Set unused data to zero if size of config is reduced

If the size of bootconf.txt is reduced then set the unused data
to all ones instead of leaving garbage at the end. The EEPROM
header contains the actual file but this makes it easier to
verify the image and makes overreads more obvious.

rpi-eeprom-config

@@ -62,6 +62,15 @@ class BootloaderImage(object):
 
         struct.pack_into('>L', self._bytes, hdr_offset + 4, new_len)
         struct.pack_into(("%ds" % len(new_config_bytes)), self._bytes, hdr_offset + 4 + FILE_HDR_LEN, new_config_bytes)
+
+        # If the new config is smaller than the old config then set any old
+        # data which is now unused to all ones (erase value)
+        pad_start = hdr_offset + 4 + FILE_HDR_LEN + len(new_config_bytes)
+        pad = 0
+        while pad < (length - len(new_config_bytes)):
+            struct.pack_into('B', self._bytes, pad_start + pad, 0xff)
+            pad = pad + 1
+
         if self._out is not None:
            self._out.write(self._bytes)
            self._out.close()

test/test-rpi-eeprom-config

@@ -4,19 +4,61 @@ set -e
 
 script_dir="$(cd "$(dirname "$0")" && pwd)"
 die() {
+   echo "ERROR"
    echo "$@" >&2
    exit 1
 }
 
 TMP_CONFIG=""
 TMP_EEPROM=""
+TMP_EEPROM2=""
 cleanup() {
-   rm -f "${TMP_CONFIG}" -f "${TMP_EEPROM}"
+   rm -f "${TMP_CONFIG}" -f "${TMP_EEPROM}" "${TMP_EEPROM2}"
    TMP_CONFIG=""
    TMP_EEPROM=""
+   TMP_EEPROM2=""
 }
 trap cleanup EXIT
 
+check_reduce_size()
+{
+   # Verify that unused bytes are set to 0xff if the size of the config file is reduced.
+   echo "check_update $1 $2"
+
+   image="${script_dir}/$1"
+   conf="${script_dir}/$2"
+
+   # Check that the production config file can be read correctly
+   image_md5="$(md5sum "${image}" | awk '{print $1}')"
+   TMP_CONFIG="$(mktemp)"
+
+   # Generate a new EEPROM with larger config file
+   cp -f "${conf}" "${TMP_CONFIG}"
+   echo "Add some random text to the config file" >> "${TMP_CONFIG}"
+
+   TMP_EEPROM="$(mktemp)"
+   "${script_dir}/../rpi-eeprom-config" \
+      "${image}" \
+      --config "${TMP_CONFIG}" \
+      --out "${TMP_EEPROM}"
+
+   # Check that the new image is different
+   actual_md5="$(md5sum "${TMP_EEPROM}" | awk '{print $1}')"
+   [ "${image_md5}" != "${actual_md5}" ] || die "EEPROM images should be different"
+
+   # Re-apply the original configuration and make sure the image is the same
+   TMP_EEPROM2="$(mktemp)"
+   "${script_dir}/../rpi-eeprom-config" \
+      "${TMP_EEPROM}" \
+      --config "${conf}" \
+      --out "${TMP_EEPROM2}"
+
+   # Check that applying the EEPROM config file again gets back to the original image
+   actual_md5="$(md5sum "${TMP_EEPROM2}" | awk '{print $1}')"
+   [ "${image_md5}" = "${actual_md5}" ] || die "Image should be the same as original"
+
+}
+
 check_loopback()
 {
    echo "check_update $1 $2"
@@ -30,7 +72,7 @@ check_loopback()
    [ "${actual_md5}" = "${expected_md5}" ] || die "Config-read: checksum mismatch"
 
    # Check that overwriting the config section produces an identical binary
-   TMP_EEPROM=$(mktemp)
+   TMP_EEPROM="$(mktemp)"
    "${script_dir}/../rpi-eeprom-config" \
       "${image}" \
       --config "${conf}" \
@@ -73,3 +115,6 @@ cleanup
 
 check_update "../firmware/critical/pieeprom-2019-07-15.bin" "pieeprom-2019-07-15-freeze.bin" "bootconf-2019-07-15-freeze.txt"
 cleanup
+
+check_reduce_size "../firmware/critical/pieeprom-2019-05-10.bin" "bootconf-2019-05-10.txt"
+cleanup