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

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/

2.6.26, which helps but a bound on the change.)

(See also this thread: <URL:
http://comments.gmane.org/gmane.linux.kernel/1224071

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

It was explicitly prohibited since 2.6.39:

commit aae8a97d3ec30788790d1720b71d76fd8eb44b73

Author: Aneesh Kumar K.V <[email protected]>
Date:   Sat Jan 29 18:43:27 2011 +0530

fs: Don't allow to create hardlink for deleted file

Add inode->i_nlink == 0 check in VFS. Some of the file systems do this internally. A followup patch will remove those instance. This is needed to ensure that with link by handle we don't allow to create hardlink of an unlinked file. The check also prevent a race between unlink and link

index 83e92ba..33be51a 100644
@@ -2906,7 +2906,11 @@ int vfs_link(struct dentry *old_dentry, struct inode *dir, struct dentry *new_de
 		return error;
 
 	mutex_lock(&inode->i_mutex);
-	error = dir->i_op->link(old_dentry, dir, new_dentry);
+	/* Make sure we don't allow creating hardlink to an unlinked file */
+	if (inode->i_nlink == 0)
+		error =  -ENOENT;
+	else
+		error = dir->i_op->link(old_dentry, dir, new_dentry);
 	mutex_unlock(&inode->i_mutex);
 	if (!error)
 		fsnotify_link(dir, inode, new_dentry);

<snip>

Thanks for digging this for me!

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

Well, the question if this counts as userspace-visible change.