Arch Linux

musbur

Member

Registered: 2022-01-14

Posts: 69

Why does mkinitcpio put wrong timestamp on UKI?

I was chasing some bug in my UKI and at some point was unsure if I had even booted the right image. The image seemed way out of date but it turned out that it was indeeed "fresh." Long story short, why is the filesystem timestamp on the UKI December 19th when inside it there is actually a Linux kernel from Dec 27th and the whole thing was created on Dec 29?

~# ls -l /boot/EFI/Linux/arch-linux.efi
-rwxr-xr-x 1 root root 40977408 Dec 19 22:36 /boot/EFI/Linux/arch-linux.efi
~# strings /boot/EFI/Linux/arch-linux.efi | grep linux@archlinux
6.12.7-arch1-1 (linux@archlinux) #1 SMP PREEMPT_DYNAMIC Fri, 27 Dec 2024 14:24:37 +0000
6.12.7-arch1-1 (linux@archlinux) (gcc (GCC) 14.2.1 20240910, GNU ld (GNU Binutils) 2.43.0) #1 SMP PREEMPT_DYNAMIC Fri, 27 Dec 2024 14:24:37 +0000
~# ls -l /boot/vmlinuz-linux
-rwxr-xr-x 1 root root 13812224 Dec 29 21:27 /boot/vmlinuz-linux

I can delete boot/EFI/Linux/arch-linux.efi, then do mkinitcpio, and it's right back there with an mtime from 10 days ago. Weird.

dimich

Member

From: Kharkiv, Ukraine

Registered: 2009-11-03

Posts: 313

Re: Why does mkinitcpio put wrong timestamp on UKI?

What are mount options for /boot?

$ mount -l | grep /boot

$ grep /boot /etc/fstab

Does mtime become correct after

# touch /boot/EFI/Linux/arch-linux.efi

?

musbur

Member

Registered: 2022-01-14

Posts: 69

Re: Why does mkinitcpio put wrong timestamp on UKI?

This is interesting: stat shows the correct creation time (birth) of the file. I can change all other dates using touch, but the mkinitcpio process deliberately "backdates" the created UKI. See shell session below.

~# mount -l | grep boot
/dev/nvme0n1p1 on /boot type vfat (rw,relatime,fmask=0022,dmask=0022,codepage=437,iocharset=ascii,shortname=mixed,utf8,errors=remount-ro) [BOOT]
~# stat /boot/EFI/Linux/arch-linux.efi
  File: /boot/EFI/Linux/arch-linux.efi
  Size: 40977408        Blocks: 80040      IO Block: 4096   regular file
Device: 259,1   Inode: 13          Links: 1
Access: (0755/-rwxr-xr-x)  Uid: (    0/    root)   Gid: (    0/    root)
Access: 2024-12-29 01:00:00.000000000 +0100
Modify: 2024-12-19 22:36:14.000000000 +0100
Change: 2024-12-19 22:36:14.000000000 +0100
 Birth: 2024-12-29 21:27:21.240000000 +0100
~# touch /boot/EFI/Linux/arch-linux.efi
~# stat /boot/EFI/Linux/arch-linux.efi
  File: /boot/EFI/Linux/arch-linux.efi
  Size: 40977408        Blocks: 80040      IO Block: 4096   regular file
Device: 259,1   Inode: 13          Links: 1
Access: (0755/-rwxr-xr-x)  Uid: (    0/    root)   Gid: (    0/    root)
Access: 2024-12-31 01:00:00.000000000 +0100
Modify: 2024-12-31 13:25:16.000000000 +0100
Change: 2024-12-31 13:25:16.000000000 +0100
 Birth: 2024-12-29 21:27:21.240000000 +0100
~# mkinitcpio -P linux
==> Building image from preset: /etc/mkinitcpio.d/linux.preset: 'default'
[...]
==> Unified kernel image generation successful
~# stat /boot/EFI/Linux/arch-linux.efi
  File: /boot/EFI/Linux/arch-linux.efi
  Size: 40977408        Blocks: 80040      IO Block: 4096   regular file
Device: 259,1   Inode: 29          Links: 1
Access: (0755/-rwxr-xr-x)  Uid: (    0/    root)   Gid: (    0/    root)
Access: 2024-12-31 01:00:00.000000000 +0100
Modify: 2024-12-19 22:36:14.000000000 +0100
Change: 2024-12-19 22:36:14.000000000 +0100
 Birth: 2024-12-31 13:26:02.320000000 +0100
~#

dimich

Member

From: Kharkiv, Ukraine

Registered: 2009-11-03

Posts: 313

Re: Why does mkinitcpio put wrong timestamp on UKI?

the mkinitcpio process deliberately "backdates" the created UKI.

Interesting. Mkinitcpio doesn't generate .efi by itself, it invokes ukify. Signing tool also may be involved, some additional ALPM hooks as well.
Out of curiosity I'd play with strace or bpftrace to watch processes which may do syscalls affecting modification time.

musbur

Member

Registered: 2022-01-14

Posts: 69

Re: Why does mkinitcpio put wrong timestamp on UKI?

According to the Wiki, mkinitcpio and ukify are alternatives to make the UKI (I don't have systemd-ukify installed). "strace mkinitcpio" hasn't turned up anything interesting yet because most action seems to be done by subprocesses. I may lose interest in this topc before I get to the root of it...

Head_on_a_Stick

Member

From: The Wirral

Registered: 2014-02-20

Posts: 8,738

Website

Re: Why does mkinitcpio put wrong timestamp on UKI?

If systemd-ukify is not installed mkinitcpio will use objcopy instead: https://gitlab.archlinux.org/archlinux/ … heads#L529

No idea about the timestamps though, that is strange

EDIT: systemd-ukify is just a Python wrapper for objcopy & sbsign.

Last edited by Head_on_a_Stick (2025-01-01 11:17:41)

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Para todos todo, para nosotros nada

musbur

Member

Registered: 2022-01-14

Posts: 69

Re: Why does mkinitcpio put wrong timestamp on UKI?

mkinitramfs uses objcopy with -p, and I found this in objcopy's manpage:

       -p
       --preserve-dates
           Set the access and modification dates of the output file to
           be the same as those of the input file.

           This option also copies the date stored in a PE format file's
           header, unless the SOURCE_DATE_EPOCH environment variable is
           defined.  If it is defined then this variable will be used as
           the date stored in the header, interpreted as the number of
           seconds since the Unix epoch.

So there's something about timestamps that is important with respect to initramfs which seems to have to do with determinism. If you do the same thing twice you want the bit-exact same output, so if you package a whole filesystem including timestamps in one file, that file will differ depending on when you did that unless you reset all timestamps inside the filesystem to the same value each time. But I wouldn't have thought that that also applies to the initramfs file itself, living in an "external" fs. I have decided to terminate my research at this point.

nl6720

The Evil Wiki Admin

Registered: 2016-07-02

Posts: 667

Re: Why does mkinitcpio put wrong timestamp on UKI?

mkinitcpio runs objcopy with /usr/lib/systemd/boot/efi/linuxx64.efi.stub (or /usr/lib/systemd/boot/efi/linuxia32.efi.stub) as the input file. The kernel and etc. are simply added as sections.
Check the timestamps of that file instead.