Re: [PATCH 1/2] Btrfs: fiemap: pass correct bytenr when fm_extent_count is zero

[robbieko <robbieko@synology.com>]

Nikolay Borisov 於 2018-03-07 18:19 寫到:
> On  7.03.2018 10:20, robbieko wrote:
>> From: Robbie Ko <robbieko@synology.com>
>> 
>>  # mount /dev/vdb5 /mnt/btrfs
>>  # dd if=/dev/zero bs=16K count=4 oflag=dsync of=/mnt/btrfs/file
>>  # xfs_io -c "fiemap -v" /mnt/btrfs/file
>>  /mnt/btrfs/file:
>>  EXT: FILE-OFFSET      BLOCK-RANGE      TOTAL FLAGS
>>    0: [0..127]:        25088..25215       128   0x1
>> 
>> Run fiemap with fm_extent_count set to 0, we'll get wrong value 4
>> instead of 1.
> 
> Wrong value 4 instead of 1 for which exact column, the flags? State 
> this
> explicitly.
> 
> Also this seems a bit bogus since fiemap's documentation states:
> 
> If fm_extent_count is zero, then the fm_extents[] array is ignored (no
> extents will be returned), and the fm_mapped_extents count will hold 
> the
> number of extents needed in fm_extents[] to hold the file's current 
> mapping.
> 
> So when fm_extent_count we shouldn't really be returning anything from
> kernel.
> 

Sorry I did not explain clearly.
The value is fm_mapped_extents.
If fm_extent_count  is zero, the fm_mapped_extents count will hold the 
number of extents needed.


> 
>> 
>> [REASON]
>> When fm_extent_count is 0, disko is not initialized correctly,
>> The value is 0 in this case, not the right bytenr.
> 
> This is too sparse, be more explicit i.e. that disko=0 is passed to
> emit_fiemap_extent which then leads to issues.
> 
>> 
>> [FIX]
>> Use correct disko.
>> 
>> Signed-off-by: Robbie Ko <robbieko@synology.com>
>> ---
>>  fs/btrfs/extent_io.c | 4 +---
>>  1 file changed, 1 insertion(+), 3 deletions(-)
>> 
>> diff --git a/fs/btrfs/extent_io.c b/fs/btrfs/extent_io.c
>> index 012d638..066b6df 100644
>> --- a/fs/btrfs/extent_io.c
>> +++ b/fs/btrfs/extent_io.c
>> @@ -4567,7 +4567,7 @@ int extent_fiemap(struct inode *inode, struct 
>> fiemap_extent_info *fieinfo,
>>  			offset_in_extent = em_start - em->start;
>>  		em_end = extent_map_end(em);
>>  		em_len = em_end - em_start;
>> -		disko = 0;
>> +		disko = em->block_start + offset_in_extent;
>>  		flags = 0;
>> 
>>  		/*
>> @@ -4590,8 +4590,6 @@ int extent_fiemap(struct inode *inode, struct 
>> fiemap_extent_info *fieinfo,
>>  			u64 bytenr = em->block_start -
>>  				(em->start - em->orig_start);
>> 
>> -			disko = em->block_start + offset_in_extent;
>> -
>>  			/*
>>  			 * As btrfs supports shared space, this information
>>  			 * can be exported to userspace tools via
>> --
>> 1.9.1
>> 
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