since when does linkat() on deleted /proc/$PID/fd/$num return ENOENT ?

[David Madore <david+ml@madore.org>]

It used to be the case (last time I checked was around late 2008 or
early 2009) that deleted entries from /proc/$PID/fd/ could be linked
back to the filesystem by using linkat(,,,,AT_SYMLINK_FOLLOW).

Now this just returns ENOENT.

I'd like to understand when, how and why this change took place.  What
commit introduced it and was it a deliberate move (e.g., because the
feature was a security issue of itself, or came into conflict with
something else) or was it accidental?  Does it depend on the /proc
filesystem itself or on the target filesystem where the deleted file
used to reside?

(There's a Reddit thread, <URL:
http://www.reddit.com/r/programming/comments/7yx6f/how_to_undelete_any_open_deleted_file_in_linux/
 >, where some people are reporting ENOENT on 2.6.27 or perhaps even
2.6.26, which helps but a bound on the change.)

(See also this thread: <URL:
http://comments.gmane.org/gmane.linux.kernel/1224071
 >, where the question is not answered, however.)

-- 
     David A. Madore
   ( http://www.madore.org/~david/ )