[Bug 2146538] Re: Kernel image build is not reproducible due to uname

** Tags added: kernel-daily-bug

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https://bugs.launchpad.net/bugs/2146538

Title:
Kernel image build is not reproducible due to uname

Status in linux package in Ubuntu:
New

Bug description:
Dear linux package maintainers,

A build of the linux-image-unsigned performed by launchpad/Ubuntu can
currently not be fully reproduced by a local build.

One of the issues is that the kernel build embeds information about
the build system and time in the uname string:

For example:

```
sudo file /boot/vmlinuz-6.19.0-6-generic
/boot/vmlinuz-6.19.0-6-generic: Linux kernel x86 boot executable, bzImage, version 6.19.0-6-generic (buildd@lcy02-amd64-071) #6-Ubuntu SMP PREEMPT_DYNAMIC Wed Feb 18 15:48:21 UTC 2026, RO-rootFS, Normal VGA, setup size 512*39, syssize 0x104200, jump 0x26c 0x8cd88ec0fc8cd239 instruction, protocol 2.15, from protected-mode code at offset 0x2cc 0x1008b9f bytes ZST compressed, relocatable, handover offset 0x1035640, legacy 64-bit entry point, can be above 4G, 32-bit EFI handoff entry point, 64-bit EFI handoff entry point, EFI kexec boot support, xloadflags bit 5, max cmdline size 2047, init_size 0x44ba000

uname --all
Linux ubuntu 6.19.0-6-generic #6-Ubuntu SMP PREEMPT_DYNAMIC Wed Feb 18 15:48:21 UTC 2026 x86_64 GNU/Linux
```


The `buildd@lcy02-amd64-071` depend on the build machine, and the date `Wed Feb 18 15:48:21 UTC 2026` is **not** set based on the SOURCE_DATE_EPOCH variable but set depending on when the build is performed.


Debian solves this by setting the following variables during the build, it would be nice if the Ubuntu kernels could follow this:

KBUILD_BUILD_USER: for example to `kernel-team`
KBUILD_BUILD_HOST: for example to `lists.ubuntu.com`
KBUILD_BUILD_TIMESTAMP: to a date based on or derived from SOURCE_DATE_EPOCH

On Debian this results in an output of `file` similar to the
following, and allows the package builds to be reproduced:

```
/usr/lib/modules/6.18.5+deb14-amd64/vmlinuz: Linux kernel x86 boot executable, bzImage, version 6.18.5+deb14-amd64 (debian-kernel@lists.debian.org) #1 SMP PREEMPT_DYNAMIC Debian 6.18.5-1 (2026-01-16)
```


<https://sources.debian.org/src/linux/6.19.8-1/debian/rules.real#L43>:

```
SOURCE_DATE_UTC_ISO := $(shell date -u -d '@$(SOURCE_DATE_EPOCH)' +%Y-%m-%d)
...
setup_env += DISTRIBUTION_OFFICIAL_BUILD=1 DISTRIBUTOR="$(DEB_VENDOR)" DISTRIBUTION_VERSION="$(SOURCEVERSION)" KBUILD_BUILD_TIMESTAMP="@$(SOURCE_DATE_EPOCH)" KBUILD_BUILD_VERSION_TIMESTAMP="$(DEB_VENDOR) $(SOURCEVERSION) ($(SOURCE_DATE_UTC_ISO))" KBUILD_BUILD_USER="$(word 1,$(subst @, ,$(MAINTAINER)))" KBUILD_BUILD_HOST="$(word 2,$(subst @, ,$(MAINTAINER)))"
```

It would be nice if these 3 variables could be set similarly to
Debian, allowing the Ubuntu package build to be reproduced locally.

Thank you,
Jonathan Hettwer

ProblemType: Bug
DistroRelease: Ubuntu 26.04
Package: linux-image-unsigned-7.0.0-10-generic (not installed)
ProcVersionSignature: Ubuntu 6.19.0-6.6-generic 6.19.2
Uname: Linux 6.19.0-6-generic x86_64
AlsaDevices:
total 0
crw-rw---- 1 root audio 116, 1 Mar 24 18:02 seq
crw-rw---- 1 root audio 116, 33 Mar 24 18:02 timer
AplayDevices: Error: [Errno 2] No such file or directory: 'aplay'
ApportVersion: 2.33.1-0ubuntu3
Architecture: amd64
ArecordDevices: Error: [Errno 2] No such file or directory: 'arecord'
AudioDevicesInUse: Error: command ['fuser', '-v', '/dev/snd/seq', '/dev/snd/timer'] failed with exit code 1:
CRDA: N/A
CasperMD5CheckResult: unknown
CloudArchitecture: x86_64
CloudBuildName: server
CloudID: nocloud
CloudName: unknown
CloudPlatform: nocloud
CloudSerial: 20260221
CloudSubPlatform: config-disk (/dev/sr0)
CurrentDmesg: Error: command ['dmesg'] failed with exit code 1: dmesg: read kernel buffer failed: Operation not permitted
Date: Thu Mar 26 16:43:11 2026
Lsusb: Error: command ['lsusb'] failed with exit code 1:
Lsusb-t:

Lsusb-v: Error: command ['lsusb', '-v'] failed with exit code 1:
MachineType: QEMU Standard PC (Q35 + ICH9, 2009)
PciMultimedia:

ProcEnviron:
LANG=C.UTF-8
PATH=(custom, user)
SHELL=/bin/bash
TERM=tmux-256color
XDG_RUNTIME_DIR=<set>
ProcFB: 0 bochs-drmdrmfb
ProcKernelCmdLine: BOOT_IMAGE=/vmlinuz-6.19.0-6-generic root=LABEL=cloudimg-rootfs ro console=tty1 console=ttyS0
RelatedPackageVersions:
firmware-sof N/A
linux-firmware N/A
RfKill: Error: [Errno 2] No such file or directory: 'rfkill'
SourcePackage: linux
UpgradeStatus: No upgrade log present (probably fresh install)
WifiSyslog:

acpidump:

dmi.bios.date: 02/06/2015
dmi.bios.release: 0.0
dmi.bios.vendor: EFI Development Kit II / OVMF
dmi.bios.version: 0.0.0
dmi.chassis.type: 1
dmi.chassis.vendor: QEMU
dmi.chassis.version: pc-q35-7.2
dmi.modalias: dmi:bvnEFIDevelopmentKitII/OVMF:bvr0.0.0:bd02/06/2015:br0.0:svnQEMU:pnStandardPC(Q35+ICH9,2009):pvrpc-q35-7.2:cvnQEMU:ct1:cvrpc-q35-7.2:sku:
dmi.product.name: Standard PC (Q35 + ICH9, 2009)
dmi.product.version: pc-q35-7.2
dmi.sys.vendor: QEMU

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