Re: [PATCH 0/1] devpts: use dynamic_dname() to generate proc name

Eric W. Biederman
2017-08-24T03:11:28Z

Linus Torvalds <torvalds@linux-foundation.org> writes:

> On Wed, Aug 23, 2017 at 6:49 PM, Linus Torvalds
> <torvalds@linux-foundation.org> wrote:
>>
>> Argh. And it's *not* fairly straightforward, because the
>> tty_operations "ioctl()" function pointer only gets 'struct tty *'.
>>
>> So in the TIOCGPTPEER path, we don't actually have access to the file
>> pointer of the fd we're doing the ioctl on.
>>
>> And that's where the 'struct path' to the 'ptmx' node is - which we
>> need to then look up the 'pts' directory.
>>
>> How very annoying. I think that's why we did it all at ptmx_open()
>> time, because then we had all the information.
>
> Anyway, the revert is pushed out. So we're back to the old behavior
> that gives the wrong pathname in /proc.
>
> And I think I can handle the lack of a 'struct file *' to the ioctl
> operations by just special-casing TIOCGPTPEER directly in tty_ioctl()
> itself.
>
> That's where we handle "generic" tty ioctls, and doing pty stuff there
> is kind of wrong, but pty's are special.
>
> But I think I'll leave it for tomorrow. So Eric, if you feel like
> looking at this, I'd appreciate it.

This is so far untested (except for compiling) but I think this will
work.

I factor out devpts_ptmx_path out of devpts_acquire so the code
doesn't have to do unnecessary and confusing work, and add the
new function devpts_mnt.

I revert the code to keep anything except a dentry in
tty->link->driver_data.

And reduce the peer opening to a single function ptm_open_peer.

It takes lines of code but the result is very straightforward code.

Eric

drivers/tty/pty.c | 63 ++++++++++++++++++++---------------------------
drivers/tty/tty_io.c | 3 +++
fs/devpts/inode.c | 60 +++++++++++++++++++++++++++++++++-----------
include/linux/devpts_fs.h | 10 ++++++++
4 files changed, 85 insertions(+), 51 deletions(-)
diff --git a/drivers/tty/pty.c b/drivers/tty/pty.c
index 284749fb0f6b..269e6ea65a33 100644
--- a/drivers/tty/pty.c
+++ b/drivers/tty/pty.c
@@ -69,13 +69,8 @@ static void pty_close(struct tty_struct *tty, struct file *filp)
#ifdef CONFIG_UNIX98_PTYS
if (tty->driver == ptm_driver) {
mutex_lock(&devpts_mutex);
- if (tty->link->driver_data) {
- struct path *path = tty->link->driver_data;
-
- devpts_pty_kill(path->dentry);
- path_put(path);
- kfree(path);
- }
+ if (tty->link->driver_data)
+ devpts_pty_kill(tty->link->driver_data);
mutex_unlock(&devpts_mutex);
}
#endif
@@ -607,25 +602,25 @@ static inline void legacy_pty_init(void) { }
static struct cdev ptmx_cdev;

/**
- * pty_open_peer - open the peer of a pty
- * @tty: the peer of the pty being opened
+ * ptm_open_peer - open the peer of a pty
+ * @master: the open struct file of the ptmx device node
+ * @tty: the master of the pty being opened
+ * @flags: the flags for open
*
- * Open the cached dentry in tty->link, providing a safe way for userspace
- * to get the slave end of a pty (where they have the master fd and cannot
- * access or trust the mount namespace /dev/pts was mounted inside).
+ * Provide a race free way for userspace to open the slave end of a pty
+ * (where they have the master fd and cannot access or trust the mount
+ * namespace /dev/pts was mounted inside).
*/
-static struct file *pty_open_peer(struct tty_struct *tty, int flags)
-{
- if (tty->driver->subtype != PTY_TYPE_MASTER)
- return ERR_PTR(-EIO);
- return dentry_open(tty->link->driver_data, flags, current_cred());
-}
-
-static int pty_get_peer(struct tty_struct *tty, int flags)
+int ptm_open_peer(struct file *master, struct tty_struct *tty, int flags)
{
int fd = -1;
struct file *filp = NULL;
int retval = -EINVAL;
+ struct path path;
+
+ if ((tty->driver->type != TTY_DRIVER_TYPE_PTY) ||
+ (tty->driver->subtype != PTY_TYPE_MASTER))
+ return -EIO;

fd = get_unused_fd_flags(0);
if (fd < 0) {
@@ -633,7 +628,16 @@ static int pty_get_peer(struct tty_struct *tty, int flags)
goto err;
}

- filp = pty_open_peer(tty, flags);
+ /* Compute the slave's path */
+ path.mnt = devpts_mnt(filp);
+ if (IS_ERR(path.mnt)) {
+ retval = PTR_ERR(path.mnt);
+ goto err_put;
+ }
+ path.dentry = tty->link->driver_data;
+
+ filp = dentry_open(&path, flags, current_cred());
+ mntput(path.mnt);
if (IS_ERR(filp)) {
retval = PTR_ERR(filp);
goto err_put;
@@ -662,8 +666,6 @@ static int pty_unix98_ioctl(struct tty_struct *tty,
return pty_get_pktmode(tty, (int __user *)arg);
case TIOCGPTN: /* Get PT Number */
return put_user(tty->index, (unsigned int __user *)arg);
- case TIOCGPTPEER: /* Open the other end */
- return pty_get_peer(tty, (int) arg);
case TIOCSIG: /* Send signal to other side of pty */
return pty_signal(tty, (int) arg);
}
@@ -791,7 +793,6 @@ static int ptmx_open(struct inode *inode, struct file *filp)
{
struct pts_fs_info *fsi;
struct tty_struct *tty;
- struct path *pts_path;
struct dentry *dentry;
int retval;
int index;
@@ -845,26 +846,16 @@ static int ptmx_open(struct inode *inode, struct file *filp)
retval = PTR_ERR(dentry);
goto err_release;
}
- /* We need to cache a fake path for TIOCGPTPEER. */
- pts_path = kmalloc(sizeof(struct path), GFP_KERNEL);
- if (!pts_path)
- goto err_release;
- pts_path->mnt = filp->f_path.mnt;
- pts_path->dentry = dentry;
- path_get(pts_path);
- tty->link->driver_data = pts_path;
+ tty->link->driver_data = dentry;

retval = ptm_driver->ops->open(tty, filp);
if (retval)
- goto err_path_put;
+ goto err_release;

tty_debug_hangup(tty, "opening (count=%d)\n", tty->count);

tty_unlock(tty);
return 0;
-err_path_put:
- path_put(pts_path);
- kfree(pts_path);
err_release:
tty_unlock(tty);
// This will also put-ref the fsi
diff --git a/drivers/tty/tty_io.c b/drivers/tty/tty_io.c
index 974b13d24401..ba3371449a5c 100644
--- a/drivers/tty/tty_io.c
+++ b/drivers/tty/tty_io.c
@@ -2518,6 +2518,9 @@ long tty_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
case TIOCSSERIAL:
tty_warn_deprecated_flags(p);
break;
+ case TIOCGPTPEER:
+ retval = ptm_open_peer(file, tty, (int)arg);
+ break;
default:
retval = tty_jobctrl_ioctl(tty, real_tty, file, cmd, arg);
if (retval != -ENOIOCTLCMD)
diff --git a/fs/devpts/inode.c b/fs/devpts/inode.c
index 108df2e3602c..6e8816cf7d54 100644
--- a/fs/devpts/inode.c
+++ b/fs/devpts/inode.c
@@ -133,37 +133,67 @@ static inline struct pts_fs_info *DEVPTS_SB(struct super_block *sb)
return sb->s_fs_info;
}

-struct pts_fs_info *devpts_acquire(struct file *filp)
+static int devpts_ptmx_path(struct path *path)
{
- struct pts_fs_info *result;
- struct path path;
struct super_block *sb;
- int err;
-
- path = filp->f_path;
- path_get(&path);
+ int err = 0;

/* Has the devpts filesystem already been found? */
- sb = path.mnt->mnt_sb;
+ sb = path->mnt->mnt_sb;
if (sb->s_magic != DEVPTS_SUPER_MAGIC) {
/* Is a devpts filesystem at "pts" in the same directory? */
- err = path_pts(&path);
- if (err) {
- result = ERR_PTR(err);
+ err = path_pts(path);
+ if (err)
goto out;
- }

/* Is the path the root of a devpts filesystem? */
- result = ERR_PTR(-ENODEV);
- sb = path.mnt->mnt_sb;
+ err = -ENODEV;
+ sb = path->mnt->mnt_sb;
if ((sb->s_magic != DEVPTS_SUPER_MAGIC) ||
- (path.mnt->mnt_root != sb->s_root))
+ (path->mnt->mnt_root != sb->s_root))
goto out;
}

+out:
+ return err;
+}
+
+struct vfsmount *devpts_mnt(struct file *filp)
+{
+ struct path path;
+ int err;
+
+ path = filp->f_path;
+ path_get(&path);
+
+ err = devpts_ptmx_path(&path);
+ if (err) {
+ path_put(&path);
+ path.mnt = ERR_PTR(err);
+ }
+ return path.mnt;
+}
+
+struct pts_fs_info *devpts_acquire(struct file *filp)
+{
+ struct pts_fs_info *result;
+ struct path path;
+ struct super_block *sb;
+ int err;
+
+ path = filp->f_path;
+ path_get(&path);
+
+ err = devpts_ptmx_path(&path);
+ if (err) {
+ result = ERR_PTR(err);
+ goto out;
+ }
+
/*
* pty code needs to hold extra references in case of last /dev/tty close
*/
+ sb = path.mnt->mnt_sb;
atomic_inc(&sb->s_active);
result = DEVPTS_SB(sb);

diff --git a/include/linux/devpts_fs.h b/include/linux/devpts_fs.h
index 277ab9af9ac2..e27c548acfb0 100644
--- a/include/linux/devpts_fs.h
+++ b/include/linux/devpts_fs.h
@@ -19,6 +19,7 @@

struct pts_fs_info;

+struct vfsmount *devpts_mnt(struct file *);
struct pts_fs_info *devpts_acquire(struct file *);
void devpts_release(struct pts_fs_info *);

@@ -32,6 +33,15 @@ void *devpts_get_priv(struct dentry *);
/* unlink */
void devpts_pty_kill(struct dentry *);

+/* in pty.c */
+int ptm_open_peer(struct file *master, struct tty_struct *tty, int flags);
+
+#else
+static inline int
+ptm_open_peer(struct file *master, struct tty_struct *tty, int flags)
+{
+ return -EIO;
+}
#endif