[PATCH 3/3] mke2fs: prevent creation of unmountable ext4 with large flex_bg count

In mke2fs command, if flex_bg count is too large to filesystem blocks count,
unmountable ext4 which has the out of filesystem block offset is created (Case1).
Moreover this large flex_bg count causes an unintentional metadata layout
(bmap-imap-itable-bmap-imap-itable .. in block group) (Case2).

To fix these issues and keep healthy flex_bg layout, disallow creating ext4
with obviously large flex_bg count to filesystem blocks count.

Steps to reproduce:
(Case1)
1.
# mke2fs -t ext4 -b 4096 -O ^resize_inode -G $((2**20)) DEV 2130483

2.
# mount -t ext4 DEV MP
mount: wrong fs type, bad option, bad superblock on /dev/sdb4,
       missing codepage or helper program, or other error
       In some cases useful info is found in syslog - try
       dmesg | tail  or so

3.
# dumpe2fs DEV
<snip>
Block count:              2130483
<snip>
Flex block group size:    1048576 
<snip>
Group 65: (Blocks 2129920-2130482) [INODE_UNINIT]
  Checksum 0x4cb3, unused inodes 8080
  Block bitmap at 67 (bg #0 + 67), Inode bitmap at 1048643 (bg #32 + 67)
  Inode table at 2129979-2130483 (+59)              <---- 2130483 is out of FS!
  65535 free blocks, 8080 free inodes, 0 directories, 8080 unused inodes
  Free blocks: 
  Free inodes: 525201-533280

(Case2)
1.
# mke2fs -t ext4 -G 2147483648 DEV 3145728

2.
# debugfs -R stats DEV
<snip>
Block count:              786432
<snip>
Flex block group size:    2147483648
<snip>
 Group  0: block bitmap at 193, inode bitmap at 194, inode table at 195    <-- 
           20233 free blocks, 8181 free inodes, 2 used directories, 8181 unused inodes
           [Checksum 0xa597]
 Group  1: block bitmap at 707, inode bitmap at 708, inode table at 709    <--
           32575 free blocks, 8192 free inodes, 0 used directories, 8192 unused inodes
           [Inode not init, Block not init, Checksum 0x196f]
 Group  2: block bitmap at 1221, inode bitmap at 1222, inode table at 1223 <--
           32768 free blocks, 8192 free inodes, 0 used directories, 8192 unused inodes
           [Inode not init, Block not init, Checksum 0x856f]
<snip>

Signed-off-by: Akira Fujita <redacted>
---
lib/ext2fs/initialize.c |   24 ++++++++++++++++++++++++
 1 file changed, 24 insertions(+)

diff --git a/lib/ext2fs/initialize.c b/lib/ext2fs/initialize.c
index 75fbf8e..34753d0 100644
--- a/lib/ext2fs/initialize.c
+++ b/lib/ext2fs/initialize.c

@@ -91,8 +91,10 @@ errcode_t ext2fs_initialize(const char *name, int flags,
 	unsigned int	rem;
 	unsigned int	overhead = 0;
 	unsigned int	ipg;
+	unsigned int	flexbg_size;
 	dgrp_t		i;
 	blk64_t		free_blocks;
+	blk64_t		flexbg_overhead;
 	blk_t		numblocks;
 	int		rsv_gdt;
 	int		csum_flag;

@@ -419,6 +421,28 @@ ipg_retry:
 	}
 
 	/*
+	 * Calculate the flex_bg related metadata blocks count.
+	 * It includes the boot block, the super block,
+	 * the block group descriptors, the reserved gdt blocks,
+	 * the block bitmaps, the inode bitmaps and the inode tables.
+	 * This is a simple check, so that the backup superblock and
+	 * other feature related blocks are not considered.
+	 */
+	flexbg_size = 1 << fs->super->s_log_groups_per_flex;
+	flexbg_overhead = super->s_first_data_block + 1 +
+		fs->desc_blocks + super->s_reserved_gdt_blocks +
+		(__u64)flexbg_size * (2 + fs->inode_blocks_per_group);
+
+	/*
+	 * Disallow creating ext4 which breaks flex_bg metadata layout
+	 * obviously.
+	 */
+	if (flexbg_overhead > ext2fs_blocks_count(fs->super)) {
+		retval = EXT2_ET_INVALID_ARGUMENT;
+		goto cleanup;
+	}
+
+	/*
 	 * At this point we know how big the filesystem will be.  So
 	 * we can do any and all allocations that depend on the block
 	 * count.