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Add /etc/xattr.conf in the sandbox to workaround cross-FS copy issue #3754

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Add /etc/xattr.conf in the sandbox to workaround cross-FS copy issue #3754

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Add /etc/xattr.conf in the sandbox to workaround cross-FS copy issue
gdonval May 30, 2025
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30 changes: 29 additions & 1 deletion mkosi/tree.py
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Original file line number Diff line number Diff line change
Expand Up @@ -148,7 +148,35 @@ def copy() -> None:
if src.is_dir():
cmdline += ["--no-target-directory"]

run(cmdline, sandbox=sandbox(options=options))
# A valid /etc/xattr.conf is necessary to work around cross-filesystem copy failures:
# https://github.com/systemd/mkosi/issues/3732.
# A bug is opened with coreutils:
# https://debbugs.gnu.org/cgi/bugreport.cgi?bug=78623
# When the bug closes, this can collapse to the following with appropriate adjustments:
#
# run(cmdline, sandbox=sandbox(options=options))
#
with tempfile.TemporaryDirectory() as tmp:
import textwrap

p = Path(tmp) / "xattr.conf"
p.write_text(
textwrap.dedent(
"""
* skip # Default: ignore all xattr but
system.* permissions # allow well-known 'system'
trusted.* permissions # allow well-known 'trusted'
user.* permissions # allow well-known 'user'
security.* permissions # allow well-known 'security'
# but
trusted.SGI_* skip # skip XFS-specific namespace (not portable)
user.Beagle.* skip # skip Beagle index data (not portable)
security.evm skip # skip evm (may only be written by kernel)
"""
)
)
Comment on lines +163 to +177
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Can we make this a bit smarter and also copy the btrfs attrs if we detect the destination filesystem is also btrfs?

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@gdonval gdonval Jun 5, 2025 •
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The only btrfs xattr I know about is btrfs.compression, which is definitely something I think should be set by systemd-repart (Compression=...), not copied over from whatever host configuration is.

Normal file attributes (chattr) are not affected by this barred one: btrfs that calculates the c-flag on the fly depending on btrfs.compression xattr, which we've established is not something we should be copying over anyway.

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Have we established that? I don't remember agreeing to that. If a file is compressed on one btrfs filesystem, why shouldn't it be compressed after copying?

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If a file is compressed on one btrfs filesystem, why shouldn't it be compressed after copying?

Because the one who "chose" to compress it was myself from 7 years ago, not the package manager.

On my system, I, myself, chose to btrfs set property set / compression "zstd", 7 years ago when I installed my local system. And now, this is inherited by mkosi's workspace folder (as xattr btrfs.compression = zstd).

Do we want to persist local arbitrary choices or do we want that choice to be explicit made in repart.d files?

If you want btrfs.compression to be copied, you need to make sure you start with a clean slate (i.e. clean file system) so that you can ensure this was an explicit packaging choice.

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I can make it smarter, but if we do, we might as well make a few checks on a single canary file before copying the tree and use cp -a, which will opportunistically keep as much metadata as possible.

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@gdonval gdonval Jun 6, 2025 •
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I see the problem... systemd-repart does not set btrfs.compression. Compression=zstd seems to actually do nothing with btrfs: it's supposed to use a mkfs.btrfs flag to force compression whilst copying files but compsize reports no compression was ever attempted. That definitely is suboptimal. Proof:

image

Debug shows the same:

image

root-x86-64 is as generated by systemd-repart with Compression=zstd whereas new-btrfs-root was generated with sudo mkfs.btrfs --compress=zstd -L "new-btrfs-root" -r ".../root-x86-64/" btrfs.img.

Not a bug in mkosi, just a constraint added to a very hairy problem...

local_options = options + ["--ro-bind", p, "/etc/xattr.conf"]
run(cmdline, sandbox=sandbox(options=local_options))

# Subvolumes always have inode 256 so we can use that to check if a directory is a subvolume.
if (
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