[PATCH 5/8] btrfs: check if the fsid in the primary sb and copy sb are same

[Anand Jain <anand.jain@oracle.com>]

During the btrfs dev scan make sure that other copies of superblock
contain the same fsid as the primary SB. So that we bring to the
user notice if the superblock has been overwritten.

 mkfs.btrfs -fq /dev/sdc
 mkfs.btrfs -fq /dev/sdb
 dd if=/dev/sdb of=/dev/sdc count=4K skip=64K seek=64K obs=1 ibs=1
 mount /dev/sdc /btrfs

Caveat: Pls note that older btrfs-progs do not wipe the non-overwriting
stale superblock like copy2 if a smaller mkfs.btrfs -b  <size>  is created.
So this patch in the kernel will report error. The workaround is to wipe
the superblock manually, like
 dd if=/dev/zero of=<dev> seek=274877906944 ibs=1 obs=1 count4K
OR apply btrfs-progs patch
  btrfs-progs: wipe copies of the stale superblock beyond -b size
which shall find and wipe the non overwriting superblock.

Signed-off-by: Anand Jain <anand.jain@oracle.com>
---
 fs/btrfs/volumes.c | 60 ++++++++++++++++++++++++++++++++++++++++++------------
 1 file changed, 47 insertions(+), 13 deletions(-)

diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
index ed22f0a3d239..45dd0674571b 100644
--- a/fs/btrfs/volumes.c
+++ b/fs/btrfs/volumes.c
@@ -1198,40 +1198,74 @@ static int btrfs_read_disk_super(struct block_device *bdev, u64 bytenr,
 int btrfs_scan_one_device(const char *path, fmode_t flags, void *holder,
 			  struct btrfs_fs_devices **fs_devices_ret)
 {
+	struct btrfs_super_block *disk_super_primary;
 	struct btrfs_super_block *disk_super;
 	struct btrfs_device *device;
 	struct block_device *bdev;
 	struct page *page;
 	int ret = 0;
-	u64 bytenr;
+	int i;
 
-	/*
-	 * we would like to check all the supers, but that would make
-	 * a btrfs mount succeed after a mkfs from a different FS.
-	 * So, we need to add a special mount option to scan for
-	 * later supers, using BTRFS_SUPER_MIRROR_MAX instead
-	 */
-	bytenr = btrfs_sb_offset(0);
 	flags |= FMODE_EXCL;
 
 	bdev = blkdev_get_by_path(path, flags, holder);
 	if (IS_ERR(bdev))
 		return PTR_ERR(bdev);
 
-	ret = btrfs_read_disk_super(bdev, bytenr, &page, &disk_super);
-	if (ret)
+	disk_super_primary = kzalloc(sizeof(*disk_super_primary), GFP_KERNEL);
+	if (!disk_super_primary) {
+		ret = -ENOMEM;
 		goto error_bdev_put;
+	}
+
+	/*
+	 * We would like to check all the supers and use one good copy,
+	 * but that would make a btrfs mount succeed after a mkfs from
+	 * a different FS.
+	 * So, we need to add a special mount option to scan for
+	 * later supers, using BTRFS_SUPER_MIRROR_MAX instead.
+	 * So, just validate if all copies of the superblocks are ok
+	 * and have the same fsid.
+	 */
+	for (i = 0; i < BTRFS_SUPER_MIRROR_MAX; i++) {
+		u64 bytenr = btrfs_sb_offset(i);
+
+		ret = btrfs_read_disk_super(bdev, bytenr, &page, &disk_super);
+		if (ret) {
+			if (i == 0)
+				goto error_kfree;
+			/* copy2 is optional */
+			ret = 0;
+			continue;
+		}
+
+		if (i == 0) {
+			memcpy(disk_super_primary, disk_super,
+			       sizeof(*disk_super_primary));
+			btrfs_release_disk_super(page);
+			continue;
+		} else if (memcmp(disk_super_primary->fsid, disk_super->fsid,
+			   BTRFS_FSID_SIZE)) {
+			pr_err("BTRFS (device %pg): superblock fsid missmatch "\
+				"primary %pU copy%d %pU", bdev,
+				disk_super_primary->fsid, i, disk_super->fsid);
+			ret = -EINVAL;
+			btrfs_release_disk_super(page);
+			goto error_kfree;
+		}
+		btrfs_release_disk_super(page);
+	}
 
 	mutex_lock(&uuid_mutex);
-	device = device_list_add(path, disk_super);
+	device = device_list_add(path, disk_super_primary);
 	if (IS_ERR(device))
 		ret = PTR_ERR(device);
 	else
 		*fs_devices_ret = device->fs_devices;
 	mutex_unlock(&uuid_mutex);
 
-	btrfs_release_disk_super(page);
-
+error_kfree:
+	kfree(disk_super_primary);
 error_bdev_put:
 	blkdev_put(bdev, flags);
 
-- 
2.7.0