Bug#1032610: reliably composable initramfs - zero-pad output to allow concatenation

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From	Ben Hutchings <ben@decadent.org.uk>
Newsgroups	linux.debian.bugs.dist, linux.debian.kernel
Subject	Bug#1032610: reliably composable initramfs - zero-pad output to allow concatenation
Date	Mon, 17 Nov 2025 00:30:01 +0100
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Control: tag -1 - patch

On Fri, 10 Mar 2023 01:54:47 +0000 Birgit Edel <edelbitter@posteo.de> wrote:
> Package: initramfs-tools-core
> Version: 0.140
> Severity: wishlist
> File: /usr/sbin/mkinitramfs
> Tags: patch
> 
> mkinitramfs supports compression that neither encodes size, nor is 
> guaranteed to signal the end of compressed archive. Linux refuses to 
> parse additional archives beyond the first compressed one - unless they 
> start at 4-byte boundary like uncompressed ones do.
> 
> I request
> A) to modify mkninitramfs to append such padding and
> B) to add tests to ensure concatenation keeps working (even after, say, 
> compression algo/lvl changes)
[...]

Currently unmkinitramfs is not able to unpack an initramfs image that
has multiple compressed parts.  Since you want mkinitramfs to support
this option, I would expect to see a patch that also makes unmkinitramfs
support it.

(There is an open merge request to make unmkinitramfs use 3cpio where
available:
<https://salsa.debian.org/kernel-team/initramfs-tools/-/merge_requests/172>.
I haven't tested whether 3cpio supports multiple compressed parts; if it
does then only the tests need to be updated to cover this case.)

Ben.

-- 
Ben Hutchings
Beware of programmers who carry screwdrivers. - Leonard Brandwein

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Bug#1032610: reliably composable initramfs - zero-pad output to allow concatenation Ben Hutchings <ben@decadent.org.uk> - 2025-11-17 00:30 +0100
  Bug#1032610: reliably composable initramfs - zero-pad output to allow concatenation Birgit Edel <edelbitter@posteo.de> - 2025-11-17 14:20 +0100

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