pax: Ustar MTIME header field is too small

[Geoff Adams <gadams+netbsd%avernus.com@localhost>]

I have some files that have timestamps outside the range of the traditional 
32-bit unix timestamp. As I understand it, the standardized pax enhancements 
the ustar format specifically allow for this, using extended attributes that 
store the timestamps as decimal numbers as UTF-8 text. It appears that our pax 
does not support generating pax output with these extended attributes. (In 
fact, it doesn't support a '-x pax' output option, which I note that some other 
pax implementations do.)

The following test case demonstrates the problem:

netbsd$ touch -t 192001251150 foo
netbsd$ ls -l foo
-rw-r--r--  1 gadams  wheel  0 Jan 25  1920 foo
netbsd$ pax -w foo > foo.pax
pax: Ustar MTIME header field is too small for foo
zsh: 5847 exit 1     pax -w foo > foo.pax

And the produced archive is simply 10240 bytes of 0x0.

What's very interesting, however, is that I can create a pax archive on an 
OpenSolaris machine containing that file and expand it successfully on NetBSD, 
and the date is maintained:

os$ touch -t 192001251150 bar
os$ pax -w bar > bar.pax
 ( copy pax file to NetBSD machine )
netbsd$ pax -r -pe < ~/bar.pax
netbsd$ ls -l bar
-rw-r--r--  1 gadams  wheel    0 Jan 25  1920 bar

So, our pax supports reading the extended attributes, but refuses to produce 
them. The version from pkgsrc (which I assume would be as up-to-date as the 
latest -current) behaves identically. Is this a known behavior? Should I look 
into adding support? (I haven't really cracked the code open, yet.) As a quick 
solution to my immediate need, should I just look into finding and building 
some third-party pax?

(Side note: I'm trying to move these files from a NetBSD machine to an 
OpenSolaris machine. Our ffs dump format is not compatible with Solaris's 
ufsrestore, so that leaves archive programs like tar/pax. NFS, at least v3, 
can't handle these dates either. Nor can the cpio formats pax supports. Are 
there are other options?)

Thanks,
- Geoff