linux-kernel - Re: [RFC v10][PATCH 09/13] Restore open file descriprtors

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Date:	Fri, 28 Nov 2008 11:27:45 +0000
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Oren Laadan <orenl@...columbia.edu>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...l.org>,
	containers@...ts.linux-foundation.org,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	linux-api@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
	Serge Hallyn <serue@...ibm.com>,
	Dave Hansen <dave@...ux.vnet.ibm.com>,
	Ingo Molnar <mingo@...e.hu>, "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [RFC v10][PATCH 09/13] Restore open file descriprtors

On Wed, Nov 26, 2008 at 08:04:40PM -0500, Oren Laadan wrote:

> +/**
> + * cr_attach_get_file - attach (and get) lonely file ptr to a file descriptor
> + * @file: lonely file pointer
> + */
> +static int cr_attach_get_file(struct file *file)
> +{
> +	int fd = get_unused_fd_flags(0);
> +
> +	if (fd >= 0) {
> +		fsnotify_open(file->f_path.dentry);
> +		fd_install(fd, file);
> +		get_file(file);
> +	}
> +	return fd;
> +}

What happens if another thread closes the descriptor in question between
fd_install() and get_file()?

> +	fd = cr_attach_file(file);	/* no need to cleanup 'file' below */
> +	if (fd < 0) {
> +		filp_close(file, NULL);
> +		ret = fd;
> +		goto out;
> +	}
> +
> +	/* register new <objref, file> tuple in hash table */
> +	ret = cr_obj_add_ref(ctx, file, parent, CR_OBJ_FILE, 0);
> +	if (ret < 0)
> +		goto out;

Who said that file still exists at that point?

BTW, there are shitloads of races here - references to fd and struct file *
are mixed in a way that breaks *badly* if descriptor table is played with
by another thread.
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