linux-ext4 - Re: [PATCH] e2fsck: Let end_blk to be the maximum value of u32.

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Date:	Wed, 16 May 2012 09:04:52 -0500
From:	Eric Sandeen <sandeen@...hat.com>
To:	Tao Ma <tm@....ma>
CC:	linux-ext4@...r.kernel.org, "Theodore Ts'o" <tytso@....edu>
Subject: Re: [PATCH] e2fsck: Let end_blk to be the maximum value of u32.

On 5/16/12 3:50 AM, Tao Ma wrote:
> From: Tao Ma <boyu.mt@...bao.com>
> 
> Now we can use fallocate to create a large file while keep the size
> to be small. It will cause the e2fsck complain about it. The test
> script is simple and I have pasted it here.
> 
> DEVICE=/dev/sdb1
> mount -t ext4 $DEVICE /mnt/ext4
> for((i=0;i<10;i++))do fallocate -n -o $[$i*8192] -l 4096 /mnt/ext4/a;done
> umount $DEVICE
> e2fsck -fn $DEVICE

Should this be put into an e2fsprogs regression test?

> The error message will be like this:
> e2fsck 1.42.3 (14-May-2012)
> Pass 1: Checking inodes, blocks, and sizes
> Inode 12 has zero length extent
> 	(invalid logical block 0, physical block 32775)
> Clear? no
> 
> Inode 12, i_blocks is 88, should be 0.  Fix? no
> 
> Pass 2: Checking directory structure
> Pass 3: Checking directory connectivity
> Pass 4: Checking reference counts
> Pass 5: Checking group summary information
> Block bitmap differences:  -(8231--8232) -(32770--32778)
> Fix? no
> 
> Now actually the end_blk can be any value which is less than
> u32, so make end_blk be the maximum value of u32.
> 
> Cc: Theodore Ts'o <tytso@....edu>
> Signed-off-by: Tao Ma <boyu.mt@...bao.com>
> ---
>  lib/ext2fs/extent.c |    4 +---
>  1 files changed, 1 insertions(+), 3 deletions(-)
> 
> diff --git a/lib/ext2fs/extent.c b/lib/ext2fs/extent.c
> index eb096d6..e2815c2 100644
> --- a/lib/ext2fs/extent.c
> +++ b/lib/ext2fs/extent.c
> @@ -253,9 +253,7 @@ extern errcode_t ext2fs_extent_open2(ext2_filsys fs, ext2_ino_t ino,
>  		ext2fs_le16_to_cpu(eh->eh_entries);
>  	handle->path[0].max_entries = ext2fs_le16_to_cpu(eh->eh_max);
>  	handle->path[0].curr = 0;
> -	handle->path[0].end_blk =
> -		(EXT2_I_SIZE(handle->inode) + fs->blocksize - 1) >>
> -		 EXT2_BLOCK_SIZE_BITS(fs->super);

Hm, so this picked the actual last block of the file, whereas

> +	handle->path[0].end_blk = ((((unsigned long long) 1) << 32) - 1);

this gives it an upper bound... why is that ok?  It's been a long time since
I looked at this code, but some explanation in the commit and in code
comments would be helpful.

If end_blk can be any value less than u32, what is its purpose?

-Eric

>  	handle->path[0].visit_num = 1;
>  	handle->level = 0;
>  	handle->magic = EXT2_ET_MAGIC_EXTENT_HANDLE;

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