Re: [PATCH 3/4] devpts: Make the newinstance option historical

[Al Viro <viro-3bDd1+5oDREiFSDQTTA3OLVCufUGDwFn@public.gmane.org>]

On Sun, Sep 23, 2012 at 05:19:06AM +0100, Al Viro wrote:
> On Sat, Sep 22, 2012 at 08:50:44PM -0700, Eric W. Biederman wrote:
> 
> > +struct inode *devpts_redirect(struct file *filp)
> > +{
> > +	struct inode *inode;
> > +	struct file *filp2;
> > +
> > +	/* Is the inode already a devpts inode? */
> > +	inode = filp->f_dentry->d_inode;
> > +	if (filp->f_dentry->d_sb->s_magic == DEVPTS_SUPER_MAGIC)
> > +		goto out;
> > +
> > +	/* Is f_dentry->d_parent usable? */
> > +	inode = ERR_PTR(-ENODEV);
> > +	if (filp->f_vfsmnt->mnt_root == filp->f_dentry)
> > +		goto out;
> > +
> > +	/* Is there a devpts inode we can use instead? */
> > +	
> > +	filp2 = file_open_root(filp->f_dentry->d_parent, filp->f_vfsmnt,
> > +			       "pts/ptmx", O_PATH);
> > +	if (!IS_ERR(filp2)) {
> > +		if (filp2->f_dentry->d_sb->s_magic == DEVPTS_SUPER_MAGIC) {
> > +			struct path old;
> > +			old = filp->f_path;
> > +			filp->f_path = filp2->f_path;
> > +			inode = filp->f_dentry->d_inode;
> > +			path_get(&filp->f_path);
> > +			path_put(&old);
> 
> You are welcome to supply an analysis of the reasons why ->open() pulling
> such tricks will not break all kinds of code in VFS.
> > +		}
> > +		fput(filp2);
> 
> ... starting with "what happens when some joker binds /dev/ptmx on
> /dev/pts/ptmx"

Or does mknod /tmp/ptmx c 5 2; mkdir /tmp/pts; ln /tmp/ptmx /tmp/pts/ptmx,
for that matter...