bcm27xx: Add padding after writing rootfs to image.

This addresses #9113 by adding up to 1MB padding after writing the
rootfs image. On boot mount_root will probe for existing filesystems
after the rootfs image data. Without overwriting the initial free
space left on the rootfs partition, OpenWrt might incorrectly detect
an exising filesystem and fails to mount it, resulting in a bricked
device as the overlayfs will not be mountend and settings will not be
available.

Fixes #9113.

Signed-off-by: Orne Brocaar <[email protected]>
Link: https://github.com/openwrt/openwrt/pull/19997
Signed-off-by: Robert Marko <[email protected]>

target/linux/bcm27xx/image/gen_rpi_sdcard_img.sh

@@ -24,5 +24,34 @@ set $(ptgen -o $OUTPUT -h $head -s $sect -l $align -t $kernel_type -p ${BOOTFSSI
 BOOTOFFSET="$(($1 / 512))"
 ROOTFSOFFSET="$(($3 / 512))"
 
+# In most cases, the rootfs image size is smaller than the total space
+# available in the rootfs. OpenWrt will use the remaining space for storing
+# the fs backing the overlayfs which holds configuration changes etc.
+# To avoid that OpenWrt incorrectly detects a filesystem in the case
+# there is remaining data from a previous installation, we need to clear
+# the first N bytes directly after the rootfs image, to make sure
+# that any initial bytes that are or look like fs superblocks are cleared
+# out. 
+ROOTFSSIZE="$(($4 / 512))"
+ROOTFSIMGSIZE="$((($(wc -c < $ROOTFS) + 511) / 512))"
+ROOTFSPADDINGSIZE="$(($ROOTFSSIZE - $ROOTFSIMGSIZE))"
+ROOTFSPADDINGOFFSET="$(($ROOTFSOFFSET + $ROOTFSIMGSIZE))"
+
+# Reduce padding to max 1MB, which is enough to clear out any superblocks
+# of previous file-systems or any data that might look like fs superblocks.
+if [ "$ROOTFSPADDINGSIZE" -gt 2048 ]; then
+  ROOTFSPADDINGSIZE="2048"
+fi
+
+# Write the bootfs.
 dd bs=512 if="$BOOTFS" of="$OUTPUT" seek="$BOOTOFFSET" conv=notrunc
-dd bs=512 if="$ROOTFS" of="$OUTPUT" seek="$ROOTFSOFFSET" conv=notrunc
+
+# Write the rootfs.
+# The `sync` is important, as in case $ROOTFS is not a multiple of bs, it will
+# assure that remaining bytes are set to 0x00.
+dd bs=512 if="$ROOTFS" of="$OUTPUT" seek="$ROOTFSOFFSET" conv=notrunc,sync
+
+# Add padding after rootfs.
+if [ "$ROOTFSPADDINGSIZE" -gt 0 ]; then
+  dd bs=512 if=/dev/zero of="$OUTPUT" seek="$ROOTFSPADDINGOFFSET" count="$ROOTFSPADDINGSIZE" conv=notrunc
+fi