This has been reported by me to bug-gnu-utils.mit.edu and logged 
as a cpio maintenance request. I thought you might like to know.

I have seen corrupted files as a result of using the pass-through option 
in cpio. The corruption occurs when active/in-use (and growing) files are 
being copied. The problem is that the file size has grown since the time 
of the stat() call (made in process_copy_pass). This will give you more 
bytes than you expect in the last read and as a result on the next call to 
copy_files_disk_to_disk() you will have "input_size" not starting at zero 
and you will get the ending bytes from the last file copied into the 
current file. 

So the problem is two-fold 1) you do not get the complete file copied; and 
2) you get parts of one file copied to another!

For my purposes, I have rewritten copy_files_disk_to_disk() to ignore the 
num_bytes input parameter and to just copy until EOF. The problem might 
also appear in other parts of the cpio source. I have rewritten it to 
serve my purposes for the moment (to suit my needs given the heavy 
schedule pressure I am under). I think you should make a more thorough 
code inspection to insure all problems of this nature are fixed 
accordingly in the complete cpio source.