Bug#998840: Strange warning when deleting subvolume as non-root user

[Adam Borowski <kilobyte@angband.pl>]

----- Forwarded message from Boris Kolpackov <boris@codesynthesis.com> -----
Date: Mon, 8 Nov 2021 16:01:53 +0200
From: Boris Kolpackov <boris@codesynthesis.com>
Subject: Bug#998840: Strange warning when deleting subvolume as non-root user

Package: btrfs-progs
Version: 5.14.1-1

After upgrading from btrfs-progs 5.7-1 with Linux kernel 5.7.6 to
btrfs-progs 5.14.1-1 with Linux kernel 5.14.9-2 (from current testing)
I see the following strange new warning when deleting the subvolume as
non-root user (the filesystem is mounted with user_subvol_rm_allowed):

WARNING: cannot read default subvolume id: Operation not permitted

The subvolume deletion succeeds.

To reproduce, mount a btrfs filesystem with user_subvol_rm_allowed,
say, on /mnt and then (assuming non-root user is called nonroot):

# chown nonroot:nonroot /mnt
# su - nonroot
$ cd /mnt
$ btrfs subvol create test
Create subvolume './test'
$ btrfs subvol delete test
WARNING: cannot read default subvolume id: Operation not permitted
Delete subvolume (no-commit): '/mnt/test'

Any idea what's going on here and how to get rid of this warning?
It adds quite a bit of noise to our logs.


----- End forwarded message -----


Reproduces for me both on 5.14 kernel+progs and 5.15.


Meow!
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