linux-kernel - Re: Finding hardlinks

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Message-ID: <Pine.LNX.4.64.0701040032460.31506@artax.karlin.mff.cuni.cz>
Date:	Thu, 4 Jan 2007 00:43:20 +0100 (CET)
From:	Mikulas Patocka <mikulas@...ax.karlin.mff.cuni.cz>
To:	Frank van Maarseveen <frankvm@...nkvm.com>
Cc:	Bryan Henderson <hbryan@...ibm.com>,
	Arjan van de Ven <arjan@...radead.org>,
	Jan Harkes <jaharkes@...cmu.edu>,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	Miklos Szeredi <miklos@...redi.hu>,
	Pavel Machek <pavel@...e.cz>
Subject: Re: Finding hardlinks

On Wed, 3 Jan 2007, Frank van Maarseveen wrote:

> On Wed, Jan 03, 2007 at 01:09:41PM -0800, Bryan Henderson wrote:
>>> On any decent filesystem st_ino should uniquely identify an object and
>>> reliably provide hardlink information. The UNIX world has relied upon
>> this
>>> for decades. A filesystem with st_ino collisions without being hardlinked
>>> (or the other way around) needs a fix.
>>
>> But for at least the last of those decades, filesystems that could not do
>> that were not uncommon.  They had to present 32 bit inode numbers and
>> either allowed more than 4G files or just didn't have the means of
>> assigning inode numbers with the proper uniqueness to files.  And the sky
>> did not fall.  I don't have an explanation why,
>
> I think it's mostly high end use and high end users tend to understand
> more. But we're going to see more really large filesystems in "normal"
> use so..
>
> Currently, large file support is already necessary to handle dvd and
> video. It's also useful for images for virtualization. So the failing stat()
> calls should already be a thing of the past with modern distributions.

As long as glibc compiles by default with 32-bit ino_t, the problem exists 
and is severe --- programs handling large files, such as coreutils, tar, 
mc, mplayer, already compile with 64-bit ino_t and off_t, but the user (or 
script) may type something like:

cat >file.c <<EOF
#include <sys/types.h>
#include <sys/stat.h>
main()
{
 	int h;
 	struct stat st;
 	if ((h = creat("foo", 0600)) < 0) perror("creat"), exit(1);
 	if (fstat(h, &st)) perror("stat"), exit(1);
 	close(h);
 	return 0;
}
EOF
gcc file.c; ./a.out

--- and you certainly do not want this to fail (unless you are out of disk 
space).

The difference is, that with 32-bit program and 64-bit off_t, you get 
deterministic failure on large files, with 32-bit program and 64-bit 
ino_t, you get random failures.

Mikulas
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