[PATCH 06/10] Define mknod_ptmx()

Serge E. Hallyn serue at us.ibm.com
Wed Sep 24 11:50:46 PDT 2008


Quoting sukadev at us.ibm.com (sukadev at us.ibm.com):
> 
> >From df5d5863f8601baa25237b60c3fcd614ee0b34aa Mon Sep 17 00:00:00 2001
> From: Sukadev Bhattiprolu <sukadev at us.ibm.com>
> Date: Tue, 9 Sep 2008 10:23:07 -0700
> Subject: [PATCH 06/10] Define mknod_ptmx()
> 
> /dev/ptmx is closely tied to the devpts filesystem. An open of /dev/ptmx,
> allocates the next pty index and the associated device shows up in the
> devpts fs as /dev/pts/n.
> 
> Wih multiple instancs of devpts filesystem, during an open of /dev/ptmx
> we would be unable to determine which instance of the devpts is being
> accessed.
> 
> So we move the 'ptmx' node into /dev/pts and use the inode of the 'ptmx'
> node to identify the superblock and hence the devpts instance.  This patch
> adds ability for the kernel to internally create the [ptmx, c, 5:2] device
> when mounting devpts filesystem.  Since the ptmx node in devpts is new and
> may surprise some userspace scripts, the default permissions for the new
> node is 0000.  These permissions can be changed either using chmod or by
> remounting with the new '-o ptmxmode=0666' mount option.
> 
> Changelog[v4]:
> 	- Change default permissions of pts/ptmx node to 0000.
> 	- Move code for ptmxmode under #ifdef CONFIG_DEVPTS_MULTIPLE_INSTANCES.
> 
> Changelog[v3]:
> 	- Rename ptmx_mode to ptmxmode (for consistency with 'newinstance')
> 
> Changelog[v2]:
> 	- [H. Peter Anvin] Remove mknod() system call support and create the
> 	  ptmx node internally.
> 
> Changelog[v1]:
> 	- Earlier version of this patch enabled creating /dev/pts/tty as
> 	  well. As pointed out by Al Viro and H. Peter Anvin, that is not
> 	  really necessary.
> 
> Signed-off-by: Sukadev Bhattiprolu <sukadev at us.ibm.com>
> ---
>  fs/devpts/inode.c |   76 +++++++++++++++++++++++++++++++++++++++++++++++++++-
>  1 files changed, 74 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/devpts/inode.c b/fs/devpts/inode.c
> index 7ae60aa..17e14f5 100644
> --- a/fs/devpts/inode.c
> +++ b/fs/devpts/inode.c
> @@ -27,6 +27,13 @@
>  #define DEVPTS_SUPER_MAGIC 0x1cd1
> 
>  #define DEVPTS_DEFAULT_MODE 0600
> +/*
> + * ptmx is a new node in /dev/pts and will be unused in legacy (single-
> + * instance) mode. To prevent surprises in user space, set permissions of
> + * ptmx to 0. Use 'chmod' or remount with '-o ptmxmode' to set meaningful
> + * permissions.
> + */
> +#define DEVPTS_DEFAULT_PTMX_MODE 0000
>  #define PTMX_MINOR	2
> 
>  extern int pty_limit;			/* Config limit on Unix98 ptys */
> @@ -40,10 +47,11 @@ struct pts_mount_opts {
>  	uid_t   uid;
>  	gid_t   gid;
>  	umode_t mode;
> +	umode_t ptmxmode;
>  };
> 
>  enum {
> -	Opt_uid, Opt_gid, Opt_mode,
> +	Opt_uid, Opt_gid, Opt_mode, Opt_ptmxmode,
>  	Opt_err
>  };
> 
> @@ -51,6 +59,9 @@ static match_table_t tokens = {
>  	{Opt_uid, "uid=%u"},
>  	{Opt_gid, "gid=%u"},
>  	{Opt_mode, "mode=%o"},
> +#ifdef CONFIG_DEVPTS_MULTIPLE_INSTANCES
> +	{Opt_ptmxmode, "ptmxmode=%o"},
> +#endif
>  	{Opt_err, NULL}
>  };
> 
> @@ -81,6 +92,7 @@ static int parse_mount_options(char *data, struct pts_mount_opts *opts)
>  	opts->uid     = 0;
>  	opts->gid     = 0;
>  	opts->mode    = DEVPTS_DEFAULT_MODE;
> +	opts->ptmxmode = DEVPTS_DEFAULT_PTMX_MODE;
> 
>  	while ((p = strsep(&data, ",")) != NULL) {
>  		substring_t args[MAX_OPT_ARGS];
> @@ -109,6 +121,13 @@ static int parse_mount_options(char *data, struct pts_mount_opts *opts)
>  				return -EINVAL;
>  			opts->mode = option & S_IALLUGO;
>  			break;
> +#ifdef CONFIG_DEVPTS_MULTIPLE_INSTANCES
> +		case Opt_ptmxmode:
> +			if (match_octal(&args[0], &option))
> +				return -EINVAL;
> +			opts->ptmxmode = option & S_IALLUGO;
> +			break;
> +#endif
>  		default:
>  			printk(KERN_ERR "devpts: called with bogus options\n");
>  			return -EINVAL;
> @@ -118,6 +137,55 @@ static int parse_mount_options(char *data, struct pts_mount_opts *opts)
>  	return 0;
>  }
> 
> +#ifdef CONFIG_DEVPTS_MULTIPLE_INSTANCES
> +static int mknod_ptmx(struct super_block *sb)
> +{
> +	struct dentry *root;
> +	struct dentry *dentry;
> +	struct inode *inode;
> +	struct pts_fs_info *fsi = DEVPTS_SB(sb);
> +	struct pts_mount_opts *opts = &fsi->mount_opts;
> +	int mode;
> +
> +	root = sb->s_root;
> +	dentry = lookup_one_len("ptmx", root, 4);
> +	if (IS_ERR(dentry)) {
> +		printk(KERN_ERR "Unable to alloc dentry for ptmx node\n");
> +		return -ENOMEM;
> +	}
> +
> +	if (dentry->d_inode) {
> +		printk(KERN_ERR "'ptmx' (ino %lu) exists in this devpts\n",
> +				dentry->d_inode->i_ino);
> +		return 0;
> +	}
> +
> +	/*
> +	 * Create a new 'ptmx' node in this mount of devpts.
> +	 */
> +	inode = new_inode(sb);
> +	if (!inode) {
> +		printk(KERN_ERR "Unable to alloc inode for ptmx node\n");
> +		dput(dentry);
> +		return -ENOMEM;
> +	}
> +
> +	inode->i_uid = inode->i_gid = 0;
> +	inode->i_blocks = 0;
> +	inode->i_mtime = inode->i_atime = inode->i_ctime = CURRENT_TIME;
> +
> +	mode = S_IFCHR|opts->ptmxmode;
> +	init_special_inode(inode, mode, MKDEV(TTYAUX_MAJOR, 2));
> +
> +	d_add(dentry, inode);

Actually, you never do dput this dentry.

At the moment you need to dput it here, then once you cache it, you
don't dput it here, but dput the cache one at devpts_kill_sb.  Right?

> +	printk(KERN_DEBUG "Created ptmx node in devpts ino %lu\n",
> +			inode->i_ino);
> +
> +	return 0;
> +}
> +#endif
> +
>  static int devpts_remount(struct super_block *sb, int *flags, char *data)
>  {
>  	struct pts_fs_info *fsi = DEVPTS_SB(sb);
> @@ -136,6 +204,9 @@ static int devpts_show_options(struct seq_file *seq, struct vfsmount *vfs)
>  	if (opts->setgid)
>  		seq_printf(seq, ",gid=%u", opts->gid);
>  	seq_printf(seq, ",mode=%03o", opts->mode);
> +#ifdef CONFIG_DEVPTS_MULTIPLE_INSTANCES
> +	seq_printf(seq, ",ptmxmode=%03o", opts->ptmxmode);
> +#endif
> 
>  	return 0;
>  }
> @@ -156,6 +227,7 @@ static void *new_pts_fs_info(void)
> 
>  	ida_init(&fsi->allocated_ptys);
>  	fsi->mount_opts.mode = DEVPTS_DEFAULT_MODE;
> +	fsi->mount_opts.ptmxmode = DEVPTS_DEFAULT_PTMX_MODE;
> 
>  	return fsi;
>  }
> @@ -211,7 +283,7 @@ static void devpts_kill_sb(struct super_block *sb)
>  	struct pts_fs_info *fsi = DEVPTS_SB(sb);
> 
>  	kfree(fsi);
> -	kill_anon_super(sb);
> +	kill_litter_super(sb);
>  }
> 
>  static struct file_system_type devpts_fs_type = {
> -- 
> 1.5.2.5


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