Date:	Mon, 26 Jan 2009 22:53:21 +0100
From:	Christoph Hellwig <hch@....de>
To:	Sukadev Bhattiprolu <sukadev@...ux.vnet.ibm.com>
Cc:	"H. Peter Anvin" <hpa@...or.com>, linux-kernel@...r.kernel.org
Subject: Re: devpts multiple instances feedback

On Sat, Jan 03, 2009 at 05:15:44PM +0100, Christoph Hellwig wrote:
> This is a little untested patch to massage the mount code into about
> how it should look like:

ping?

> 
> Index: linux-2.6/fs/devpts/inode.c
> ===================================================================
> --- linux-2.6.orig/fs/devpts/inode.c	2009-01-03 16:52:22.823672077 +0100
> +++ linux-2.6/fs/devpts/inode.c	2009-01-03 17:14:24.153711598 +0100
> @@ -329,106 +329,6 @@ static int compare_init_pts_sb(struct su
>  }
>  
>  /*
> - * Safely parse the mount options in @data and update @opts.
> - *
> - * devpts ends up parsing options two times during mount, due to the
> - * two modes of operation it supports. The first parse occurs in
> - * devpts_get_sb() when determining the mode (single-instance or
> - * multi-instance mode). The second parse happens in devpts_remount()
> - * or new_pts_mount() depending on the mode.
> - *
> - * Parsing of options modifies the @data making subsequent parsing
> - * incorrect. So make a local copy of @data and parse it.
> - *
> - * Return: 0 On success, -errno on error
> - */
> -static int safe_parse_mount_options(void *data, struct pts_mount_opts *opts)
> -{
> -	int rc;
> -	void *datacp;
> -
> -	if (!data)
> -		return 0;
> -
> -	/* Use kstrdup() ?  */
> -	datacp = kmalloc(PAGE_SIZE, GFP_KERNEL);
> -	if (!datacp)
> -		return -ENOMEM;
> -
> -	memcpy(datacp, data, PAGE_SIZE);
> -	rc = parse_mount_options((char *)datacp, PARSE_MOUNT, opts);
> -	kfree(datacp);
> -
> -	return rc;
> -}
> -
> -/*
> - * Mount a new (private) instance of devpts.  PTYs created in this
> - * instance are independent of the PTYs in other devpts instances.
> - */
> -static int new_pts_mount(struct file_system_type *fs_type, int flags,
> -		void *data, struct vfsmount *mnt)
> -{
> -	int err;
> -	struct pts_fs_info *fsi;
> -	struct pts_mount_opts *opts;
> -
> -	printk(KERN_NOTICE "devpts: newinstance mount\n");
> -
> -	err = get_sb_nodev(fs_type, flags, data, devpts_fill_super, mnt);
> -	if (err)
> -		return err;
> -
> -	fsi = DEVPTS_SB(mnt->mnt_sb);
> -	opts = &fsi->mount_opts;
> -
> -	err = parse_mount_options(data, PARSE_MOUNT, opts);
> -	if (err)
> -		goto fail;
> -
> -	err = mknod_ptmx(mnt->mnt_sb);
> -	if (err)
> -		goto fail;
> -
> -	return 0;
> -
> -fail:
> -	dput(mnt->mnt_sb->s_root);
> -	deactivate_super(mnt->mnt_sb);
> -	return err;
> -}
> -
> -/*
> - * Check if 'newinstance' mount option was specified in @data.
> - *
> - * Return: -errno  	on error (eg: invalid mount options specified)
> - * 	 : 1 		if 'newinstance' mount option was specified
> - * 	 : 0 		if 'newinstance' mount option was NOT specified
> - */
> -static int is_new_instance_mount(void *data)
> -{
> -	int rc;
> -	struct pts_mount_opts opts;
> -
> -	if (!data)
> -		return 0;
> -
> -	rc = safe_parse_mount_options(data, &opts);
> -	if (!rc)
> -		rc = opts.newinstance;
> -
> -	return rc;
> -}
> -
> -/*
> - * get_init_pts_sb()
> - *
> - *     This interface is needed to support multiple namespace semantics in
> - *     devpts while preserving backward compatibility of the current 'single-
> - *     namespace' semantics. i.e all mounts of devpts without the 'newinstance'
> - *     mount option should bind to the initial kernel mount, like
> - *     get_sb_single().
> - *
>   *     Mounts with 'newinstance' option create a new private namespace.
>   *
>   *     But for single-mount semantics, devpts cannot use get_sb_single(),
> @@ -436,20 +336,43 @@ static int is_new_instance_mount(void *d
>   *     the most recent mount of devpts. But that recent mount may be a
>   *     'newinstance' mount and get_sb_single() would pick the newinstance
>   *     super-block instead of the initial super-block.
> - *
> - *     This interface is identical to get_sb_single() except that it
> - *     consistently selects the 'single-namespace' superblock even in the
> - *     presence of the private namespace (i.e 'newinstance') super-blocks.
>   */
> -static int get_init_pts_sb(struct file_system_type *fs_type, int flags,
> -		void *data, struct vfsmount *mnt)
> +static int devpts_get_sb(struct file_system_type *fs_type, int flags,
> +		const char *dev_name, void *data, struct vfsmount *mnt)
>  {
> +	struct pts_mount_opts opts = { 0, };
>  	struct super_block *s;
>  	int error;
>  
> -	s = sget(fs_type, compare_init_pts_sb, set_anon_super, NULL);
> -	if (IS_ERR(s))
> -		return PTR_ERR(s);
> +	if (data) {
> +		error = parse_mount_options(data, PARSE_MOUNT, &opts);
> +		if (error)
> +			return error;
> +	}
> +
> +	if (opts.newinstance) {
> +		/*
> +		 * Mount a new (private) instance of devpts.  PTYs created
> +		 * in this instance are independent of the PTYs in other devpts
> +		 * instances.
> +		 */
> +
> +		printk(KERN_NOTICE "devpts: creating new instance\n");
> +
> +		s = sget(fs_type, NULL, set_anon_super, NULL);
> +		if (IS_ERR(s))
> +			return PTR_ERR(s);
> +	} else {
> +		/*
> +		 * Mount or remount the initial kernel mount of devpts.
> +		 *
> +		 * This type of mount maintains the legacy, single-instance
> +		 * semantics, while the kernel still allows multiple-instances.
> +		 */
> +		s = sget(fs_type, compare_init_pts_sb, set_anon_super, NULL);
> +		if (IS_ERR(s))
> +			return PTR_ERR(s);
> +	}
>  
>  	if (!s->s_root) {
>  		s->s_flags = flags;
> @@ -461,46 +384,26 @@ static int get_init_pts_sb(struct file_s
>  		}
>  		s->s_flags |= MS_ACTIVE;
>  	}
> -	do_remount_sb(s, flags, data, 0);
> -	return simple_set_mnt(mnt, s);
> -}
> -
> -/*
> - * Mount or remount the initial kernel mount of devpts. This type of
> - * mount maintains the legacy, single-instance semantics, while the
> - * kernel still allows multiple-instances.
> - */
> -static int init_pts_mount(struct file_system_type *fs_type, int flags,
> -		void *data, struct vfsmount *mnt)
> -{
> -	int err;
> -
> -	err = get_init_pts_sb(fs_type, flags, data, mnt);
> -	if (err)
> -		return err;
> -
> -	err = mknod_ptmx(mnt->mnt_sb);
> -	if (err) {
> -		dput(mnt->mnt_sb->s_root);
> -		deactivate_super(mnt->mnt_sb);
> -	}
> -
> -	return err;
> -}
> -
> -static int devpts_get_sb(struct file_system_type *fs_type,
> -	int flags, const char *dev_name, void *data, struct vfsmount *mnt)
> -{
> -	int new;
>  
> -	new = is_new_instance_mount(data);
> -	if (new < 0)
> -		return new;
> +	/*
> +	 * Copy over mount options structure into the superblock.
> +	 */
> +	memcpy(&DEVPTS_SB(mnt->mnt_sb)->mount_opts, &opts, sizeof(opts));
> +
> +	error = simple_set_mnt(mnt, s);
> +	if (error)
> +		return error;
> +
> +	error = mknod_ptmx(mnt->mnt_sb);
> +	if (error)
> +		goto out_dput;
>  
> -	if (new)
> -		return new_pts_mount(fs_type, flags, data, mnt);
> +	return 0;
>  
> -	return init_pts_mount(fs_type, flags, data, mnt);
> + out_dput:
> +	dput(mnt->mnt_sb->s_root);
> +	deactivate_super(mnt->mnt_sb);
> +	return error;
>  }
>  #else
>  /*
---end quoted text---
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