Re: [PATCH 1/2] Check if last partition ends at GPT backup header

[Henning Schild <henning.schild@siemens.com>]

Am Tue, 8 Nov 2022 12:28:36 +0100
schrieb "T. Schaffner" <tobias.schaffner@siemens.com>:

> From: Tobias Schaffner <tobias.schaffner@siemens.com>
> 
> The GPT backup header has a fixed size of 33 512 byte blocks. Check
> if the last partition ends at the block before the start of the GUID
> partition table backup header. If so the partition is fully expanded.
> 
> Signed-off-by: Tobias Schaffner <tobias.schaffner@siemens.com>
> ---
>  .../files/expand-last-partition.sh            | 21
> +++++++++---------- 1 file changed, 10 insertions(+), 11 deletions(-)
> 
> diff --git
> a/meta/recipes-support/expand-on-first-boot/files/expand-last-partition.sh
> b/meta/recipes-support/expand-on-first-boot/files/expand-last-partition.sh
> index 57055cc..0d662cc 100755 ---
> a/meta/recipes-support/expand-on-first-boot/files/expand-last-partition.sh
> +++
> b/meta/recipes-support/expand-on-first-boot/files/expand-last-partition.sh
> @@ -22,24 +22,23 @@ if [ "${ROOT_DEV}" = "${BOOT_DEV}" ]; then exit 1
> fi -# this value is in blocks. Normally a block has 512 bytes.
> -BUFFER_SIZE=32768
>  BOOT_DEV_NAME=${BOOT_DEV##*/}
> +LAST_PART_NAME="$(lsblk -l -o NAME "${BOOT_DEV}" | tail -1)"
> +LAST_PART="/dev/${LAST_PART_NAME}"
> +
>  DISK_SIZE="$(cat /sys/class/block/"${BOOT_DEV_NAME}"/size)"
> -ALL_PARTS_SIZE=0
> -for PARTITION in
> /sys/class/block/"${BOOT_DEV_NAME}"/"${BOOT_DEV_NAME}"*; do
> -	PART_SIZE=$(cat "${PARTITION}"/size)
> -	ALL_PARTS_SIZE=$((ALL_PARTS_SIZE + PART_SIZE))
> -done
> +LAST_PART_SIZE="$(cat /sys/class/block/"${LAST_PART_NAME}"/size)"
> +LAST_PART_START="$(cat /sys/class/block/"${LAST_PART_NAME}"/start)"
> +
> +# The GUID partition table stores its backup in the last 33 blocks
> of the table. +# Therefore the last partition ends 33 before the end
> of the disk if expanded. +GPT_BACKUP_SIZE=33
>  
> -MINIMAL_SIZE=$((ALL_PARTS_SIZE + BUFFER_SIZE))
> -if [ "$DISK_SIZE" -lt "$MINIMAL_SIZE" ]; then
> +if [ $((LAST_PART_START + LAST_PART_SIZE + GPT_BACKUP_SIZE)) -lt
> "${DISK_SIZE}" ]; then echo "Disk is practically already full, doing
> nothing." >&2 exit 0

There was a good reason for that. I think when for alignment reasons or
so it looks like there is a little bit of space left. But the resize
would then not work or we would in fact try a shrink and fail.
Here you seem the switch the logic from "practically full" to "really
full" and remove that margin where a resize would be tried but would
fail.

The trick with last_part start + size looks nicer than summing up the
whole size.

Henning

>  fi
>  
> -LAST_PART="$(sfdisk -d "${BOOT_DEV}" 2>/dev/null | tail -1 | cut -d
> ' ' -f 1)" -
>  # Transform the partition table as follows:
>  #
>  # - Remove any 'last-lba' header so sfdisk uses the entire available
> space.