user_subvol_rm_allowed? Is there a user_subvol_create_deny|allowed?

[Nicholas D Steeves <nsteeves@gmail.com>]

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Dear btrfs community,

Please accept my apologies in advance if I missed something in recent
btrfs development; my MUA tells me I'm ~1500 unread messages
out-of-date. :/

I recently read about "mount -t btrfs -o user_subvol_rm_allowed" while
doing reading up on LXC handling of snapshots with the btrfs backend.
Is this mount option per-subvolume, or per volume?

Also, what mechanisms to restrict a user's ability to create an
arbitrarily large number of snapshots?  Is there a
user_subvol_create_deny|allowed?  What I've read about the inverse
correlation between number of subvols to performance, a potentially
hostile user could cause an IO denial of service or potentially even
trigger an ENOSPC.

From what I gather, the following will reproduce the hypothetical
issue related to my question:

# as root
btrfs sub create /some/dir/subvol
chown some-user /some/dir/subvol

# as some-user
cd /home/dir/subvol
cp -ar --reflink=always /some/big/files ./
COUNT=1
while [ 0 -lt 1 ]; do
  btrfs sub snap ./ ./snapshot-$COUNT
  COUNT=COUNT+1
  sleep 2   # --maybe unnecessary
done

--

I hope there's something I've misunderstood or failed to read!

Please CC me so your reply will hit my main inbox :-)
Nicholas

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