linux-kernel - Re: Finding hardlinks

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Date:	Wed, 20 Dec 2006 12:44:42 +0100
From:	Miklos Szeredi <miklos@...redi.hu>
To:	mikulas@...ax.karlin.mff.cuni.cz
CC:	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: Finding 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?

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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