[openwrt/openwrt] bcm27xx: Add padding after writing rootfs to image.

[LEDE Commits]

robimarko pushed a commit to openwrt/openwrt.git, branch main:
https://git.openwrt.org/20aeef1ef845871a385f2c9437efdf5f8432a446

commit 20aeef1ef845871a385f2c9437efdf5f8432a446
Author: Orne Brocaar <info at brocaar.com>
AuthorDate: Tue Sep 9 10:04:59 2025 +0100

    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 <info at brocaar.com>
    Link: https://github.com/openwrt/openwrt/pull/19997
    Signed-off-by: Robert Marko <robimarko at gmail.com>
---
 target/linux/bcm27xx/image/gen_rpi_sdcard_img.sh | 31 +++++++++++++++++++++++-
 1 file changed, 30 insertions(+), 1 deletion(-)

diff --git a/target/linux/bcm27xx/image/gen_rpi_sdcard_img.sh b/target/linux/bcm27xx/image/gen_rpi_sdcard_img.sh
index 8d33e4f54c..492347a61e 100755
--- a/target/linux/bcm27xx/image/gen_rpi_sdcard_img.sh
+++ b/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