Finding hardlinks

[Mikulas Patocka <mikulas@artax.karlin.mff.cuni.cz>]

Hi

I've came across this problem: how can a userspace program (such as for 
example "cp -a") tell that two files form a hardlink? Comparing inode 
number will break on filesystems that can have more than 2^32 files (NFS3, 
OCFS, SpadFS; kernel developers already implemented iget5_locked for the 
case of colliding inode numbers). Other possibilities:

--- compare not only ino, but all stat entries and make sure that
	i_nlink > 1?
	--- is not 100% reliable either, only lowers failure probability
--- create a hardlink and watch if i_nlink is increased on both files?
	--- doesn't work on read-only filesystems
--- compare file content?
	--- "cp -a" won't then corrupt data at least, but will create
	hardlinks where they shouldn't be.

Is there some reliable way how should "cp -a" command determine that? 
Finding in kernel whether two dentries point to the same inode is trivial 
but I am not sure how to let userspace know ... am I missing something?

Mikulas
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