ocfs2: validate bg_free_bits_count after update

This patch adds a safe check to ensure bg_free_bits_count doesn't exceed
bg_bits in a group descriptor. This is to avoid on disk corruption that was
seen recently.

debugfs: group <52803072>
Group Chain: 179 Parent Inode: 11 Generation: 2959379682
CRC32: 00000000 ECC: 0000
## Block# Total Used Free Contig Size
0 52803072 32256 4294965350 34202 18207 4032
......

Signed-off-by: Srinivas Eeda <[e-mail]>
Signed-off-by: Joel Becker <[e-mail]>

fs/ocfs2/suballoc.c

···
 	}
 
 	le16_add_cpu(&bg->bg_free_bits_count, -num_bits);
+	if (le16_to_cpu(bg->bg_free_bits_count) > le16_to_cpu(bg->bg_bits)) {
+		ocfs2_error(alloc_inode->i_sb, "Group descriptor # %llu has bit"
+			    " count %u but claims %u are freed. num_bits %d",
+			    (unsigned long long)le64_to_cpu(bg->bg_blkno),
+			    le16_to_cpu(bg->bg_bits),
+			    le16_to_cpu(bg->bg_free_bits_count), num_bits);
+		return -EROFS;
+	}
 	while(num_bits--)
 		ocfs2_set_bit(bit_off++, bitmap);
 
···
 				(unsigned long *) undo_bg->bg_bitmap);
 	}
 	le16_add_cpu(&bg->bg_free_bits_count, num_bits);
+	if (le16_to_cpu(bg->bg_free_bits_count) > le16_to_cpu(bg->bg_bits)) {
+		ocfs2_error(alloc_inode->i_sb, "Group descriptor # %llu has bit"
+			    " count %u but claims %u are freed. num_bits %d",
+			    (unsigned long long)le64_to_cpu(bg->bg_blkno),
+			    le16_to_cpu(bg->bg_bits),
+			    le16_to_cpu(bg->bg_free_bits_count), num_bits);
+		return -EROFS;
+	}
 
 	if (undo_fn)
 		jbd_unlock_bh_state(group_bh);