On Wed, 10 Feb 2021 08:09:14 +0800
Qu Wenruo <quwenruo.bt...@gmx.com> wrote:
> On 2021/2/10 上午1:33, Marek Behún wrote:
> > When the btrfs_read_fs_root() function is searching a ROOT_ITEM with
> > location key offset other than -1, it currently fails via BUG_ON.
> >
> > The offset can have other value than -1, though. This can happen for
> > example if a subvolume is renamed:
> >
> > $ btrfs subvolume create X && sync
> > Create subvolume './X'
> > $ btrfs inspect-internal dump-tree /dev/root | grep -B 2 'name: X$
> > location key (270 ROOT_ITEM 18446744073709551615) type DIR
> > transid 283 data_len 0 name_len 1
> > name: X
> > $ mv X Y && sync
> > $ btrfs inspect-internal dump-tree /dev/root | grep -B 2 'name: Y$
> > location key (270 ROOT_ITEM 0) type DIR
> > transid 285 data_len 0 name_len 1
> > name: Y
> >
> > As can be seen the offset changed from -1ULL to 0.
>
>
> Offset for subvolume ROOT_ITEM can be other values, especially for
> snapshot that offset is the transid when it get created.
>
> But the problem is, if we call btrfs_read_fs_root() for subvolume tree,
> the offset of the key really doesn't matter, the only important thing is
> the objectid.
>
> Thus we use that BUG_ON() to catch careless callers.
>
> Would you please provide a case where we wrongly call
> btrfs_read_fs_root() with incorrect offset inside btrfs-progs/uboot?
>
> I believe that would be the proper way to fix.
Qu,
this can be triggered in U-Boot when listing a directory containing a
subvolume that was renamed:
- create a subvolume && sync
- rename subvolume && sync
- umount, reboot, list the directory containing the subvolume in
u-boot
It will also break when you want to read a file that has a subvolume in
it's path (e.g. `read mmc 0 0x10000000 /renamed-subvol/file`).
I found out this btrfs-progs commit:
https://github.com/kdave/btrfs-progs/commit/10f1af0fe7de5a0310657993c7c21a1d78087e56
This commit ensures that while searching a directory recursively, when
a ROOT_ITEM is encountered, the offset of its location is changed to -1
before passing the location to btrfs_read_fs_root().
So maybe we could do this in u-boot as well, but why do this? Linux'
btrfs driver does not check whether the offset is -1. So why do it here?
BTW, Qu, I think we have to change the BUG_ON code in U-Boot's btrfs
driver. BUG_ON in U-Boot calls a complete SOC reset. We can't break
whole U-Boot simply because btrfs partition contains broken data.
U-Boot commands must fail in such a case, not reset the SOC.
Marek
