btrfs-progs: avoid implicit scan for backup SB

When a disk containing btrfs is overwritten with other FS, ext4
for example it doesn't overwrite 2nd and 3rd copy of the btrfs SB.
And btrfs_read_dev_super() would look for backup SB when primary
SB isn't found. This causes the problem as in the reproducer below.

In kernel we avoid this by _not_ reading backup SB implicitly,
this patch would port the same to btrfs-progs.

reproducer:
mkfs.btrfs /dev/sde
mkfs.ext4 /dev/sde
mount /dev/sde /ext4
btrfs-convert /dev/sde (is successful (bug))

with this patch
::
btrfs-convert /dev/sde
/dev/sde is mounted

Signed-off-by: Anand Jain <Anand.Jain@oracle.com>
Signed-off-by: Chris Mason <clm@fb.com>

1 files changed, 8 insertions, 1 deletions

diff --git a/disk-io.c b/disk-io.c
index 0bd1bb01..19b95a72 100644
--- a/disk-io.c
+++ b/disk-io.c
@@ -1206,7 +1206,14 @@ int btrfs_read_dev_super(int fd, struct btrfs_super_block *sb, u64 sb_bytenr)
 		return 0;
 	}
 
-	for (i = 0; i < BTRFS_SUPER_MIRROR_MAX; 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
+	*/
+
+	for (i = 0; i < 1; i++) {
 		bytenr = btrfs_sb_offset(i);
 		ret = pread64(fd, &buf, sizeof(buf), bytenr);
 		if (ret < sizeof(buf))