From bbd57d8dd29009b1bfa18783a5778535961ba64a Mon Sep 17 00:00:00 2001
From: Thales Maciel <thales@thalesmaciel.com>
Date: Fri, 30 Jan 2026 12:13:35 -0300
Subject: [PATCH] Use hostname-safe VM names

---
 README.md |  12 ++++--
 namegen   |   2 +-
 run.sh    | 107 +++++++++++++++++++++++++++++++++++++++++++++++++++---
 3 files changed, 110 insertions(+), 11 deletions(-)

diff --git a/README.md b/README.md
index 8574b7a..61f6657 100644
--- a/README.md
+++ b/README.md
@@ -5,6 +5,8 @@ Minimal Firecracker launcher.
 ## Requirements
 - Linux host with KVM (`/dev/kvm` access)
 - `sudo`, `ip`, `curl`, `ssh`
+- `dmsetup`, `losetup`, `blockdev` (device-mapper snapshot for rootfs)
+- `e2cp`, `e2rm` (writes hostname and resolv.conf into rootfs snapshot)
 
 ## Files
 - `firecracker`: Firecracker binary
@@ -21,17 +23,19 @@ Minimal Firecracker launcher.
 ```
 ./run.sh --name calm_otter --vcpu 4 --ram 2048 --home-size 6G
 ```
-- `--name`: must be unique and match `[a-z0-9][a-z0-9_-]{0,63}`.
+- `--name`: must be unique and match `[a-z0-9][a-z0-9-]{0,63}`.
 - `--vcpu`: defaults to 2, max 16.
 - `--ram`: MiB, defaults to 1024, max 32768.
 - `--rootfs`: path to the base rootfs image (default: `./rootfs.ext4`).
 - `--kernel`: path to the kernel image (default: `./vmlinux`).
 - `--home-size`: M/G suffixes supported (default: 2G).
+- `--var-size`: M/G suffixes supported (default: 2G).
 
 ## Storage Layout
-- `rootfs.ext4` is mounted read-only as `/` and shared across VMs.
-- Each VM gets a writable ext4 disk mounted at `/home`.
-- The base image must include an `/etc/fstab` entry for `/dev/vdb` → `/home`.
+- `rootfs.ext4` is used as the read-only origin for a per-VM device-mapper snapshot mounted as `/`.
+- Each VM gets writable ext4 disks mounted at `/home` and `/var`.
+- The base image must include `/etc/fstab` entries for `/dev/vdb` → `/home` and `/dev/vdc` → `/var`.
+- `/run` and `/tmp` should be tmpfs via `/etc/fstab`.
 
 ## SSH
 ```
diff --git a/namegen b/namegen
index ed133a8..830632a 100755
--- a/namegen
+++ b/namegen
@@ -412,4 +412,4 @@ rand() {
 
 adjective="${ADJECTIVES[$(rand ${#ADJECTIVES[@]})]}"
     substantive="${SUBSTANTIVES[$(rand ${#SUBSTANTIVES[@]})]}"
-        printf '%s_%s' "$adjective" "$substantive"
+        printf '%s-%s' "$adjective" "$substantive"
diff --git a/run.sh b/run.sh
index 171635f..685290d 100755
--- a/run.sh
+++ b/run.sh
@@ -10,12 +10,13 @@ usage() {
 Usage: ./run.sh [options]
 
 Options:
-  --name <name>        VM name (lowercase letters, digits, -, _)
+  --name <name>        VM name (lowercase letters, digits, -)
   --vcpu <count>       vCPU count (default: 2)
   --ram <mib>          RAM in MiB (default: 1024)
   --rootfs <path>      Root filesystem image (default: ./rootfs.ext4)
   --kernel <path>      Kernel image (default: ./vmlinux)
   --home-size <size>   Home disk size (e.g. 4G, 10240M)
+  --var-size <size>    Var disk size (e.g. 4G, 10240M)
   -h, --help           Show this help
 EOF
 }
@@ -40,16 +41,19 @@ CIDR="24"
 DEFAULT_VCPU=2
 DEFAULT_RAM=1024
 DEFAULT_HOME_SIZE="2G"
+DEFAULT_VAR_SIZE="2G"
 MIN_VCPU=1
 MAX_VCPU=16
 MIN_RAM=256
 MAX_RAM=32768
 MAX_DISK_BYTES=$((128 * 1024 * 1024 * 1024))
 DNS_SERVER="1.1.1.1"
+COW_SIZE="2G"
 
 VCPU_COUNT="$DEFAULT_VCPU"
 RAM_MIB="$DEFAULT_RAM"
 HOME_SIZE="$DEFAULT_HOME_SIZE"
+VAR_SIZE="$DEFAULT_VAR_SIZE"
 KERNEL="$DEFAULT_KERNEL"
 ROOTFS="$DEFAULT_ROOTFS"
 VM_NAME=""
@@ -109,6 +113,10 @@ while [[ $# -gt 0 ]]; do
       HOME_SIZE="${2:-}"
       shift 2
       ;;
+    --var-size)
+      VAR_SIZE="${2:-}"
+      shift 2
+      ;;
     -h|--help)
       usage
       exit 0
@@ -149,6 +157,16 @@ if (( HOME_BYTES > MAX_DISK_BYTES )); then
   exit 1
 fi
 
+VAR_BYTES=""
+if ! VAR_BYTES="$(parse_disk_size "$VAR_SIZE")"; then
+  log "invalid --var-size value: $VAR_SIZE"
+  exit 1
+fi
+if (( VAR_BYTES > MAX_DISK_BYTES )); then
+  log "var-size exceeds max of $((MAX_DISK_BYTES / 1024 / 1024 / 1024))G"
+  exit 1
+fi
+
 if [[ -z "$VM_NAME" ]]; then
   if [[ ! -x "$NAMEGEN" ]]; then
     log "name generator not found: $NAMEGEN"
@@ -167,7 +185,7 @@ if [[ -z "$VM_NAME" ]]; then
   exit 1
 fi
 
-if ! [[ "$VM_NAME" =~ ^[a-z0-9][a-z0-9_-]{0,63}$ ]]; then
+if ! [[ "$VM_NAME" =~ ^[a-z0-9][a-z0-9-]{0,63}$ ]]; then
   log "invalid --name value: $VM_NAME"
   exit 1
 fi
@@ -213,6 +231,12 @@ VM_STARTED=0
 CLEANUP_ON_EXIT=0
 KEEP_VM_DIR_ON_FAIL=1
 HOME_PATH="$VM_DIR/home.ext4"
+VAR_PATH="$VM_DIR/var.ext4"
+COW_FILE="$VM_DIR/cow.ext4"
+BASE_LOOP=""
+COW_LOOP=""
+DM_NAME="fc-rootfs-$VM_TAG"
+DM_DEV=""
 
 cleanup() {
   local exit_code=$?
@@ -227,6 +251,15 @@ cleanup() {
     sudo ip link del "$TAP_DEV" 2>/dev/null || true
   fi
   rm -f "${API_SOCK:-}"
+  if [[ -n "${DM_NAME:-}" ]]; then
+    sudo dmsetup remove "$DM_NAME" 2>/dev/null || true
+  fi
+  if [[ -n "${COW_LOOP:-}" ]]; then
+    sudo losetup -d "$COW_LOOP" 2>/dev/null || true
+  fi
+  if [[ -n "${BASE_LOOP:-}" ]]; then
+    sudo losetup -d "$BASE_LOOP" 2>/dev/null || true
+  fi
   if [[ -n "${VM_DIR:-}" && -d "$VM_DIR" ]]; then
     if [[ "$KEEP_VM_DIR_ON_FAIL" -eq 0 || "$VM_STARTED" -eq 1 || "$CLEANUP_ON_EXIT" -eq 1 ]]; then
       rm -rf "$VM_DIR"
@@ -262,11 +295,57 @@ else
 fi
 
 if ! command -v mkfs.ext4 >/dev/null 2>&1; then
-  log "mkfs.ext4 required to create home disk"
+  log "mkfs.ext4 required to create home/var disks"
   exit 1
 fi
 truncate -s "$HOME_BYTES" "$HOME_PATH"
 mkfs.ext4 -F "$HOME_PATH" >/dev/null
+truncate -s "$VAR_BYTES" "$VAR_PATH"
+mkfs.ext4 -F "$VAR_PATH" >/dev/null
+
+if ! command -v dmsetup >/dev/null 2>&1 || ! command -v losetup >/dev/null 2>&1 || ! command -v blockdev >/dev/null 2>&1; then
+  log "dmsetup, losetup, and blockdev are required for rootfs snapshots"
+  exit 1
+fi
+if ! command -v e2cp >/dev/null 2>&1 || ! command -v e2rm >/dev/null 2>&1; then
+  log "e2cp and e2rm are required to set hostname and resolv.conf"
+  exit 1
+fi
+
+COW_BYTES="$(parse_disk_size "$COW_SIZE")"
+if [[ -z "$COW_BYTES" ]]; then
+  log "invalid COW size: $COW_SIZE"
+  exit 1
+fi
+
+BASE_LOOP="$(sudo losetup -f --show --read-only "$ROOTFS")"
+truncate -s "$COW_BYTES" "$COW_FILE"
+COW_LOOP="$(sudo losetup -f --show "$COW_FILE")"
+SECTORS="$(sudo blockdev --getsz "$BASE_LOOP")"
+sudo dmsetup create "$DM_NAME" --table "0 $SECTORS snapshot $BASE_LOOP $COW_LOOP P 8"
+DM_DEV="/dev/mapper/$DM_NAME"
+
+RESOLV_TMP="$VM_DIR/resolv.conf"
+HOSTNAME_TMP="$VM_DIR/hostname"
+HOSTS_TMP="$VM_DIR/hosts"
+printf 'nameserver %s\n' "$DNS_SERVER" >"$RESOLV_TMP"
+printf '%s\n' "$VM_NAME" >"$HOSTNAME_TMP"
+printf '127.0.0.1 localhost\n127.0.1.1 %s\n' "$VM_NAME" >"$HOSTS_TMP"
+sudo e2rm "$DM_DEV:/etc/resolv.conf" >/dev/null 2>&1 || true
+sudo e2rm "$DM_DEV:/etc/hostname" >/dev/null 2>&1 || true
+sudo e2rm "$DM_DEV:/etc/hosts" >/dev/null 2>&1 || true
+sudo e2cp "$RESOLV_TMP" "$DM_DEV:/etc/resolv.conf" >/dev/null 2>&1 || {
+  log "failed to write /etc/resolv.conf into rootfs snapshot"
+  exit 1
+}
+sudo e2cp "$HOSTNAME_TMP" "$DM_DEV:/etc/hostname" >/dev/null 2>&1 || {
+  log "failed to write /etc/hostname into rootfs snapshot"
+  exit 1
+}
+sudo e2cp "$HOSTS_TMP" "$DM_DEV:/etc/hosts" >/dev/null 2>&1 || {
+  log "failed to write /etc/hosts into rootfs snapshot"
+  exit 1
+}
 
 # Host bridge
 if ! ip link show "$BR_DEV" >/dev/null 2>&1; then
@@ -333,7 +412,7 @@ log "configuring machine"
 
 # Boot source
 log "configuring boot source"
-KCMD="console=ttyS0 reboot=k panic=1 pci=off root=/dev/vda rw ip=${GUEST_IP}::${BR_IP}:${DNS_SERVER}:255.255.255.0::eth0:off hostname=${VM_NAME}"
+KCMD="console=ttyS0 reboot=k panic=1 pci=off root=/dev/vda rw ip=${GUEST_IP}::${BR_IP}:255.255.255.0::eth0:off:${DNS_SERVER} hostname=${VM_NAME}"
 
 "${CURL_CMD[@]}" --unix-socket "$API_SOCK" -X PUT http://localhost/boot-source \
   -H "Content-Type: application/json" \
@@ -348,9 +427,9 @@ log "attaching root filesystem"
   -H "Content-Type: application/json" \
   -d "{
     \"drive_id\": \"rootfs\",
-    \"path_on_host\": \"$ROOTFS\",
+    \"path_on_host\": \"$DM_DEV\",
     \"is_root_device\": true,
-    \"is_read_only\": true
+    \"is_read_only\": false
   }" >/dev/null
 
 # Home filesystem
@@ -364,6 +443,17 @@ log "attaching home filesystem"
     \"is_read_only\": false
   }" >/dev/null
 
+# Var filesystem
+log "attaching var filesystem"
+"${CURL_CMD[@]}" --unix-socket "$API_SOCK" -X PUT http://localhost/drives/var \
+  -H "Content-Type: application/json" \
+  -d "{
+    \"drive_id\": \"var\",
+    \"path_on_host\": \"$VAR_PATH\",
+    \"is_root_device\": false,
+    \"is_read_only\": false
+  }" >/dev/null
+
 # Network interface
 log "configuring network interface"
 "${CURL_CMD[@]}" --unix-socket "$API_SOCK" -X PUT http://localhost/network-interfaces/eth0 \
@@ -391,6 +481,11 @@ guest_ip=$GUEST_IP
 tap=$TAP_DEV
 api_sock=$API_SOCK
 log=$LOG_FILE
+base_loop=$BASE_LOOP
+cow_file=$COW_FILE
+cow_loop=$COW_LOOP
+dm_name=$DM_NAME
+dm_dev=$DM_DEV
 EOF
 
 log "vm started successfully"