Re: [PATCH U-BOOT 1/3] btrfs: fix offset within btrfs_read_extent_reg()

[Qu Wenruo]

On 2023/4/18 10:05, Dominique Martinet wrote:
Qu Wenruo wrote on Tue, Apr 18, 2023 at 09:58:37AM +0800:
The subject can be changed to "btrfs: fix offset when reading compressed
extents".
The original one is a little too generic.

Ok.

btrfs_file_read()
   -> btrfs_read_extent_reg
      (aligned part loop from 0x40480000 to 0x40ba0000, 128KB at a time)
       last read had 0x4000 bytes in extent, but aligned_end - cur limited
       the read to 0x3000 (offset 0x720000)
   -> read_and_truncate_page
     -> btrfs_read_extent_reg
        reading the last 0x1000 bytes from offset 0x73000 (end of the
        previous 0x4000) into 0x40ba3000
        here key.offset = 0x70000 so we need to use it to recover the
        0x3000 offset.

You can use "btrfs ins dump-tree" to provide a much easier to read on-disk
data layout.

key.offset is the offset within the read call, not the offset on disk.
The file on disk looks perfectly normal, the condition to trigger the
bug is to have a file which size is not sector-aligned and where the
last extent is bigger than a sector.

Yes, I understand the runtime offset is involved.

But you're still trying to explain the situation with involved on-disk 
file extent, right?

In that case it's way easier to go something like this:

  We can have a compressed file extent like this:

        item 6 key (257 EXTENT_DATA 0) itemoff 15816 itemsize 53
                generation 7 type 1 (regular)
                extent data disk byte 13631488 nr 4096
                extent data offset 0 nr 32768 ram 32768
                extent compression 1 (zlib)

  Then if we try to read file range [4K, 8K) of inode 257 in Uboot, then
  we got corrupted data.

At least that's the preferred way to explain in btrfs community (with 
on-disk thus static part, then runtime part).


I had a look at dump-tree early on and couldn't actually find my file in
there, now the problem is understood it should be easy to make a
reproducer so I'll add this info and commands needed to reproduce (+ a
link to a fs image just in case) in v2

The reproducer is super easy.

 # mkfs.btrfs -f $dev
 # mount $dev -o compress $mnt
 # xfs_io -f -c "pwrite -i /dev/urandom 0 16K" $mnt/file
 # umount $mnt

Then in uboot
 # load host 0 $addr file 0x1000 0x1000
 # md $addr

And compare to regular xxd from kernel.

  
        /* Preallocated or hole , fill @dest with zero */
        if (btrfs_file_extent_type(leaf, fi) == BTRFS_FILE_EXTENT_PREALLOC ||
@@ -454,9 +456,7 @@ int btrfs_read_extent_reg(struct btrfs_path *path,
        if (btrfs_file_extent_compression(leaf, fi) == BTRFS_COMPRESS_NONE) {
                u64 logical;
-               logical = btrfs_file_extent_disk_bytenr(leaf, fi) +
-                         btrfs_file_extent_offset(leaf, fi) +
-                         offset - key.offset;
+               logical = btrfs_file_extent_disk_bytenr(leaf, fi) + offset;

This is touching non-compressed path, which is very weird as your commit
message said this part is correct.

my rationale is that since this was considered once but forgotten the
other time it'll be easy to add yet another code path that forgets it
later, but I guess it won't change much anyway -- I'll fix the patch
making it explicit again.

OK, that makes sense now.

Thanks,
Qu



Thanks,