Re: [PATCH 11/28] ext4: correctly calculate number of blocks for fiemap

[Eric Sandeen <sandeen@redhat.com>]

tytso@mit.edu wrote:
> On Thu, Mar 04, 2010 at 02:40:22PM +0900, Akira Fujita wrote:
>> <user-space>
>> filefrag:
>> 	fiemap->fm_start(0) fiemap->fm_length(~0ULL)
>>
>> <kernel-space>
>> fs/ioctl.c ioctl_fimap():
>>
>> filemap_check_ranges():
>> 	len(~0ULL)
>> 	new_len(4398046511103 = s_maxbytes)  <--- Because 'len > s_maxbytes'
>>
>> fs/ext4/extents.c ext4_fiemap():
>> 	last_blk = start(0) + len(4398046511103) - 1 >> s_blocksize_bits(11)
>> 	         = 4294967295 (0xFFFFFFFF)
>> 	len_blks = 4294967295 + 1 (0xFFFFFFFF + 0x00000001)
>> 		 = 4294967296 (0x100000000)  <--- _OVERFLOW!!_
>> 	
>> ext4_ext_walk_space():
>> 	num = 0
>>
>> This overflow leads to incorrect output like the below,
>> even though 2 extents exist.
> 
> Akira-san,
> 
> Thank you for your clear explanation; you're absolutely correct.  I've
> replaced the patch with the following, which I think is a bit clearer.
> I've tested to make sure it does the right thing, including a number
> of corner cases.

Ted, I think those testcases you used would be great xfstests candidates,
hint hint.  :)

-Eric

> Regards,
> 
> 					- Ted
> 
> commit aca92ff6f57c000d1b4523e383c8bd6b8269b8b1
> Author: Leonard Michlmayr <leonard.michlmayr@gmail.com>
> Date:   Thu Mar 4 17:07:28 2010 -0500
> 
>     ext4: correctly calculate number of blocks for fiemap
>     
>     ext4_fiemap() rounds the length of the requested range down to
>     blocksize, which is is not the true number of blocks that cover the
>     requested region.  This problem is especially impressive if the user
>     requests only the first byte of a file: not a single extent will be
>     reported.
>     
>     We fix this by calculating the last block of the region and then
>     subtract to find the number of blocks in the extents.
>     
>     Signed-off-by: Leonard Michlmayr <leonard.michlmayr@gmail.com>
>     Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
> 
> diff --git a/fs/ext4/extents.c b/fs/ext4/extents.c
> index bd80891..7d54850 100644
> --- a/fs/ext4/extents.c
> +++ b/fs/ext4/extents.c
> @@ -3768,7 +3768,6 @@ int ext4_fiemap(struct inode *inode, struct fiemap_extent_info *fieinfo,
>  		__u64 start, __u64 len)
>  {
>  	ext4_lblk_t start_blk;
> -	ext4_lblk_t len_blks;
>  	int error = 0;
>  
>  	/* fallback to generic here if not in extents fmt */
> @@ -3782,8 +3781,14 @@ int ext4_fiemap(struct inode *inode, struct fiemap_extent_info *fieinfo,
>  	if (fieinfo->fi_flags & FIEMAP_FLAG_XATTR) {
>  		error = ext4_xattr_fiemap(inode, fieinfo);
>  	} else {
> +		ext4_lblk_t len_blks;
> +		__u64 last_blk;
> +
>  		start_blk = start >> inode->i_sb->s_blocksize_bits;
> -		len_blks = len >> inode->i_sb->s_blocksize_bits;
> +		last_blk = (start + len - 1) >> inode->i_sb->s_blocksize_bits;
> +		if (last_blk >= EXT_MAX_BLOCK)
> +			last_blk = EXT_MAX_BLOCK-1;
> +		len_blks = ((ext4_lblk_t) last_blk) - start_blk + 1;
>  
>  		/*
>  		 * Walk the extent tree gathering extent information.
> --
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