bin/zbm-kcl: use proper offset when overwriting EFI KCL

The overwrite will use the gap between the .cmdline section and the next
higher section as the maximum size for new KCLs. In practice, alignment
requirements in the stub loader mean that the maximum KCL size is likely
to be 4 kB.

bin/zbm-kcl

@@ -212,17 +212,95 @@ save_kcl_zfs() {
   return 0
 }
 
+find_cmdline_gap() {
+  local file
+  file="${1}"
+
+  if [ ! -r "${file}" ]; then
+    zerror "unable to read object file '${file}'"
+    return 1
+  fi
+
+  if ! OBJDATA="$(objdump -h -w "$1")"; then
+    zerror "failed to parse object file; is objdump available?"
+    return 1
+  fi
+
+  local ready idx name size vma rem offsets cmdoff
+  offsets=( )
+  cmdoff=
+  ready=
+  # shellcheck disable=SC2034
+  while read -r idx name size vma rem; do
+    # Object header table begins with header labeling columns
+    if [ "${idx,,}" = "idx" ] && [ "${vma,,}" = "vma" ]; then
+      ready="yes"
+      continue
+    fi
+
+    # Ignore all lines until the header line has been encountered
+    [ -n "${ready}" ] || continue
+
+    # Make sure the index field is integral
+    [ "${idx}" -eq "${idx}" ] >/dev/null 2>&1 || continue
+
+    # Validate the VMA field, which should be hex
+    vma="${vma,,}"
+    # Field should not start with 0x, but tolerate it anyway
+    vma="${vma#0x}"
+
+    if ! vma="$(( "0x${vma}" ))"; then
+      zerror "invalid VMA for section '${name}'"
+      return 1
+    fi
+
+    if [ "${name,,}" = ".cmdline" ]; then
+      cmdoff="${vma}"
+    else
+      offsets+=( "${vma}" )
+    fi
+  done <<< "${OBJDATA}"
+
+  if [ -z "${cmdoff}" ]; then
+    zerror "file '${file}' contains no .cmdline section"
+    return 1
+  fi
+
+  local gap mingap
+  gap=
+  mingap=
+  for vma in "${offsets[@]}"; do
+    [ "${vma}" -gt "${cmdoff}" ] >/dev/null 2>&1 || continue;
+    gap="$(( vma - cmdoff ))"
+    if [ -z "${mingap}" ] || [ "${gap}" -lt "${mingap}" ]; then
+      mingap="${gap}"
+    fi
+  done
+
+  if [ -z "${mingap}" ]; then
+    zerror "unable to determine .cmdline gap size"
+    return 1
+  fi
+
+  printf "%X %X\n" "${cmdoff}" "${mingap}"
+  return 0
+}
+
 save_kcl_efi() {
-  local kcl efi kclfile
+  local kcl efi kclfile kcloff kclgap kclsize
   efi="${1}"
 
-  if [ ! -r "${efi}" ]; then
-    zerror "unable to read EFI excutable '${efi}'"
+  # Find offset and space available for the cmdline to replace;
+  # this seems to be 4 kB with the x86_64 stub loader alignment
+  if ! kclsize="$(find_cmdline_gap "${efi}")"; then
+    zerror "failed to determine offset data for KCL"
     return 1
   fi
 
-  if ! objout="$(objcopy --remove-section .cmdline "${efi}" 2>&1)"; then
-    zerror "failed to clear existing KCL from EFI executable"
+  read -r kcloff kclgap <<< "${kclsize}"
+
+  if [ -z "${kcloff}" ] || [ -z "${kclgap}" ]; then
+    zerror "offset data for KCL appears invalid; aborting"
     return 1
   fi
 
@@ -239,14 +317,27 @@ save_kcl_efi() {
   # Dracut also adds a null terminator
   echo -ne "\x00" >> "${kclfile}"
 
-  if [ -s "${kclfile}" ]; then
-    local objargs
-    objargs=( --add-section ".cmdline=${kclfile}"
-              --change-section-vma ".cmdline=0x30000" )
-    if ! objout="$(objcopy "${objargs[@]}" "${efi}" 2>&1)"; then
-      zerror "failed to write new KCL to EFI executable"
-      return 1
-    fi
+  if ! kclsize="$(stat -c %s "${kclfile}")"; then
+    zerror "failed to determine new KCL size; is stat available?"
+    return 1
+  fi
+
+  if ! [ "${kclsize}" -le "$(( "0x${kclgap}" ))" ] >/dev/null 2>&1; then
+    zerror "new KCL size exceeds space available in EFI file '${efi}'; aborting"
+    return 1
+  fi
+
+  if ! objout="$(objcopy --remove-section .cmdline "${efi}" 2>&1)"; then
+    zerror "failed to clear existing KCL from EFI executable"
+    return 1
+  fi
+
+  local objargs
+  objargs=( --add-section ".cmdline=${kclfile}"
+            --change-section-vma ".cmdline=0x${kcloff}" )
+  if ! objout="$(objcopy "${objargs[@]}" "${efi}" 2>&1)"; then
+    zerror "failed to write new KCL to EFI executable"
+    return 1
   fi
 
   return 0