Re: [PATCH] scripts/make-gpt.sh: allow setting partition size

[Demi Marie Obenour <demiobenour@gmail.com>]

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On 11/27/25 12:40, Alyssa Ross wrote:
> We want non-minimally-sized partitions to leave space for updates.
> All such partitions will also be labelled, so we can just add another
> optional field at the end.
> 
> Since we don't parse partition specifications in sh, we can't keep a
> running total any more, so instead we just go through the table at the
> end and add up all the sizes, taking advantage of our knowledge that
> the size will always be the last thing in each line in our tables.
> 
> Signed-off-by: Alyssa Ross <hi@alyssa.is>
> ---
> This is an alternative to Demi's implementation[1], which required Bash
> features.
> 
> [1]: https://spectrum-os.org/lists/archives/spectrum-devel/20251126-updates-v5-3-fd746748febd@gmail.com/
> 
>  scripts/make-gpt.sh      | 28 +++++++++++++++-------------
>  scripts/sfdisk-field.awk | 22 +++++++++++++++++-----
>  2 files changed, 32 insertions(+), 18 deletions(-)
> 
> diff --git a/scripts/make-gpt.sh b/scripts/make-gpt.sh
> index 96f0d2c..07eac50 100755
> --- a/scripts/make-gpt.sh
> +++ b/scripts/make-gpt.sh
> @@ -1,10 +1,10 @@
>  #!/bin/sh -eu
>  #
> -# SPDX-FileCopyrightText: 2021-2023 Alyssa Ross <hi@alyssa.is>
> +# SPDX-FileCopyrightText: 2021-2023, 2025 Alyssa Ross <hi@alyssa.is>
>  # SPDX-FileCopyrightText: 2022 Unikie
>  # SPDX-License-Identifier: EUPL-1.2+
>  #
> -# usage: make-gpt.sh GPT_PATH PATH:PARTTYPE[:PARTUUID[:PARTLABEL]]...
> +# usage: make-gpt.sh GPT_PATH PATH:PARTTYPE[:PARTUUID[:PARTLABEL[:PARTMiB]]]...
>  
>  ONE_MiB=1048576
>  
> @@ -28,7 +28,8 @@ fillPartition() {
>  	lseek -S 1 "$start" cat "$3" 1<>"$1"
>  }
>  
> -# Prints the partition path from a PATH:PARTTYPE[:PARTUUID[:PARTLABEL]] string.
> +# Prints the partition path from a
> +# PATH:PARTTYPE[:PARTUUID[:PARTLABEL[:PARTMiB]]] string.
>  partitionPath() {
>  	awk -F: '{print $1}' <<EOF
>  $1
> @@ -40,21 +41,22 @@ scriptsDir="$(dirname "$0")"
>  out="$1"
>  shift
>  
> -nl='
> -'
> -table="label: gpt"
> +table=$(for partition; do
> +	awk -f "$scriptsDir/sfdisk-field.awk" \
> +		-v partition="$partition" \
> +		-v size="$(sizeMiB "$(partitionPath "$partition")")"
> +done)
>  
>  # Keep 1MiB free at the start, and 1MiB free at the end.
> -gptBytes=$((ONE_MiB * 2))
> -for partition; do
> -	sizeMiB="$(sizeMiB "$(partitionPath "$partition")")"
> -	table="$table${nl}size=${sizeMiB}MiB,$(awk -f "$scriptsDir/sfdisk-field.awk" -v partition="$partition")"
> -	gptBytes="$((gptBytes + sizeMiB * ONE_MiB))"
> -done
> +# Here we rely on sfdisk-field.awk always putting size last
> +gptMiB=$(printf "%s\n" "$table" | awk -F= '{size += $NF} END {print size + 2}')

Needs -o pipefail, though I believe this will be in the next revision
of POSIX if it isn't already.  You can avoid that with some awk magic
but the script gets harder to read.

>  rm -f "$out"
> -truncate -s "$gptBytes" "$out"
> +truncate -s "${gptMiB}M" "$out"
> +
>  sfdisk --no-reread --no-tell-kernel "$out" <<EOF
> +label: gpt
> +sector-size: $ONE_MiB

I'm concerned about this line.  GPT sector sizes aren't just used
to compute partition sizes.  They are part of the on-disk format,
and moving an image from a disk to a different disk with a different
sector size requires fixing on-disk data structures.  I wrote a
tool to do this for Qubes OS, but it is also possible to generate
a polyglot GPT that works for both 512-byte and 4096-byte sectors.
I recommend we use this for the installer eventually.

That said, if the installer boots with this it should not have a problem.

>  $table
>  EOF
>  
> diff --git a/scripts/sfdisk-field.awk b/scripts/sfdisk-field.awk
> index e13c86d..78b438e 100644
> --- a/scripts/sfdisk-field.awk
> +++ b/scripts/sfdisk-field.awk
> @@ -1,7 +1,7 @@
>  #!/usr/bin/awk -f
>  #
>  # SPDX-License-Identifier: EUPL-1.2+
> -# SPDX-FileCopyrightText: 2022, 2024 Alyssa Ross <hi@alyssa.is>
> +# SPDX-FileCopyrightText: 2022, 2024-2025 Alyssa Ross <hi@alyssa.is>
>  
>  BEGIN {
>  	types["root.aarch64"] = "b921b045-1df0-41c3-af44-4c6f280d3fae"
> @@ -9,12 +9,11 @@ BEGIN {
>  	types["verity.aarch64"] = "df3300ce-d69f-4c92-978c-9bfb0f38d820"
>  	types["verity.x86_64"] = "2c7357ed-ebd2-46d9-aec1-23d437ec2bf5"
>  
> -	# Field #1 is the partition path, which make-gpt.sh will turn into
> -	# the size field.  Since it's handled elsewhere, we skip that
> -	# first field.
> +	# Field #1 is the partition path, which is read by make-gpt.sh
> +	# but not relevant for running sfdisk, so skip it.
>  	skip=1
>  
> -	split("type uuid name", keys)
> +	split("type uuid name size", keys)
>  	split(partition, fields, ":")
>  
>  	arch = ENVIRON["ARCH"]
> @@ -31,8 +30,21 @@ BEGIN {
>  		if (keys[n - skip] == "type") {
>  			if (uuid = types[fields[n] "." arch])
>  				fields[n] = uuid
> +		} else if (keys[n - skip] == "size") {
> +			if (fields[n] < size) {
> +				printf "%s MiB partition content is too big for %s MiB partition\n",
> +					size, fields[n] > "/dev/stderr"
> +				exit 1
> +			}
> +
> +			size = fields[n]
> +			continue # Handled at the end.
>  		}
>  
>  		printf "%s=%s,", keys[n - skip], fields[n]
>  	}
> +
> +	# Always output a size field, either supplied in input or
> +	# default value of the size variable.
> +	printf "size=%s\n", size
>  }
> 
> base-commit: c43e5c63a028994d5f66a15db19f415bf3cb7736

This should work if I read the code correctly, so if it passes tests:

Acked-by: Demi Marie Obenour <demiobenour@gmail.com>

The check for not fitting in the partition is a nice improvement over
my patch.
-- 
Sincerely,
Demi Marie Obenour (she/her/hers)

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