Re: [PATCH] fs/btrfs: fix a bug that U-boot fs btrfs implementation doesn't handle NO_HOLE feature correctly

[Tom Rini <trini@konsulko.com>]

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On Mon, Dec 27, 2021 at 02:11:14PM +0800, Qu Wenruo wrote:

> [BUG]
> When passing a btrfs with NO_HOLE feature to U-boot, and if one file
> contains holes, then the hash of the file is not correct in U-boot:
> 
>  # mkfs.btrfs -f test.img	# Since v5.15, mkfs defaults to NO_HOLES
>  # mount test.img /mnt/btrfs
>  # xfs_io -f -c "pwrite 0 4k" -c "pwrite 8k 4k" /mnt/btrfs/file
>  # md5sum /mnt/btrfs/file
>  277f3840b275c74d01e979ea9d75ac19  /mnt/btrfs/file
>  # umount /mnt/btrfs
>  # ./u-boot
>  => host bind 0 /home/adam/test.img
>  => ls host 0
>  <   >      12288  Mon Dec 27 05:35:23 2021  file
>  => load host 0 0x1000000 file
>  12288 bytes read in 0 ms
>  => md5sum 0x1000000 0x3000
>  md5 for 01000000 ... 01002fff ==> 855ffdbe4d0ccc5acab92e1b5330e4c1
> 
> The md5sum doesn't match at all.
> 
> [CAUSE]
> In U-boot btrfs implementation, the function btrfs_read_file() has the
> following iteration for file extent iteration:
> 
> 	/* Read the aligned part */
> 	while (cur < aligned_end) {
> 		ret = lookup_data_extent(root, &path, ino, cur, &next_offset);
> 		if (ret < 0)
> 			goto out;
> 		if (ret > 0) {
> 			/* No next, direct exit */
> 			if (!next_offset) {
> 				ret = 0;
> 				goto out;
> 			}
> 		}
> 		/* Read file extent */
> 
> But for NO_HOLES features, hole extents will not have any extent item
> for it.
> Thus if @cur is at a hole, lookup_data_extent() will just return >0, and
> update @next_offset.
> 
> But we still believe there is some data to read for @cur for ret > 0
> case, causing we read extent data from the next file extent.
> 
> This means, what we do for above NO_HOLES btrfs is:
> - Read 4K data from disk to file offset [0, 4K)
>   So far the data is still correct
> 
> - Read 4K data from disk to file offset [4K, 8K)
>   We didn't skip the 4K hole, but read the data at file offset [8K, 12K)
>   into file offset [4K, 8K).
> 
>   This causes the checksum mismatch.
> 
> [FIX]
> Add extra check to skip to the next non-hole range after
> lookup_data_extent().
> 
> Signed-off-by: Qu Wenruo <wqu@suse.com>

Applied to u-boot/master, thanks!

-- 
Tom

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