linux-kernel - Re: Finding hardlinks

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Date:	Thu, 4 Jan 2007 01:12:24 +0100
From:	Frank van Maarseveen <frankvm@...nkvm.com>
To:	Mikulas Patocka <mikulas@...ax.karlin.mff.cuni.cz>
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 Thu, Jan 04, 2007 at 12:43:20AM +0100, Mikulas Patocka wrote:
> On Wed, 3 Jan 2007, Frank van Maarseveen wrote:
> >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.

What's (technically) the problem with changing the gcc default?

Alternatively we could make the error deterministic in various ways. Start
st_ino numbering from 4G (except for a few special ones maybe such
as root/mounts). Or make old and new programs look differently at the
ELF level or by sys_personality() and/or check against a "ino64" mount
flag/filesystem feature. Lots of possibilities.

-- 
Frank
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