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The one thing I noticed is that there is a random PARTUUID for the X86_64
target, and that it gets regenerated on every (re)build. This makes the
image different every time, even if nothing has changed.
PARTUUID can't be the source of the rebuild as PARTUUID is not target or
dependency in the make's build pipeline. The (re)build times here are the same,
with or without ptgen's random signature.
It is probably true that a different PARTUUID would not cause a rebuild of some
source files.
But then again, a different PARTUUID would probably cause a rebuild of the final image file, even if nothing else has changed. There is no need for
such a rebuild if the image contents have not changed in the meantime.
Besides, like I mentioned in my first mail, this random PARTUUID is an obstacle for repeatable builds. In my opinion, repeatable builds is a desirable
goal. It can help troubleshoot some problems, but it is especially interesting from a security point of view. Two independent parties can verify that
a binary was created from the stated sources. This particular UUID issue is listed here as a problem:
https://reproducible-builds.org/docs/system-images/
Is there a reason why this partition ID has to be random? Such things make it
impossible to do repeatable builds.
It's UUID, so I think it should be unique. It seems like you could use
CONFIG_TARGET_ROOTFS_PARTNAME for fixed root partition name, but by looking at
the code, it might probably need some love to get it working properly.
Thanks for pointing out CONFIG_TARGET_ROOTFS_PARTNAME.
However, you did not actually give a reason why this OpenWrt PARTUUID has to be
random at build time.
A unique ID is certainly interesting for USB sticks or external disks that need to be recognised when mounted again. Or if you have several partitions
on a rather dynamic hard disk. But the OpenWrt system partition probably does not need such an ID. Even if it did, maybe for the benefit of other
operating systems in a multiboot environment, it does not make sense to regenerate this PARTUUID every time. The partition IDs on Linux do not change
when the files inside the partition change.
If you think about it, a fixed PARTUUID is a bad idea in any case. You could install the same OpenWrt image on 2 different partitions on your hard
disk, and then both copies would have the same PARTUUID, which actually beats the purpose of such UUIDs. Setting a PARTUUID should probably be done by
the installer, and not be a random but fixed value embedded during image build time.
Regards,
rdiez
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