On 22.03.2018 15:01, Anand Jain wrote:
> 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
> users 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
>
> Signed-off-by: Anand Jain <anand.jain@xxxxxxxxxx>
> ---
> fs/btrfs/volumes.c | 43 ++++++++++++++++++++++++++++++++-----------
> 1 file changed, 32 insertions(+), 11 deletions(-)
>
> diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
> index 87d183accdb2..b79e3ffd2047 100644
> --- a/fs/btrfs/volumes.c
> +++ b/fs/btrfs/volumes.c
> @@ -1203,24 +1203,45 @@ int btrfs_scan_one_device(const char *path, fmode_t flags, void *holder,
> struct block_device *bdev;
> struct page *page;
> int ret = 0;
> - u64 bytenr;
> + int i;
> + u8 fsid_tmp[BTRFS_FSID_SIZE];
>
> - /*
> - * 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);
>
> - if (btrfs_read_disk_super(bdev, bytenr, &page, &disk_super)) {
> - ret = -EINVAL;
> - 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);
> +
> + if (btrfs_read_disk_super(bdev, bytenr, &page, &disk_super)) {
> + if (i != 0)
> + continue;
> + ret = -EINVAL;
> + goto error_bdev_put;
> + }
> + if (i == 0) {
> + memcpy(fsid_tmp, disk_super->fsid, BTRFS_FSID_SIZE);
> + continue;
> + }
> +
> + if (memcmp(fsid_tmp, disk_super->fsid, BTRFS_FSID_SIZE)) {
> + pr_err("BTRFS: (device %pg) SB copy#%d fsid %pU "\
> + "doesn't match with the primary SB fsid %pU\n",
> + bdev, i, disk_super->fsid, fsid_tmp);
> + ret = -EINVAL;
> + goto error_bdev_put;
> + }
So the code per-se makes sense. However, have you came across such a
case in the real world or are you just fixing a theoretical (but real)
issue?
> }
>
> mutex_lock(&uuid_mutex);
>
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