Reproducible XFS Filesystems Builds for VMs

[Luca DiMaio]

Hello list,

I am a Software Engineer at Chainguard working on reproducible builds
for VMs, and we're happy to say that we've successfully implemented
reproducible disk images with EFI+EXT4 partitions using the following
methods:

- For FAT32 partitions: `mkfs.vfat --invarian -i $EFI_UUID` with
`$SOURCE_DATE_EPOCH` and populating via mtools
- For EXT4 partitions: `mkfs.ext4 -E hash_seed=$EXT4_HASH_SEED -U
$ROOTFS_UUID` with `$SOURCE_DATE_EPOCH` plus the `-d
/path/to/rootfs.tar.gz` to populate it

We're interested in also supplying XFS rootfs, which seems to not be
rb-friendly.
I had some success on having reproducible empty XFS partitions by
using a combination of `libfaketime`
to enforce `$SOURCE_DATE_EPOCH` and a custom library that overwrites
the libc's `getrandom()`:

```
~$ export LD_PRELOAD="./deterministic_rng.so /usr/lib/faketime/libfaketime.so.1"
~$ mkfs.xfs \
-b size=4096 \
-d agcount=4 \
-d noalign \
-i attr=2 \
-i projid32bit=1 \
-i size=512 \
-l size=67108864 \
-l su=4096 \
-l version=2 \
-m crc=1 \
-m finobt=1 \
-m uuid=$ROOTFS_UUID \
-n size=16384 \
-n version=2 disk1.img
~$ mkfs.xfs \
-b size=4096 \
-d agcount=4 \
-d noalign \
-i attr=2 \
-i projid32bit=1 \
-i size=512 \
-l size=67108864 \
-l su=4096 \
-l version=2 \
-m crc=1 \
-m finobt=1 \
-m uuid=$ROOTFS_UUID \
-n size=16384 \
-n version=2 disk2.img
~$ md5sum disk*
c68c202163dcb862762fc01970f6c8b4  disk1.img
c68c202163dcb862762fc01970f6c8b4  disk2.img
```

But this all becomes aleatory when we try to populate the filesystem,
by using mount + untar the source tar.gz

Right now I've opened a thread on the XFS Mailing list, to ask about how to
proceed about this, and if they're interested in making XFS
reproducible-friendly
Here: https://lore.kernel.org/linux-xfs/CAKBQhKVi6FWNWJH2PWUA4Ue=aSrvVcR_r2aJOUh45Nd0YdnxVA@mail.gmail.com/T/#u


I'd like to hear your inputs on the approach here, or also showing
interest in the
XFS thread if anyone else is interested in this.

Regards,
L.