Re: [PATCH v2] scripts/make-gpt.sh: allow setting partition size

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

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On 11/28/25 16:35, Alyssa Ross wrote:
> On Fri, Nov 28, 2025 at 04:31:21PM -0500, Demi Marie Obenour wrote:
>> On 11/28/25 16:26, Alyssa Ross wrote:
>>> Demi Marie Obenour <demiobenour@gmail.com> writes:
>>>
>>>> On 11/28/25 16:14, Alyssa Ross wrote:
>>>>> Demi Marie Obenour <demiobenour@gmail.com> writes:
>>>>>
>>>>>> On 11/28/25 08:22, 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>
>>>>>>> Message-ID: <20251127174054.2056835-2-hi@alyssa.is>
>>
>> (snip)
>>
>>>>>>>  # Keep 1MiB free at the start, and 1MiB free at the end.
>>>>>>> -gptBytes=$((ONE_MiB * 2))
>>>>>>> -for partition; do
>>>>>>> -	sizeMiB="$(sizeMiB "${partition%%:*}")"
>>>>>>> -	table="$table${nl}size=${sizeMiB}MiB,$(awk -f "$scriptsDir/sfdisk-field.awk" -v partition="$partition")"
>>>>>>> -	gptBytes="$((gptBytes + sizeMiB * ONE_MiB))"
>>>>>>> -done
>>>>>>> +gptMiB=2
>>>>>>> +while read -r partition; do
>>>>>>> +	# Here we rely on sfdisk-field.awk always putting size last.
>>>>>>> +	: $((gptMiB += ${partition##*=}))
>>>>>>> +done <<EOF
>>>>>>> +$table
>>>>>>> +EOF
>>>>>>
>>>>>> Nit: should ${partition##*=} be in double quotes?
>>>>>
>>>>> I tried changing it, but it is apparently an "arithmetic syntax error"
>>>>> to use double quotes here.
>>>>
>>>> I'd use a separate variable then.
>>>
>>> Doesn't seem to make a difference.  As an experiment:
>>>
>>> partition="size=2 + 3"
>>> size=${partition##*=}
>>> printf "%s\n" $(($size))
>>>
>>> This prints 5 even though a separate variable is used.  It doesn't seem
>>> to be possible to get $(( … )) *not* to interpret arithmetic in
>>> variables.
>>>
>>> In bash, but not busybox ash as used in Nix builds, it is possible to
>>> use double quotes like you suggested, but $(("$size")) is /still/ 5.
>>
>> That's actually required by POSIX.  What about:
>>
>> size=${partition##*=}
>> : $((gptMiB += size))
> 
> How's that better?  Arithmetic will still be expanded inside size —
> or do you have some other concern with the parameter expansion inside
> the arithmetic expansion?

Nevermind.  The only way it would be better is if the input was
explicitly validated, which might not be worthwile.  In bash, bad
$size even allows code execution in at least the default mode.
-- 
Sincerely,
Demi Marie Obenour (she/her/hers)

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