[PATCH u-boot] fs: btrfs: do not fail when offset of a ROOT_ITEM is not -1

[PATCH u-boot] fs: btrfs: do not fail when offset of a ROOT_ITEM is not -1

[Marek Behún]

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.

Do not fail in this case.

Signed-off-by: Marek Behún <marek.behun@nic.cz>
Cc: David Sterba <dsterba@suse.com>
Cc: Qu Wenruo <wqu@suse.com>
Cc: Tom Rini <trini@konsulko.com>
---
 fs/btrfs/disk-io.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
index b332ecb796..c6fdec95c1 100644
--- a/fs/btrfs/disk-io.c
+++ b/fs/btrfs/disk-io.c
@@ -732,8 +732,7 @@ struct btrfs_root *btrfs_read_fs_root(struct btrfs_fs_info *fs_info,
 		return fs_info->chunk_root;
 	if (location->objectid == BTRFS_CSUM_TREE_OBJECTID)
 		return fs_info->csum_root;
-	BUG_ON(location->objectid == BTRFS_TREE_RELOC_OBJECTID ||
-	       location->offset != (u64)-1);
+	BUG_ON(location->objectid == BTRFS_TREE_RELOC_OBJECTID);
 
 	node = rb_search(&fs_info->fs_root_tree, (void *)&objectid,
 			 btrfs_fs_roots_compare_objectids, NULL);
-- 
2.26.2

Re: [PATCH u-boot] fs: btrfs: do not fail when offset of a ROOT_ITEM is not -1

[Qu Wenruo]

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.

Thanks,
Qu
>
> Do not fail in this case.
>
> Signed-off-by: Marek Behún <marek.behun@nic.cz>
> Cc: David Sterba <dsterba@suse.com>
> Cc: Qu Wenruo <wqu@suse.com>
> Cc: Tom Rini <trini@konsulko.com>
> ---
>   fs/btrfs/disk-io.c | 3 +--
>   1 file changed, 1 insertion(+), 2 deletions(-)
>
> diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
> index b332ecb796..c6fdec95c1 100644
> --- a/fs/btrfs/disk-io.c
> +++ b/fs/btrfs/disk-io.c
> @@ -732,8 +732,7 @@ struct btrfs_root *btrfs_read_fs_root(struct btrfs_fs_info *fs_info,
>   		return fs_info->chunk_root;
>   	if (location->objectid == BTRFS_CSUM_TREE_OBJECTID)
>   		return fs_info->csum_root;
> -	BUG_ON(location->objectid == BTRFS_TREE_RELOC_OBJECTID ||
> -	       location->offset != (u64)-1);
> +	BUG_ON(location->objectid == BTRFS_TREE_RELOC_OBJECTID);
>
>   	node = rb_search(&fs_info->fs_root_tree, (void *)&objectid,
>   			 btrfs_fs_roots_compare_objectids, NULL);
>

Re: [PATCH u-boot] fs: btrfs: do not fail when offset of a ROOT_ITEM is not -1

[Marek Behun]

On Wed, 10 Feb 2021 08:09:14 +0800
Qu Wenruo <quwenruo.btrfs@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

Re: [PATCH u-boot] fs: btrfs: do not fail when offset of a ROOT_ITEM is not -1

[Qu Wenruo]

On 2021/2/10 上午9:05, Marek Behun wrote:
> On Wed, 10 Feb 2021 08:09:14 +0800
> Qu Wenruo <quwenruo.btrfs@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().

That's what I expect the code to do, but you're right, if kernel is not 
doing it anymore, I prefer the kernel behavior.

> 
> 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?

You're correct, the kernel is using new schema, btrfs_get_fs_root(), 
which only requires root objectid and completely get rid of the 
offset/type, which is far less possible to call with wrong parameters.

It would be a good timing to sync the code between kernel and 
progs/u-boot now.

> 
> 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.

Well, progs (and even kernel) is a mine-field for BUG_ON()s.

But at least for kernel, it's protected by tree-checker which rejects 
invalid on-disk data before it reaches btrfs code, thus mostly kernel 
BUG_ON()s are really hard to hit (a lot of them are even impossible to 
hit after the introduction of tree-checker), and indicate real problems.

For now, the BUG_ON()s in U-boot still indicates problems that we can't 
really solve or doesn't expect at all in btrfs realm, e.g. the BUG_ON() 
you're hitting (call sites problem).

I admit it's a pain in the ass for full SoC reset, but I don't have any 
better alternatives yet.

The mid to long term solution would be introducing tree-checker to 
U-boot, so that the remaining BUG_ON()s are really code bugs.

Thanks,
Qu

> 
> Marek
> 

Re: [PATCH u-boot] fs: btrfs: do not fail when offset of a ROOT_ITEM is not -1

[Marek Behun]

On Wed, 10 Feb 2021 09:20:11 +0800
Qu Wenruo <wqu@suse.com> wrote:

> You're correct, the kernel is using new schema, btrfs_get_fs_root(), 
> which only requires root objectid and completely get rid of the 
> offset/type, which is far less possible to call with wrong parameters.
> 
> It would be a good timing to sync the code between kernel and 
> progs/u-boot now.

So do you agree with this patch? If so, can you add Reviewed-by? Thanks.

Re: [PATCH u-boot] fs: btrfs: do not fail when offset of a ROOT_ITEM is not -1

[Qu Wenruo]

On 2021/2/11 上午12:21, Marek Behun wrote:
> On Wed, 10 Feb 2021 09:20:11 +0800
> Qu Wenruo <wqu@suse.com> wrote:
>
>> You're correct, the kernel is using new schema, btrfs_get_fs_root(),
>> which only requires root objectid and completely get rid of the
>> offset/type, which is far less possible to call with wrong parameters.
>>
>> It would be a good timing to sync the code between kernel and
>> progs/u-boot now.
>
> So do you agree with this patch? If so, can you add Reviewed-by? Thanks.
>
I mean, to change btrfs-progs interface to follow kernel
btrfs_get_fs_root() schema, just pass objectid, without the need for
btrfs_key.

Thanks,
Qu

Re: [PATCH u-boot] fs: btrfs: do not fail when offset of a ROOT_ITEM is not -1

[Tom Rini]

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On Tue, Feb 09, 2021 at 06:33:37PM +0100, 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.
> 
> Do not fail in this case.
> 
> Signed-off-by: Marek Behún <marek.behun@nic.cz>
> Cc: David Sterba <dsterba@suse.com>
> Cc: Qu Wenruo <wqu@suse.com>
> Cc: Tom Rini <trini@konsulko.com>

Applied to u-boot/master, thanks!

-- 
Tom

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