Re: Finding hardlinks

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

> > 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.

There is 32-bit __st_ino and 64-bit st_ino --- what is their purpose? Some 
old compatibility code?

> SUSv3 requires st_ino/st_dev to be unique within a system so the
> application shouldn't need to bend over backwards.

I see but kernel needs to be fixed for that. Would patches for changing 
kstat be accepted?

Mikulas

> Miklos
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