pty: Repair TIOCGPTPEER

The implementation of TIOCGPTPEER has two issues.

When /dev/ptmx (as opposed to /dev/pts/ptmx) is opened the wrong
vfsmount is passed to dentry_open.  Which results in the kernel displaying
the wrong pathname for the peer.

The second is simply by caching the vfsmount and dentry of the peer it leaves
them open, in a way they were not previously Which because of the inreased
reference counts can cause unnecessary behaviour differences resulting in
regressions.

To fix these move the ioctl into tty_io.c at a generic level allowing
the ioctl to have access to the struct file on which the ioctl is
being called.  This allows the path of the slave to be derived when
opening the slave through TIOCGPTPEER instead of requiring the path to
the slave be cached.  Thus removing the need for caching the path.

A new function devpts_ptmx_path is factored out of devpts_acquire and
used to implement a function devpts_mntget.   The new function devpts_mntget
takes a filp to perform the lookup on and fsi so that it can confirm
that the superblock that is found by devpts_ptmx_path is the proper superblock.

v2: Lots of fixes to make the code actually work
v3: Suggestions by Linus
    - Removed the unnecessary initialization of filp in ptm_open_peer
    - Simplified devpts_ptmx_path as gotos are no longer required

[ This is the fix for the issue that was reverted in commit
  143c97cc6529, but this time without breaking 'pbuilder' due to
  increased reference counts   - Linus ]

Fixes: 54ebbfb16034 ("tty: add TIOCGPTPEER ioctl")
Reported-by: Christian Brauner <christian.brauner@canonical.com>
Reported-and-tested-by: Stefan Lippers-Hollmann <s.l-h@gmx.de>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 49 insertions, 16 deletions

diff --git a/fs/devpts/inode.c b/fs/devpts/inode.c
index 108df2e3602c..7eae33ffa3fc 100644
--- a/fs/devpts/inode.c
+++ b/fs/devpts/inode.c
@@ -133,6 +133,50 @@ static inline struct pts_fs_info *DEVPTS_SB(struct super_block *sb)
 	return sb->s_fs_info;
 }
 
+static int devpts_ptmx_path(struct path *path)
+{
+	struct super_block *sb;
+	int err;
+
+	/* Has the devpts filesystem already been found? */
+	if (path->mnt->mnt_sb->s_magic == DEVPTS_SUPER_MAGIC)
+		return 0;
+
+	/* Is a devpts filesystem at "pts" in the same directory? */
+	err = path_pts(path);
+	if (err)
+		return err;
+
+	/* Is the path the root of a devpts filesystem? */
+	sb = path->mnt->mnt_sb;
+	if ((sb->s_magic != DEVPTS_SUPER_MAGIC) ||
+	    (path->mnt->mnt_root != sb->s_root))
+		return -ENODEV;
+
+	return 0;
+}
+
+struct vfsmount *devpts_mntget(struct file *filp, struct pts_fs_info *fsi)
+{
+	struct path path;
+	int err;
+
+	path = filp->f_path;
+	path_get(&path);
+
+	err = devpts_ptmx_path(&path);
+	dput(path.dentry);
+	if (err) {
+		mntput(path.mnt);
+		path.mnt = ERR_PTR(err);
+	}
+	if (DEVPTS_SB(path.mnt->mnt_sb) != fsi) {
+		mntput(path.mnt);
+		path.mnt = ERR_PTR(-ENODEV);
+	}
+	return path.mnt;
+}
+
 struct pts_fs_info *devpts_acquire(struct file *filp)
 {
 	struct pts_fs_info *result;
@@ -143,27 +187,16 @@ struct pts_fs_info *devpts_acquire(struct file *filp)
 	path = filp->f_path;
 	path_get(&path);
 
-	/* Has the devpts filesystem already been found? */
-	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);
-			goto out;
-		}
-
-		/* Is the path the root of a devpts filesystem? */
-		result = ERR_PTR(-ENODEV);
-		sb = path.mnt->mnt_sb;
-		if ((sb->s_magic != DEVPTS_SUPER_MAGIC) ||
-		    (path.mnt->mnt_root != sb->s_root))
-			goto out;
+	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);