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SubjectFinding hardlinks

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?

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