ext4: fix corruption during on-line resize

We observed a corruption during on-line resize of a file system that is
larger than 16 TiB with 4k block size. With having more then 2^32 blocks
resize_inode is turned off by default by mke2fs. The issue can be
reproduced on a smaller file system for convenience by explicitly
turning off resize_inode. An on-line resize across an 8 GiB boundary (the
size of a meta block group in this setup) then leads to a corruption:

  dev=/dev/<some_dev> # should be >= 16 GiB
  mkdir -p /corruption
  /sbin/mke2fs -t ext4 -b 4096 -O ^resize_inode $dev $((2 * 2**21 - 2**15))
  mount -t ext4 $dev /corruption

  dd if=/dev/zero bs=4096 of=/corruption/test count=$((2*2**21 - 4*2**15))
  sha1sum /corruption/test
  # 79d2658b39dcfd77274e435b0934028adafaab11  /corruption/test

  /sbin/resize2fs $dev $((2*2**21))
  # drop page cache to force reload the block from disk
  echo 1 > /proc/sys/vm/drop_caches

  sha1sum /corruption/test
  # 3c2abc63cbf1a94c9e6977e0fbd72cd832c4d5c3  /corruption/test

2^21 = 2^15*2^6 equals 8 GiB whereof 2^15 is the number of blocks per
block group and 2^6 are the number of block groups that make a meta
block group.

The last checksum might be different depending on how the file is laid
out across the physical blocks. The actual corruption occurs at physical
block 63*2^15 = 2064384 which would be the location of the backup of the
meta block group's block descriptor. During the on-line resize the file
system will be converted to meta_bg starting at s_first_meta_bg which is
2 in the example - meaning all block groups after 16 GiB. However, in
ext4_flex_group_add we might add block groups that are not part of the
first meta block group yet. In the reproducer we achieved this by
substracting the size of a whole block group from the point where the
meta block group would start. This must be considered when updating the
backup block group descriptors to follow the non-meta_bg layout. The fix
is to add a test whether the group to add is already part of the meta
block group or not.

Fixes: 01f795f9e0 ("ext4: add online resizing support for meta_bg and 64-bit file systems")
Cc:  <stable@vger.kernel.org>
Signed-off-by: Maximilian Heyne <mheyne@amazon.de>
Tested-by: Srivathsa Dara <srivathsa.d.dara@oracle.com>
Reviewed-by: Srivathsa Dara <srivathsa.d.dara@oracle.com>
Link: https://lore.kernel.org/r/20240215155009.94493-1-mheyne@amazon.de
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

fs/ext4/resize.c

@@ -1602,7 +1602,8 @@ exit_journal:
 		int gdb_num = group / EXT4_DESC_PER_BLOCK(sb);
 		int gdb_num_end = ((group + flex_gd->count - 1) /
 				   EXT4_DESC_PER_BLOCK(sb));
-		int meta_bg = ext4_has_feature_meta_bg(sb);
+		int meta_bg = ext4_has_feature_meta_bg(sb) &&
+			      gdb_num >= le32_to_cpu(es->s_first_meta_bg);
 		sector_t padding_blocks = meta_bg ? 0 : sbi->s_sbh->b_blocknr -
 					 ext4_group_first_block_no(sb, 0);