
+sync
+umount "${DISK_IMAGE_A}"
+rmdir "${TMP_MNT}"
+
+# Set up a config file for "ifup" and "ifdown" (on the host) to use.
+# All the backslashes below are needed because we're sitting inside
+# a double here-document...
+cat > "${TAP_IFUPDOWN_CONFIG}" <<!
+iface ${TAP} inet manual
+       up brctl addif virbr0 "\\\${IFACE}"
+       up ip link set dev "\\\${IFACE}" up
+       pre-down brctl delif virbr0 "\\\${IFACE}"
+       pre-down ip link del dev "\\\${IFACE}"
+       tunctl_user $(whoami)
+!
+
+# OK, bring up the tap device using our config file
+ifup -i "${TAP_IFUPDOWN_CONFIG}" "${TAP}"
+
+EnD_Of_sUdO
+
+# Finally ready to launch the UML instance.
+./linux \
+       umid="${GUEST_HOSTNAME}" \
+       ubda="${DISK_IMAGE_A}" \
+       eth0="tuntap,${TAP}" \
+       mem=1024M
+
+# When we're done, clean up.  Bring down the tap interface and
+# delete the config file.
+#
+# Note that if the above "./linux" crashes, you'll need to run the
+# following commands manually in order to clean up state.
+sudo ifdown -i "${TAP_IFUPDOWN_CONFIG}" "${TAP}"
+sudo rm -f "${TAP_IFUPDOWN_CONFIG}"
+
+exit 0