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b477cf8e97
As documented in ioctl_iflags(2):
```
FS_IMMUTABLE_FL
The file is immutable: no changes are permitted to the file contents
or metadata (permissions, timestamps, ownership, link count, and so
on). (This restriction applies even to the superuser.)
```
For example, any user cannot delete/move layer blobs when
FS_IMMUTABLE_FL is set:
``` sh
# cd /var/lib/containerd/io.containerd.snapshotter.v1.erofs/snapshots/4
# mv layer{,1}.erofs
mv: cannot move 'layer.erofs' to 'layer1.erofs': Operation not permitted
# rm layer.erofs
rm: cannot remove 'layer.erofs': Operation not permitted
```
Note that it's a best-effort approach for data loss prevention. IOWs,
just warn out if FS_IMMUTABLE_FL cannot be set anyway (e.g., due to lack
of support in the underlying filesystem.)
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>