Add ioctls to get/set the ext4 superblock uuid.

This fixes a race between changing the ext4 superblock uuid and operations
like mounting, resizing, changing features, etc.

Reviewed-by: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: Jeremy Bongio <bongiojp@gmail.com>

fs/ext4/ext4.h

@@ -724,6 +724,8 @@ enum {
 #define EXT4_IOC_GETSTATE		_IOW('f', 41, __u32)
 #define EXT4_IOC_GET_ES_CACHE		_IOWR('f', 42, struct fiemap)
 #define EXT4_IOC_CHECKPOINT		_IOW('f', 43, __u32)
+#define EXT4_IOC_GETFSUUID		_IOR('f', 44, struct fsuuid)
+#define EXT4_IOC_SETFSUUID		_IOW('f', 44, struct fsuuid)
 
 #define EXT4_IOC_SHUTDOWN _IOR ('X', 125, __u32)
 
@@ -753,6 +755,15 @@ enum {
 						EXT4_IOC_CHECKPOINT_FLAG_ZEROOUT | \
 						EXT4_IOC_CHECKPOINT_FLAG_DRY_RUN)
 
+/*
+ * Structure for EXT4_IOC_GETFSUUID/EXT4_IOC_SETFSUUID
+ */
+struct fsuuid {
+	__u32       fsu_len;
+	__u32       fsu_flags;
+	__u8        fsu_uuid[];
+};
+
 #if defined(__KERNEL__) && defined(CONFIG_COMPAT)
 /*
  * ioctl commands in 32 bit emulation

fs/ext4/ioctl.c

@@ -20,6 +20,7 @@
 #include <linux/delay.h>
 #include <linux/iversion.h>
 #include <linux/fileattr.h>
+#include <linux/uuid.h>
 #include "ext4_jbd2.h"
 #include "ext4.h"
 #include <linux/fsmap.h>
@@ -41,6 +42,15 @@ static void ext4_sb_setlabel(struct ext4_super_block *es, const void *arg)
 	memcpy(es->s_volume_name, (char *)arg, EXT4_LABEL_MAX);
 }
 
+/*
+ * Superblock modification callback function for changing file system
+ * UUID.
+ */
+static void ext4_sb_setuuid(struct ext4_super_block *es, const void *arg)
+{
+	memcpy(es->s_uuid, (__u8 *)arg, UUID_SIZE);
+}
+
 static
 int ext4_update_primary_sb(struct super_block *sb, handle_t *handle,
 			   ext4_update_sb_callback func,
@@ -1133,6 +1143,67 @@ static int ext4_ioctl_getlabel(struct ext4_sb_info *sbi, char __user *user_label
 	return 0;
 }
 
+static int ext4_ioctl_getuuid(struct ext4_sb_info *sbi,
+			struct fsuuid __user *ufsuuid)
+{
+	int ret = 0;
+	struct fsuuid fsuuid;
+	__u8 uuid[UUID_SIZE];
+
+	if (copy_from_user(&fsuuid, ufsuuid, sizeof(fsuuid)))
+		return -EFAULT;
+
+	if (fsuuid.fsu_len != UUID_SIZE || fsuuid.fsu_flags != 0)
+		return -EINVAL;
+
+	lock_buffer(sbi->s_sbh);
+	memcpy(uuid, sbi->s_es->s_uuid, UUID_SIZE);
+	unlock_buffer(sbi->s_sbh);
+
+	if (copy_to_user(&ufsuuid->fsu_uuid[0], uuid, UUID_SIZE))
+		return -EFAULT;
+	return 0;
+}
+
+static int ext4_ioctl_setuuid(struct file *filp,
+			const struct fsuuid __user *ufsuuid)
+{
+	int ret = 0;
+	struct super_block *sb = file_inode(filp)->i_sb;
+	struct fsuuid fsuuid;
+	__u8 uuid[UUID_SIZE];
+
+	if (!capable(CAP_SYS_ADMIN))
+		return -EPERM;
+
+	/*
+	 * If any checksums (group descriptors or metadata) are being used
+	 * then the checksum seed feature is required to change the UUID.
+	 */
+	if (((ext4_has_feature_gdt_csum(sb) || ext4_has_metadata_csum(sb))
+			&& !ext4_has_feature_csum_seed(sb))
+		|| ext4_has_feature_stable_inodes(sb))
+		return -EOPNOTSUPP;
+
+	if (copy_from_user(&fsuuid, ufsuuid, sizeof(fsuuid)))
+		return -EFAULT;
+
+	if (fsuuid.fsu_len != UUID_SIZE || fsuuid.fsu_flags != 0)
+		return -EINVAL;
+
+	if (copy_from_user(uuid, &ufsuuid->fsu_uuid[0], UUID_SIZE))
+		return -EFAULT;
+
+	ret = mnt_want_write_file(filp);
+	if (ret)
+		return ret;
+
+	ret = ext4_update_superblocks_fn(sb, ext4_sb_setuuid, &uuid);
+	mnt_drop_write_file(filp);
+
+	return ret;
+}
+
 static long __ext4_ioctl(struct file *filp, unsigned int cmd, unsigned long arg)
 {
 	struct inode *inode = file_inode(filp);
@@ -1515,6 +1586,10 @@ static long __ext4_ioctl(struct file *filp, unsigned int cmd, unsigned long arg)
 		return ext4_ioctl_setlabel(filp,
 					   (const void __user *)arg);
 
+	case EXT4_IOC_GETFSUUID:
+		return ext4_ioctl_getuuid(EXT4_SB(sb), (void __user *)arg);
+	case EXT4_IOC_SETFSUUID:
+		return ext4_ioctl_setuuid(filp, (const void __user *)arg);
 	default:
 		return -ENOTTY;
 	}
@@ -1592,6 +1667,8 @@ long ext4_compat_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
 	case EXT4_IOC_CHECKPOINT:
 	case FS_IOC_GETFSLABEL:
 	case FS_IOC_SETFSLABEL:
+	case EXT4_IOC_GETFSUUID:
+	case EXT4_IOC_SETFSUUID:
 		break;
 	default:
 		return -ENOIOCTLCMD;