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

Qu Wenruo
2021-12-27T12:08:55Z

On 2021/12/27 14:11, 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>
> ---
> This bug exposed another missing link, that we don't have good test
> coverage in U-boot btrfs.
>
> This is partially caused by the fact that, btrfs-progs code is not
> designed to read file contents, but just to check the cross-reference
> (aka, btrfs-check).
>
> If we really only want read-only support in U-boot, and don't ever plan
> to add write support, then I'd say the btrfs-fuse project
> (https://github.com/adam900710/btrfs-fuse/) is more suitable for U-boot.
>
> As that project already has full fs content verification selftest along
> with extra multi-device recovery tests.
> And shares the same code style between btrfs-progs/kernel.

OK, things are not that bad.

In fact, the btrfs_read_file() implementation in btrfs-fuse has the same
naming, same lookup_file_extent() (just a little naming different than
lookup_data_extent()), same parameter list.

Just without the unaligned sector handling (handled by FUSE, and it may
also be unnecessary for U-boot too), and already have the correct
handling for lookup_file_extent(), thanks to the selftest.

So this already means, it can be pretty easy for U-boot to take code
from btrfs-fuse part by part, without huge refactor again.

Thanks,
Qu

> ---
> fs/btrfs/inode.c | 8 ++++++++
> 1 file changed, 8 insertions(+)
>
> diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
> index 2c2379303d74..d00b5153336d 100644
> --- a/fs/btrfs/inode.c
> +++ b/fs/btrfs/inode.c
> @@ -717,6 +717,14 @@ int btrfs_file_read(struct btrfs_root *root, u64 ino, u64 file_offset, u64 len,
> ret = 0;
> goto out;
> }
> + /*
> + * Find a extent gap, mostly caused by NO_HOLE feature.
> + * Just to next offset directly.
> + */
> + if (next_offset > cur) {
> + cur = next_offset;
> + continue;
> + }
> }
> fi = btrfs_item_ptr(path.nodes[0], path.slots[0],
> struct btrfs_file_extent_item);