Re: [PATCH 1/2] btrfs-progs: mkfs: fix a crash when enabling extent-tree-v2

[Anand Jain <anand.jain@oracle.com>]

On 10/7/22 20:03, Qu Wenruo wrote:
> [BUG]
> When enabling extent-tree-v2 feature at mkfs time (need to enable
> experimental features), mkfs.btrfs will crash:
> 
>    # ./mkfs.btrfs  -f -O extent-tree-v2 ~/test.img
>    btrfs-progs v5.19.1
>    See http://btrfs.wiki.kernel.org for more information.
> 
>    ERROR: superblock magic doesn't match
>    NOTE: several default settings have changed in version 5.15, please make sure
>          this does not affect your deployments:
>          - DUP for metadata (-m dup)
>          - enabled no-holes (-O no-holes)
>          - enabled free-space-tree (-R free-space-tree)
> 
>    Segmentation fault (core dumped)
> 
> [CAUSE]
> The block group tree looks like this after make_btrfs() call:
> 
>    (gdb) call btrfs_print_tree(root->fs_info->block_group_root->node, 0)
>    leaf 1163264 items 1 free space 16234 generation 1 owner BLOCK_GROUP_TREE
>    leaf 1163264 flags 0x0() backref revision 1
>    checksum stored f137c1ac
>    checksum calced f137c1ac
>    fs uuid 450d4b15-4954-4574-9801-8c6d248aaec6
>    chunk uuid 4c4cc54d-f240-4aa4-b88b-bd487db43444
> 	item 0 key (1048576 BLOCK_GROUP_ITEM 4194304) itemoff 16259 itemsize 24
> 		block group used 131072 chunk_objectid 256 flags SYSTEM|single
> 						       ^^^
> 
> This looks completely sane, but notice that chunk_objectid 256.
> That 256 value is the expected one for regular non-extent-tree-v2 btrfs,
> but for extent-tree-v2, chunk_objectid is reused as the global id of
> extent tree where the block group belongs to.
> 
> With the old 256 value as chunk_objectid, btrfs will not find an extent
> tree root for the block group, and return NULL for btrfs_extent_root()
> call, and trigger segfault.
> 
> This is a regression caused by commit 1430b41427b5 ("btrfs-progs:
> separate block group tree from extent tree v2"), which doesn't take
> extent-tree-v2 on-disk format into consideration.
> 
> [FIX]
> For the initial btrfs created by make_btrfs(), all block group items
> will be in extent-tree global id 0, thus we can reset chunk_objectid to
> 0, if and only if extent-tree-v2 is enabled.
> 
> Signed-off-by: Qu Wenruo <wqu@suse.com>


Reviewed-by: Anand Jain <anand.jain@oracle.com>

Tested-by: Anand Jain <anand.jain@oracle.com>