mke2fs: set upper limit to flex_bg count

mke2fs -G option allows root user to set flex_bg count (power of 2).
However ext4 has bad metadata layout if we specify more than or equal to
2^32 to mke2fs -G, because of the 32bit shift operation
in ext2fs_allocate_group_table().

And the maximum block group count of ext4 is 2^32 -1 (ext4_group_t
s_groups_count), so diallow more than 2^32 flex_bg count.

Steps to reproduce:

   # mke2fs -t ext4 -G 4294967296 DEV

   # dumpe2fs DEV
   ...
   Flex block group size:    1          <----- flex_bg is 1!
   ...
   Group 0: (Blocks 0-32767)
     Checksum 0x4afd, unused inodes 7541
     Primary superblock at 0, Group descriptors at 1-1
     Reserved GDT blocks at 2-59
     Block bitmap at 60 (+60), Inode bitmap at 61 (+61)
     Inode table at 62-533 (+62)
     32228 free blocks, 7541 free inodes, 2 directories, 7541 unused inodes
     Free blocks: 540-32767
     Free inodes: 12-7552
   Group 1: (Blocks 32768-65535) [INODE_UNINIT]
     Checksum 0xc890, unused inodes 7552
     Backup superblock at 32768, Group descriptors at 32769-32769
     Reserved GDT blocks at 32770-32827
     Block bitmap at 32828 (+60), Inode bitmap at 32829 (+61)
     Inode table at 32830-33301 (+62)
     32234 free blocks, 7552 free inodes, 0 directories, 7552 unused inodes
     Free blocks: 33302-65535
     Free inodes: 7553-15104
   ...

Signed-off-by: Akira Fujita <a-fujita@rs.jp.nec.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Reviewed-by: "Darrick J. Wong" <darrick.wong@oracle.com>

1 files changed, 6 insertions, 0 deletions

diff --git a/misc/mke2fs.c b/misc/mke2fs.c
index d5e16c03..a08709d1 100644
--- a/misc/mke2fs.c
+++ b/misc/mke2fs.c
@@ -1572,6 +1572,12 @@ profile_error:
 					_("flex_bg size must be a power of 2"));
 				exit(1);
 			}
+			if (flex_bg_size > MAX_32_NUM) {
+				com_err(program_name, 0,
+				_("flex_bg size (%lu) must be less than"
+				" or equal to 2^31"), flex_bg_size);
+				exit(1);
+			}
 			break;
 		case 'i':
 			inode_ratio = strtoul(optarg, &tmp, 0);