linux-kernel - Re: [PATCH] devpts: Make each mount of devpts an independent filesystem.

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Message-ID: <20160420032541.GE25498@ZenIV.linux.org.uk>
Date:	Wed, 20 Apr 2016 04:25:42 +0100
From:	Al Viro <viro@...IV.linux.org.uk>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Andy Lutomirski <luto@...capital.net>, security@...ian.org,
	"security@...nel.org" <security@...nel.org>,
	"security@...ntu.com >> security" <security@...ntu.com>,
	Peter Hurley <peter@...leysoftware.com>,
	Serge Hallyn <serge.hallyn@...ntu.com>,
	Willy Tarreau <w@....eu>,
	Aurelien Jarno <aurelien@...el32.net>,
	One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>,
	Jann Horn <jann@...jh.net>, Greg KH <greg@...ah.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Jiri Slaby <jslaby@...e.com>, Florian Weimer <fw@...eb.enyo.de>
Subject: Re: [PATCH] devpts: Make each mount of devpts an independent
 filesystem.

On Tue, Apr 19, 2016 at 10:04:20PM -0500, Eric W. Biederman wrote:
> +#ifdef CONFIG_UNIX98_PTYS
> +int path_pts(struct path *path)
> +{
> +	/* Find "pts" in the same directory as the input path */
> +	struct dentry *child, *parent;
> +	struct qstr this;
> +	int ret;
> +
> +	ret = path_parent_directory(path);
> +	if (ret)
> +		return ret;
> +
> +	parent = path->dentry;
> +	if (!d_can_lookup(parent))
> +		return -ENOENT;

And how, pray tell, would a parent of anything fail to be a directory?

> +	this.name = "pts";
> +	this.len = 3;
> +	this.hash = full_name_hash(this.name, this.len);
> +	if (parent->d_flags & DCACHE_OP_HASH) {
> +		int err = parent->d_op->d_hash(parent, &this);
> +		if (err < 0)
> +			return err;
> +	}
> +	inode_lock(parent->d_inode);

What the hell for?  What does that lock on parent change for the
dcache lookup you are doing here?

> +	child = d_lookup(parent, &this);
> +	inode_unlock(parent->d_inode);
> +	if (!child)
> +		return -ENOENT;

Take a look at d_hash_and_lookup(), BTW...

> +	path->dentry = child;
> +	dput(parent);
> +	follow_mount(path);
> +	return 0;
> +}
> +#endif
> +
>  int user_path_at_empty(int dfd, const char __user *name, unsigned flags,
>  		 struct path *path, int *empty)
>  {
> diff --git a/include/linux/devpts_fs.h b/include/linux/devpts_fs.h
> index e0ee0b3000b2..486f6282b0c6 100644
> --- a/include/linux/devpts_fs.h
> +++ b/include/linux/devpts_fs.h
> @@ -17,36 +17,21 @@
>  
>  #ifdef CONFIG_UNIX98_PTYS
>  
> -int devpts_new_index(struct inode *ptmx_inode);
> -void devpts_kill_index(struct inode *ptmx_inode, int idx);
> -void devpts_add_ref(struct inode *ptmx_inode);
> -void devpts_del_ref(struct inode *ptmx_inode);
> +struct pts_fs_info;
> +
> +struct pts_fs_info *devpts_acquire(struct file *filp);
> +void devpts_release(struct pts_fs_info *fsi);
> +
> +int devpts_new_index(struct pts_fs_info *fsi);
> +void devpts_kill_index(struct pts_fs_info *fsi, int idx);
>  /* mknod in devpts */
> -struct inode *devpts_pty_new(struct inode *ptmx_inode, dev_t device, int index,
> +struct inode *devpts_pty_new(struct pts_fs_info *fsi, dev_t device, int index,
>  		void *priv);
>  /* get private structure */
>  void *devpts_get_priv(struct inode *pts_inode);
>  /* unlink */
>  void devpts_pty_kill(struct inode *inode);
>  
> -#else
> -
> -/* Dummy stubs in the no-pty case */
> -static inline int devpts_new_index(struct inode *ptmx_inode) { return -EINVAL; }
> -static inline void devpts_kill_index(struct inode *ptmx_inode, int idx) { }
> -static inline void devpts_add_ref(struct inode *ptmx_inode) { }
> -static inline void devpts_del_ref(struct inode *ptmx_inode) { }
> -static inline struct inode *devpts_pty_new(struct inode *ptmx_inode,
> -		dev_t device, int index, void *priv)
> -{
> -	return ERR_PTR(-EINVAL);
> -}
> -static inline void *devpts_get_priv(struct inode *pts_inode)
> -{
> -	return NULL;
> -}
> -static inline void devpts_pty_kill(struct inode *inode) { }
> -
>  #endif
>  
>  
> diff --git a/include/linux/namei.h b/include/linux/namei.h
> index 77d01700daf7..f29abda31e6d 100644
> --- a/include/linux/namei.h
> +++ b/include/linux/namei.h
> @@ -45,6 +45,8 @@ enum {LAST_NORM, LAST_ROOT, LAST_DOT, LAST_DOTDOT, LAST_BIND};
>  #define LOOKUP_ROOT		0x2000
>  #define LOOKUP_EMPTY		0x4000
>  
> +extern int path_pts(struct path *path);
> +
>  extern int user_path_at_empty(int, const char __user *, unsigned, struct path *, int *empty);
>  
>  static inline int user_path_at(int dfd, const char __user *name, unsigned flags,
> -- 
> 2.8.1
>