> 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?
The stat64.st_ino field is 64bit, so AFAICS you'd only need to extend
the kstat.ino field to 64bit and fix those filesystems to fill in
kstat correctly.
SUSv3 requires st_ino/st_dev to be unique within a system so the
application shouldn't need to bend over backwards.
Miklos
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