While testing the "append data" ACL permission, I wantted to try this out with cifs vfs against a Windows 10 machine as server. "tee -a" is using O_APPEND but it looks like cifs vfs is not translating O_APPEND open calls to SMB's FILE_APPEND_DATA. So files that have "append only" permission but not "write data" permission will not be "append writable" by cifs vfs in cases where they shold.