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

[Nikolay Borisov <nborisov@suse.com>]

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.


> 
> [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
> 
> --
> To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in
> the body of a message to majordomo@vger.kernel.org
> More majordomo info at  http://vger.kernel.org/majordomo-info.html
>