linux-kernel - Re: Finding hardlinks

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Date:	Fri, 5 Jan 2007 15:08:24 +0100 (CET)
From:	Mikulas Patocka <mikulas@...ax.karlin.mff.cuni.cz>
To:	Miklos Szeredi <miklos@...redi.hu>
Cc:	pavel@....cz, matthew@....cx, bhalevy@...asas.com,
	arjan@...radead.org, jaharkes@...cmu.edu,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	nfsv4@...f.org
Subject: Re: Finding hardlinks

>>> Well, sort of.  Samefile without keeping fds open doesn't have any
>>> protection against the tree changing underneath between first
>>> registering a file and later opening it.  The inode number is more
>>
>> You only need to keep one-file-per-hardlink-group open during final
>> verification, checking that inode hashing produced reasonable results.
>
> What final verification?  I wasn't just talking about 'tar' but all
> cases where st_ino might be used to check the identity of two files at
> possibly different points in time.
>
> Time A:    remember identity of file X
> Time B:    check if identity of file Y matches that of file X
>
> With samefile() if you open X at A, and keep it open till B, you can
> accumulate large numbers of open files and the application can fail.
>
> If you don't keep an open file, just remember the path, then renaming
> X will foil the later identity check.  Changing the file at this path
> between A and B can even give you a false positive.  This applies to
> 'tar' as well as the other uses.

And does it matter? If you rename a file, tar might skip it no matter of 
hardlink detection (if readdir races with rename, you can read none of the 
names of file, one or both --- all these are possible).

If you have "dir1/a" hardlinked to "dir1/b" and while tar runs you delete 
both "a" and "b" and create totally new files "dir2/c" linked to "dir2/d", 
tar might hardlink both "c" and "d" to "a" and "b".

No one guarantees you sane result of tar or cp -a while changing the tree. 
I don't see how is_samefile() could make it worse.

Mikulas

> Miklos
>
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