[PATCH 16/28] VFS: Mountpoint file descriptor attach support

[Mike Waychison <michael.waychison@sun.com>]

This patch allows an unattached mountfd to be attached to a directory given a
direct fd.

NOTE: Probably requires CAP_SYSADMIN..  depends on what we want the security
model to be.

Signed-off-by: Mike Waychison <michael.waychison@sun.com>
---

 fs/mountfd.c       |   35 +++++++++++++++++++++++++++++++++++
 include/linux/fs.h |    1 +
 2 files changed, 36 insertions(+)

Index: linux-2.6.9-quilt/fs/mountfd.c
===================================================================
--- linux-2.6.9-quilt.orig/fs/mountfd.c	2004-10-22 17:17:41.367175376 -0400
+++ linux-2.6.9-quilt/fs/mountfd.c	2004-10-22 17:17:42.011077488 -0400
@@ -5,6 +5,7 @@
 #include <linux/stat.h>
 #include <linux/init.h>
 #include <linux/security.h>
+#include <linux/namei.h>
 
 #define MFDFS_MAGIC 0x4A9F2E43
 #define MFDFS_ROOT_INO 1
@@ -251,6 +252,38 @@ out_filp:
 	return error;
 }
 
+static long mfd_attach(struct file *mountfilp, int dirfd)
+{
+	struct file *dir;
+	struct vfsmount *mnt;
+	struct nameidata nd;
+	int ret;
+
+	mnt = mntget(VFSMOUNT(mountfilp));
+
+	ret = -EBADF;
+	dir = fget(dirfd);
+	if (!dir)
+		goto out;
+
+	ret = -ENOTDIR;
+	if (!S_ISDIR(dir->f_dentry->d_inode->i_mode))
+		goto out2;
+
+	memset(&nd, 0, sizeof(nd));
+	nd.dentry = dget(dir->f_dentry);
+	nd.mnt = mntget(dir->f_vfsmnt);
+
+	ret = do_graft_mount(mnt, &nd);
+
+	path_release(&nd);
+out2:
+	put_filp(dir);
+out:
+	mntput(mnt);
+	return ret;
+}
+
 static int mfd_ioctl(struct inode *inode, struct file *filp,
 		     unsigned int cmd, unsigned long arg)
 {
@@ -263,6 +296,8 @@ static int mfd_ioctl(struct inode *inode
 		return mfd_umount(filp, 0);
 	case MOUNTFD_IOC_FORCEDUNMOUNT:
 		return mfd_umount(filp, MNT_FORCE);
+	case MOUNTFD_IOC_ATTACH:
+		return mfd_attach(filp, arg);
 	}
 	return -ENOTTY;
 }
Index: linux-2.6.9-quilt/include/linux/fs.h
===================================================================
--- linux-2.6.9-quilt.orig/include/linux/fs.h	2004-10-22 17:17:41.369175072 -0400
+++ linux-2.6.9-quilt/include/linux/fs.h	2004-10-22 17:17:42.012077336 -0400
@@ -218,6 +218,7 @@ extern int leases_enable, dir_notify_ena
 #define MOUNTFD_IOC_UNMOUNT       _IO('p', 0xa1)
 #define MOUNTFD_IOC_DETACH        _IO('p', 0xa2)
 #define MOUNTFD_IOC_FORCEDUNMOUNT _IO('p', 0xa3)
+#define MOUNTFD_IOC_ATTACH        _IOW('p', 0xa4, int)
 
 #ifdef __KERNEL__