linux-kernel - Re: Finding hardlinks

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Date:	Mon, 1 Jan 2007 23:47:06 +0100 (CET)
From:	Mikulas Patocka <mikulas@...ax.karlin.mff.cuni.cz>
To:	Pavel Machek <pavel@...e.cz>
Cc:	Arjan van de Ven <arjan@...radead.org>,
	Jan Harkes <jaharkes@...cmu.edu>,
	Miklos Szeredi <miklos@...redi.hu>,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: Finding hardlinks

Hi!

>>>> If user (or script) doesn't specify that flag, it
>>>> doesn't help. I think
>>>> the best solution for these filesystems would be
>>>> either to add new syscall
>>>> 	int is_hardlink(char *filename1, char *filename2)
>>>> (but I know adding syscall bloat may be objectionable)
>>>
>>> it's also the wrong api; the filenames may have been
>>> changed under you
>>> just as you return from this call, so it really is a
>>> "was_hardlink_at_some_point()" as you specify it.
>>> If you make it work on fd's.. it has a chance at least.
>>
>> Yes, but it doesn't matter --- if the tree changes under
>> "cp -a" command, no one guarantees you what you get.
>> 	int fis_hardlink(int handle1, int handle 2);
>> Is another possibility but it can't detect hardlinked
>> symlinks.
>
> Ugh. Is it even legal to hardlink symlinks?

Why it shoudln't be? It seems to work quite fine in Linux.

> Anyway, cp -a is not the only application that wants to do hardlink
> detection.

I tested programs for ino_t collision (I intentionally injected it) and 
found that CP from coreutils 6.7 fails to copy directories but displays 
error messages (coreutils 5 work fine). MC and ARJ skip directories with 
colliding ino_t and pretend that operation completed successfuly. FTS 
library fails to walk directories returning FTS_DC error. Diffutils, find, 
grep fail to search directories with coliding inode numbers. Tar seems 
tolerant except incremental backup (which I didn't try). All programs 
except diff were tolerant to coliding ino_t on files.

ino_t is no longer unique in many filesystems, it seems like quite serious 
data corruption possibility.

Mikulas

> 						Pavel
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