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

[Al Viro]

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"

NAK.  This violates asserts made by VFS (namely, that ->f_path is not
changed since dentry_open() has set it and until __fput() rips the thing
out) *and* by your own code (attack mentioned above, just from looking
at it for a minute).  Far too brittle...