I've attached a test program 'fsmount.c'. This can be used along with
the test script below to show a kernel bug when calling fsconfig(2)
with security options on a btrfs filesystem.

This problem only occurs using fsconfig(2) when attempting to add
security options. Setting a native btrfs option (e.g. flushoncommit)
works.

Copy the statements below into test.sh and run with the fs name. Other
fs will work such as ext4, xfs. Only btrfs will fail.

#!/bin/sh
fs_name=$1

mkdir -p /mnt/selinux-testsuite
dd if=/dev/zero of=./fstest bs=4096 count=27904
dev=`losetup -f`
losetup $dev ./fstest
mkfs.$fs_name $dev
/usr/bin/systemctl stop udisks2 # Stops crap appearing in journal log
# mount(2) works:
#mount -t $fs_name -o "rootcontext=system_u:object_r:unconfined_t:s0"
$dev /mnt/selinux-testsuite
# This native btrfs "flushoncommit" option will work with fsconfig(2):
#./fsmount $fs_name $dev  /mnt/selinux-testsuite "flushoncommit"
# This will not:
./fsmount $fs_name $dev  /mnt/selinux-testsuite
"rootcontext=system_u:object_r:unconfined_t:s0"
# rootcontext fails with journal entry: SELinux: mount invalid.
#    Same superblock, different security settings for (dev loop0, type
btrfs)
umount /mnt/selinux-testsuite
losetup -d $dev
/usr/bin/systemctl start udisks2
rm -f ./fstest