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
>