ext4: fix corruption when online resizing a 1K bigalloc fs

When a backup superblock is updated in update_backups(), the primary
superblock's offset in the group (that is, sbi->s_sbh->b_blocknr) is used
as the backup superblock's offset in its group. However, when the block
size is 1K and bigalloc is enabled, the two offsets are not equal. This
causes the backup group descriptors to be overwritten by the superblock
in update_backups(). Moreover, if meta_bg is enabled, the file system will
be corrupted because this feature uses backup group descriptors.

To solve this issue, we use a more accurate ext4_group_first_block_no() as
the offset of the backup superblock in its group.

Fixes: d77147ff ("ext4: add support for online resizing with bigalloc")
Signed-off-by: Baokun Li <libaokun1@huawei.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Cc: stable@kernel.org
Link: https://lore.kernel.org/r/20221117040341.1380702-4-libaokun1@huawei.comSigned-off-by: Theodore Ts'o <tytso@mit.edu>

fs/ext4/resize.c

@@ -1604,8 +1604,8 @@ static int ext4_flex_group_add(struct super_block *sb,
 		int meta_bg = ext4_has_feature_meta_bg(sb);
 		sector_t old_gdb = 0;
 
-		update_backups(sb, sbi->s_sbh->b_blocknr, (char *)es,
-			       sizeof(struct ext4_super_block), 0);
+		update_backups(sb, ext4_group_first_block_no(sb, 0),
+			       (char *)es, sizeof(struct ext4_super_block), 0);
 		for (; gdb_num <= gdb_num_end; gdb_num++) {
 			struct buffer_head *gdb_bh;
 
@@ -1816,7 +1816,7 @@ static int ext4_group_extend_no_check(struct super_block *sb,
 		if (test_opt(sb, DEBUG))
 			printk(KERN_DEBUG "EXT4-fs: extended group to %llu "
 			       "blocks\n", ext4_blocks_count(es));
-		update_backups(sb, EXT4_SB(sb)->s_sbh->b_blocknr,
+		update_backups(sb, ext4_group_first_block_no(sb, 0),
 			       (char *)es, sizeof(struct ext4_super_block), 0);
 	}
 	return err;