Description:
There was a misunderstanding about the enforcement details of the "Read-only"
flag introduced for SMB/CIFS compatibility, way back in 2007 in the Sun PSARC
2007/315 case.
The original authors thought enforcement of the READONLY flag should work
similarly as the IMMUTABLE flag. Unfortunately, that enforcement is
incompatible with the expectations of Windows applications using this feature
through the SMB service. Applications assume (and the MS File System Algorithm

MS-FSA confirms they should) that an SMB client can:
(a) Open an SMB handle on a file with read/write access,
(b) Set the DOS attributes to include the READONLY flag,
(c) continue to have write access via that handle.
This access model is essentially the same as a Unix/POSIX application that
creates a file (with read/write access), uses fchmod() to change the file mode
to something not granting write access (i.e. 0444), and then continues to writ

that file using the open handle it got before the mode change.
Currently, the SMB server works-around this problem in a way that will become
difficult to maintain as we implement support for SMB3 persistent handles, so
SMB depends on this fix.
I've written a test program that can be used to demonstrate this problem, and
added it to zfs-tests (tests/functional/acl/cifs/cifs_attr_004_pos).
It currently fails, but will pass when this problem fixed.
Steps to Reproduce:
  Run the test program on a ZFS file system.
Expected Results:
  Pass
Actual Results:
  Fail.