[PATCH 3/4] fsfreeze: add ioctl to create a fd for freeze control

[Fernando Luis Vazquez Cao <fernando@oss.ntt.co.jp>]

Adds a new ioctl, FIGETFREEZEFD, which freezes the indicated filesystem and
returns a file descriptor; as long as that file descriptor is held open, the
filesystem remains open.

Signed-off-by: Fernando Luis Vazquez Cao <fernando@oss.ntt.co.jp>
---

diff -urNp linux-3.0-orig/fs/compat_ioctl.c linux-3.0/fs/compat_ioctl.c
--- linux-3.0-orig/fs/compat_ioctl.c	2011-07-27 11:32:10.188008559 +0900
+++ linux-3.0/fs/compat_ioctl.c	2011-07-27 11:34:42.844014535 +0900
@@ -882,6 +882,7 @@ COMPATIBLE_IOCTL(FIBMAP)
 COMPATIBLE_IOCTL(FIGETBSZ)
 /* 'X' - originally XFS but some now in the VFS */
 COMPATIBLE_IOCTL(FIFREEZE)
+COMPATIBLE_IOCTL(FIGETFREEZEFD)
 COMPATIBLE_IOCTL(FITHAW)
 COMPATIBLE_IOCTL(FIISFROZEN)
 COMPATIBLE_IOCTL(KDGETKEYCODE)
diff -urNp linux-3.0-orig/fs/ioctl.c linux-3.0/fs/ioctl.c
--- linux-3.0-orig/fs/ioctl.c	2011-07-27 11:32:10.188008559 +0900
+++ linux-3.0/fs/ioctl.c	2011-07-27 12:24:54.116004298 +0900
@@ -14,6 +14,7 @@
 #include <linux/uaccess.h>
 #include <linux/writeback.h>
 #include <linux/buffer_head.h>
+#include <linux/anon_inodes.h>
 #include <linux/falloc.h>
 
 #include <asm/ioctls.h>
@@ -525,6 +526,80 @@ static int ioctl_fsfreeze(struct file *f
 	return freeze_super(sb);
 }
 
+struct freeze_fd_data {
+	struct super_block *sb;
+};
+
+static int freeze_fd_release(struct inode *inode, struct file *filp)
+{
+	struct freeze_fd_data *fd_data = filp->private_data;
+	struct super_block *sb = fd_data->sb;
+
+	thaw_super(sb);
+	deactivate_super(sb);
+	kfree(fd_data);
+
+	return 0;
+}
+
+static struct file_operations freeze_fd_fops = {
+	.release	= freeze_fd_release,
+	.llseek		= noop_llseek,
+};
+
+static int ioctl_get_freeze_fd(struct file *filp, int __user *argp)
+{
+	struct super_block *sb = filp->f_path.dentry->d_inode->i_sb;
+	int fd, ret;
+	struct file *file;
+	struct freeze_fd_data *fd_data;
+
+	if (!capable(CAP_SYS_ADMIN))
+		return -EPERM;
+
+	/* If filesystem doesn't support freeze feature, return. */
+	if (sb->s_op->freeze_fs == NULL)
+		return -EOPNOTSUPP;
+
+	fd_data = kmalloc(sizeof(struct freeze_fd_data), GFP_KERNEL);
+	if (!fd_data)
+		return -ENOMEM;
+
+	/* Get a file descriptor. */
+	ret = get_unused_fd_flags(O_RDWR);
+	if (ret < 0)
+		goto err_free_fd_data;
+	fd = ret;
+	fd_data->sb = sb;
+	file = anon_inode_getfile("freeze-fd",
+				  &freeze_fd_fops, fd_data, O_RDWR);
+	if (IS_ERR(file)) {
+		ret = PTR_ERR(file);
+		goto err_put_fd;
+	}
+
+	/* Increment the active counter to keep the superblock around
+	 * until the freeze fd is closed.
+	 */
+	atomic_inc(&sb->s_active);
+
+	ret = freeze_super(sb);
+	if (ret < 0)
+		goto err_deact_super;
+
+	fd_install(fd, file);
+
+	return fd;
+
+err_deact_super:
+	deactivate_super(sb);
+err_put_fd:
+	put_unused_fd(fd);
+err_free_fd_data:
+	kfree(fd_data);
+	return ret;
+}
+
 static int ioctl_fsthaw(struct file *filp)
 {
 	struct super_block *sb = filp->f_path.dentry->d_inode->i_sb;
@@ -591,6 +666,10 @@ int do_vfs_ioctl(struct file *filp, unsi
 		error = ioctl_fsfreeze(filp);
 		break;
 
+	case FIGETFREEZEFD:
+		error = ioctl_get_freeze_fd(filp, argp);
+		break;
+
 	case FITHAW:
 		error = ioctl_fsthaw(filp);
 		break;
diff -urNp linux-3.0-orig/include/linux/fs.h linux-3.0/include/linux/fs.h
--- linux-3.0-orig/include/linux/fs.h	2011-07-27 11:32:12.700001748 +0900
+++ linux-3.0/include/linux/fs.h	2011-07-27 11:57:14.832000721 +0900
@@ -326,6 +326,7 @@ struct inodes_stat_t {
 #define FITHAW		_IOWR('X', 120, int)	/* Thaw */
 #define FITRIM		_IOWR('X', 121, struct fstrim_range)	/* Trim */
 #define FIISFROZEN	_IOR('X', 122, int)	/* get sb freeze state */
+#define FIGETFREEZEFD	_IOWR('X', 123, int)
 
 #define	FS_IOC_GETFLAGS			_IOR('f', 1, long)
 #define	FS_IOC_SETFLAGS			_IOW('f', 2, long)