Comment 3 for bug 451368

SRU Justification:

When truncating inodes in the lower filesystem, eCryptfs directly
invoked vmtruncate(). As Christoph Hellwig pointed out, vmtruncate() is
a filesystem helper function, but filesystems may need to do more than
just a call to vmtruncate(). eCryptfs needs to go through
notify_change()->setattr to ensure truncate works reliable without
corruption or leaking blocks on a variety of filesystems.

See: http://thread.gmane.org/gmane.linux.file-systems/36105

Fix: With this patch , lower inode truncation is moved out of
ecryptfs_truncate() and the function is renamed to truncate_upper().
truncate_upper() updates an iattr for the lower inode to indicate
if the lower inode needs to be truncated upon return.
ecryptfs_setattr() then calls notify_change(), using the updated iattr
for the lower inode, to complete the truncation.

Without the patch we risk corrupting or leaking blocks on the lower
filesystem.

Has been soak tested with a variety of ftruncate() soak tests and also
running a full kernel build on ecryptfs with a lower ext4 filesystem.