[RFC][PATCH 6/7]: Determine pts_ns from a pty's inode

From: sukadev
Date: Tue Apr 08 2008 - 18:01:54 EST


From: Sukadev Bhattiprolu <sukadev@xxxxxxxxxx>
Subject: [RFC][PATCH 6/7]: Determine pts_ns from a pty's inode.

The devpts interfaces currently operate on a specific pts namespace
which they get from the 'current' task.

With implementation of containers and cloning of PTS namespaces, we want
to be able to access PTYs in a child-pts-ns from a parent-pts-ns. For
instance we could bind-mount and pivot-root the child container on
'/vserver/vserver1' and then access the "pts/0" of 'vserver1' using

$ echo foo > /vserver/vserver1/dev/pts/0

The task doing the above 'echo' could be in parent-pts-ns. So we find
the 'pts-ns' of the above file from the inode representing the device
rather than from the 'current' task.

Note that we need to find and hold a reference to the pts_ns to prevent
the pts_ns from being freed while it is being accessed from 'outside'.

This patch implements, 'pts_ns_from_inode()' which returns the pts_ns
using 'inode->i_sb->s_fs_info'.

Since, the 'inode' information is not visible inside devpts code itself,
this patch modifies the tty driver code to determine the pts_ns and passes
it into devpts.

Changelog [v2]:
[Serge Hallyn] Use rcu to access sb->s_fs_info.

[Serge Hallyn] Simplify handling of ptmx and tty devices by expecting
user to create them in /dev/pts (see also devpts-mknod patch)


Signed-off-by: Sukadev Bhattiprolu <sukadev@xxxxxxxxxx>
---
drivers/char/pty.c | 13 +++++-
drivers/char/tty_io.c | 96 +++++++++++++++++++++++++++++++++++++++-------
fs/devpts/inode.c | 19 +++------
include/linux/devpts_fs.h | 38 +++++++++++++++---
4 files changed, 134 insertions(+), 32 deletions(-)

Index: 2.6.25-rc8-mm1/include/linux/devpts_fs.h
===================================================================
--- 2.6.25-rc8-mm1.orig/include/linux/devpts_fs.h 2008-04-08 13:36:31.000000000 -0700
+++ 2.6.25-rc8-mm1/include/linux/devpts_fs.h 2008-04-08 13:38:08.000000000 -0700
@@ -17,6 +17,7 @@
#include <linux/nsproxy.h>
#include <linux/kref.h>
#include <linux/idr.h>
+#include <linux/fs.h>

struct pts_namespace {
struct kref kref;
@@ -26,12 +27,39 @@ struct pts_namespace {

extern struct pts_namespace init_pts_ns;

+#define DEVPTS_SUPER_MAGIC 0x1cd1
+
+static inline struct pts_namespace *current_pts_ns(void)
+{
+ return &init_pts_ns;
+}
+
+static inline struct pts_namespace *pts_ns_from_inode(struct inode *inode)
+{
+ /*
+ * If this file exists on devpts, return the pts_ns from the
+ * devpts super-block. Otherwise just use the pts-ns of the
+ * calling task.
+ */
+ if(inode->i_sb->s_magic == DEVPTS_SUPER_MAGIC)
+ return rcu_dereference(inode->i_sb->s_fs_info);
+
+ return current_pts_ns();
+}
+
+
#ifdef CONFIG_UNIX98_PTYS
-int devpts_new_index(void);
-void devpts_kill_index(int idx);
-int devpts_pty_new(struct tty_struct *tty); /* mknod in devpts */
-struct tty_struct *devpts_get_tty(int number); /* get tty structure */
-void devpts_pty_kill(int number); /* unlink */
+int devpts_new_index(struct pts_namespace *pts_ns);
+void devpts_kill_index(struct pts_namespace *pts_ns, int idx);
+
+/* mknod in devpts */
+int devpts_pty_new(struct pts_namespace *pts_ns, struct tty_struct *tty);
+
+/* get tty structure */
+struct tty_struct *devpts_get_tty(struct pts_namespace *pts_ns, int number);
+
+/* unlink */
+void devpts_pty_kill(struct pts_namespace *pts_ns, int number);

static inline void free_pts_ns(struct kref *ns_kref) { }

Index: 2.6.25-rc8-mm1/drivers/char/tty_io.c
===================================================================
--- 2.6.25-rc8-mm1.orig/drivers/char/tty_io.c 2008-04-08 09:15:56.000000000 -0700
+++ 2.6.25-rc8-mm1/drivers/char/tty_io.c 2008-04-08 14:25:11.000000000 -0700
@@ -2064,8 +2064,8 @@ static void tty_line_name(struct tty_dri
* relaxed for the (most common) case of reopening a tty.
*/

-static int init_dev(struct tty_driver *driver, int idx,
- struct tty_struct **ret_tty)
+static int init_dev(struct tty_driver *driver, struct pts_namespace *pts_ns,
+ int idx, struct tty_struct **ret_tty)
{
struct tty_struct *tty, *o_tty;
struct ktermios *tp, **tp_loc, *o_tp, **o_tp_loc;
@@ -2074,7 +2074,11 @@ static int init_dev(struct tty_driver *d

/* check whether we're reopening an existing tty */
if (driver->flags & TTY_DRIVER_DEVPTS_MEM) {
- tty = devpts_get_tty(idx);
+ tty = devpts_get_tty(pts_ns, idx);
+ if (IS_ERR(tty)) {
+ retval = PTR_ERR(tty);
+ goto end_init;
+ }
/*
* If we don't have a tty here on a slave open, it's because
* the master already started the close process and there's
@@ -2361,6 +2365,21 @@ static void release_tty(struct tty_struc
}

/*
+ * If the inode belongs to a device in devpts fs, return the pts-namespace
+ * associated with the device. Return NULL otherwise.
+ */
+struct pts_namespace *pty_pts_ns(struct tty_driver *driver, struct inode *inode)
+{
+ struct pts_namespace *pts_ns;
+
+ pts_ns = NULL;
+ if (driver->flags & TTY_DRIVER_DEVPTS_MEM)
+ pts_ns = pts_ns_from_inode(inode);
+
+ return pts_ns;
+}
+
+/*
* Even releasing the tty structures is a tricky business.. We have
* to be very careful that the structures are all released at the
* same time, as interrupts might otherwise get the wrong pointers.
@@ -2376,10 +2395,12 @@ static void release_dev(struct file *fil
int idx;
char buf[64];
unsigned long flags;
+ struct pts_namespace *pts_ns;
+ struct inode *inode;

+ inode = filp->f_path.dentry->d_inode;
tty = (struct tty_struct *)filp->private_data;
- if (tty_paranoia_check(tty, filp->f_path.dentry->d_inode,
- "release_dev"))
+ if (tty_paranoia_check(tty, inode, "release_dev"))
return;

check_tty_count(tty, "release_dev");
@@ -2392,6 +2413,12 @@ static void release_dev(struct file *fil
devpts = (tty->driver->flags & TTY_DRIVER_DEVPTS_MEM) != 0;
o_tty = tty->link;

+ /*
+ * We already have a reference to pts_ns here, so it cannot
+ * be going away.
+ */
+ pts_ns = pty_pts_ns(tty->driver, inode);
+
#ifdef TTY_PARANOIA_CHECK
if (idx < 0 || idx >= tty->driver->num) {
printk(KERN_DEBUG "release_dev: bad idx when trying to "
@@ -2569,6 +2596,10 @@ static void release_dev(struct file *fil

mutex_unlock(&tty_mutex);

+ /* drop the reference from ptmx_open/tty_open() */
+ if (devpts)
+ put_pts_ns(pts_ns);
+
/* check whether both sides are closing ... */
if (!tty_closing || (o_tty && !o_tty_closing))
return;
@@ -2634,7 +2665,7 @@ static void release_dev(struct file *fil

/* Make this pty number available for reallocation */
if (devpts)
- devpts_kill_index(idx);
+ devpts_kill_index(pts_ns, idx);
}

/**
@@ -2666,6 +2697,7 @@ static int tty_open(struct inode *inode,
int index;
dev_t device = inode->i_rdev;
unsigned short saved_flags = filp->f_flags;
+ struct pts_namespace *pts_ns;

nonseekable_open(inode, filp);

@@ -2715,10 +2747,31 @@ retry_open:
return -ENODEV;
}
got_driver:
- retval = init_dev(driver, index, &tty);
+
+ /*
+ * If this is a pty device, we maybe accessing this device from
+ * an ancestor pts-namespace. Find the pts-namespace from the
+ * device's inode and grab a reference.
+ *
+ * If pts-namespace is NULL then:
+ * - either this is not a PTY device or
+ * - this open is from an ancestor-pts-ns and the pts-ns has
+ * just been freed.
+ *
+ * If the pts-namespace is NULL for a PTY device (i.e pts-ns has
+ * been freed), init_dev() will fail the open()).
+ */
+ rcu_read_lock();
+ pts_ns = pty_pts_ns(driver, inode);
+ get_pts_ns(pts_ns);
+ rcu_read_unlock();
+
+ retval = init_dev(driver, pts_ns, index, &tty);
mutex_unlock(&tty_mutex);
- if (retval)
+ if (retval) {
+ put_pts_ns(pts_ns);
return retval;
+ }

filp->private_data = tty;
file_move(filp, &tty->tty_files);
@@ -2790,16 +2843,31 @@ static int ptmx_open(struct inode *inode
struct tty_struct *tty;
int retval;
int index;
+ struct pts_namespace *pts_ns;

nonseekable_open(inode, filp);

+ /*
+ * We maybe accessing this device from an ancestor pts-namespace.
+ * Find the pts-namespace from the device's inode and grab a
+ * reference.
+ *
+ * If pts-namespace is NULL, this open is from an ancestor-pts-ns
+ * and the pts-ns has just been freed and devpts_new_index()
+ * below will fail the open().
+ */
+ rcu_read_lock();
+ pts_ns = pts_ns_from_inode(inode);
+ get_pts_ns(pts_ns);
+ rcu_read_unlock();
+
/* find a device that is not in use. */
- index = devpts_new_index();
+ retval = index = devpts_new_index(pts_ns);
if (index < 0)
- return index;
+ goto drop_ns;

mutex_lock(&tty_mutex);
- retval = init_dev(ptm_driver, index, &tty);
+ retval = init_dev(ptm_driver, pts_ns, index, &tty);
mutex_unlock(&tty_mutex);

if (retval)
@@ -2809,7 +2877,7 @@ static int ptmx_open(struct inode *inode
filp->private_data = tty;
file_move(filp, &tty->tty_files);

- retval = devpts_pty_new(tty->link);
+ retval = devpts_pty_new(pts_ns, tty->link);
if (retval)
goto out1;

@@ -2821,7 +2889,9 @@ out1:
release_dev(filp);
return retval;
out:
- devpts_kill_index(index);
+ devpts_kill_index(pts_ns, index);
+drop_ns:
+ put_pts_ns(pts_ns);
return retval;
}
#endif
Index: 2.6.25-rc8-mm1/fs/devpts/inode.c
===================================================================
--- 2.6.25-rc8-mm1.orig/fs/devpts/inode.c 2008-04-08 13:35:43.000000000 -0700
+++ 2.6.25-rc8-mm1/fs/devpts/inode.c 2008-04-08 13:38:08.000000000 -0700
@@ -23,8 +23,6 @@
#include <linux/fsnotify.h>
#include <linux/seq_file.h>

-#define DEVPTS_SUPER_MAGIC 0x1cd1
-
#define DEVPTS_DEFAULT_MODE 0600

extern int pty_limit; /* Config limit on Unix98 ptys */
@@ -283,11 +281,10 @@ static struct dentry *get_node(struct de
return lookup_one_len(s, root, sprintf(s, "%d", num));
}

-int devpts_new_index(void)
+int devpts_new_index(struct pts_namespace *pts_ns)
{
int index;
int idr_ret;
- struct pts_namespace *pts_ns = &init_pts_ns;

retry:
if (!idr_pre_get(&pts_ns->allocated_ptys, GFP_KERNEL)) {
@@ -312,16 +309,15 @@ retry:
return index;
}

-void devpts_kill_index(int idx)
+void devpts_kill_index(struct pts_namespace *pts_ns, int idx)
{
- struct pts_namespace *pts_ns = &init_pts_ns;

down(&allocated_ptys_lock);
idr_remove(&pts_ns->allocated_ptys, idx);
up(&allocated_ptys_lock);
}

-int devpts_pty_new(struct tty_struct *tty)
+int devpts_pty_new( struct pts_namespace *pts_ns, struct tty_struct *tty)
{
int number = tty->index; /* tty layer puts index from devpts_new_index() in here */
struct tty_driver *driver = tty->driver;
@@ -330,7 +326,6 @@ int devpts_pty_new(struct tty_struct *tt
struct dentry *root;
struct vfsmount *mnt;
struct inode *inode;
- struct pts_namespace *pts_ns = &init_pts_ns;

/* We're supposed to be given the slave end of a pty */
BUG_ON(driver->type != TTY_DRIVER_TYPE_PTY);
@@ -369,13 +364,13 @@ int devpts_pty_new(struct tty_struct *tt
return 0;
}

-struct tty_struct *devpts_get_tty(int number)
+struct tty_struct *devpts_get_tty(struct pts_namespace *pts_ns, int number)
{
struct vfsmount *mnt;
struct dentry *dentry;
struct tty_struct *tty;

- mnt = init_pts_ns.mnt;
+ mnt = pts_ns->mnt;

dentry = get_node(mnt->mnt_root, number);

@@ -391,12 +386,12 @@ struct tty_struct *devpts_get_tty(int nu
return tty;
}

-void devpts_pty_kill(int number)
+void devpts_pty_kill(struct pts_namespace *pts_ns, int number)
{
struct dentry *dentry;
struct dentry *root;

- root = init_pts_ns.mnt->mnt_root;
+ root = pts_ns->mnt->mnt_root;

dentry = get_node(root, number);

Index: 2.6.25-rc8-mm1/drivers/char/pty.c
===================================================================
--- 2.6.25-rc8-mm1.orig/drivers/char/pty.c 2008-04-08 09:12:54.000000000 -0700
+++ 2.6.25-rc8-mm1/drivers/char/pty.c 2008-04-08 13:37:16.000000000 -0700
@@ -37,6 +37,9 @@ static struct tty_driver *pts_driver;

static void pty_close(struct tty_struct * tty, struct file * filp)
{
+ struct inode *inode;
+ struct pts_namespace *pts_ns;
+
if (!tty)
return;
if (tty->driver->subtype == PTY_TYPE_MASTER) {
@@ -58,8 +61,14 @@ static void pty_close(struct tty_struct
if (tty->driver->subtype == PTY_TYPE_MASTER) {
set_bit(TTY_OTHER_CLOSED, &tty->flags);
#ifdef CONFIG_UNIX98_PTYS
- if (tty->driver == ptm_driver)
- devpts_pty_kill(tty->index);
+ if (tty->driver == ptm_driver) {
+ inode = filp->f_path.dentry->d_inode;
+ rcu_read_lock();
+ pts_ns = pts_ns_from_inode(inode);
+ rcu_read_unlock();
+
+ devpts_pty_kill(pts_ns, tty->index);
+ }
#endif
tty_vhangup(tty->link);
}
--
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