linux-ext4 - Re: [PATCH] e2fsprogs: Disallow tune2fs enabling sparse_super with ext4 meta

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Date:	Thu, 6 Feb 2014 15:15:02 -0500
From:	Theodore Ts'o <tytso@....edu>
To:	Akira Fujita <a-fujita@...jp.nec.com>
Cc:	ext4 development <linux-ext4@...r.kernel.org>
Subject: Re: [PATCH] e2fsprogs: Disallow tune2fs enabling sparse_super with
 ext4 meta_bg enabled

On Fri, Jan 24, 2014 at 02:55:57AM +0000, Akira Fujita wrote:
> When meta_bg feature is enabled, group descriptor block is allocated
> every 128 block group (or every 64 block group if 64bit feature is enabled).
> 
> In such situation, files in block group more than #128 will be removed
> if sparse_super feature is enabled with tune2fs and
> afterwards necessary e2fsck running.
> 
> Because tune2fs does not reallocate group descriptor blocks
> but just set sparse_super feature.
> If ext4 has sparse_super, ext2fs_descriptor_block_loc2() called
> by e2fsck thinks the block group (e.g. #128) that it has group descriptor block
> at the head offset. But that offset is used as backup super block before.
> So e2fsck fixes ext4 based on invalid group descriptor blocks and this
> cause data lost.
> 
> The patch avoids this problem simply by disallow tune2fs enabling
> sparse_super if meta_bg is enabled.
> 
> Steps to reproduce:
> 
> 1. Create ext4 which has meta_bg, ^sparse_super and 129+ block groups.
> # mke2fs -t ext4 -O meta_bg,^resize_inode,^sparse_super DEV 17G
> # mount DEV /MP
> 
> 2. Create direcotry and files which use block group #128's metadata.
> # echo $((8192*128+1)) > /sys/fs/ext4/DEV/inode_goal
> # mkdir /MP/DIR
> # for i in $(seq 1 100); do dd if=/dev/urandom of=/MP/DIR/file$i bs=1024 count=10; done
> 
> 3. Enable sparse_super with tune2fs then execute e2fsck.
>    Data in block group #128 will be lost!!
> # umount DEV
> # tune2fs -O sparse_super DEV
> # e2fsck/e2fsck -yf DEV
> 
> Signed-off-by: Akira Fujita <a-fujita@...jp.ne.cocm>

Applied, thanks.

						- Ted
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