So NAK on both Amerigo's and Eric's patch. ATTR_FORCE is wrong.But file_remove_suid() actually adds ATTR_FORCE too, in __remove_suid()...
ATTR_FORCE here is going to force the security system to accept ALL of
the attr changes, not just the ATTR_KILL_SUID and/or ATTR_KILL_SGID you
|= in from should_remove_suid.
You need to follow esandeen's recommendation, change file_remove_suid()
to dentry_remove_suid() and then use dentry_remove_suid() in do_truncate
instead of what you are currently trying to do (and I think that's
supposed to be done under the i_mutex right?)