Before, when using shred on a device, one had to specify --exact,

Before, when using shred on a device, one had to specify --exact,
or be careful to choose a size that would not be rounded up and
exceed the maximum value;  that could result in a failure of
the final write.

(do_wipefd): --exact is now the default for non-regular
files.  Suggestion from Ben Elliston.
(usage): Say it.