Re: [PATCH] Btrfs: check btree node's nritems

[David Sterba <dsterba@suse.cz>]

On Wed, Aug 03, 2016 at 12:57:28PM -0700, Liu Bo wrote:
> When btree node (level = 1) has nritems which equals to zero,
> we can end up with panic due to insert_ptr()'s
> 
> BUG_ON(slot > nritems);
> 
> where slot is 1 and nritems is 0, as copy_for_split() calls
> insert_ptr(.., path->slots[1] + 1, ...);
> 
> A invalid value results in the whole mess, this adds the check
> for btree's node nritems so that we stop reading block when
> when something is wrong.
> 
> Signed-off-by: Liu Bo <bo.li.liu@oracle.com>
> ---
>  fs/btrfs/disk-io.c | 17 +++++++++++++++++
>  1 file changed, 17 insertions(+)
> 
> diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
> index 37d1780..a5a22be 100644
> --- a/fs/btrfs/disk-io.c
> +++ b/fs/btrfs/disk-io.c
> @@ -612,6 +612,20 @@ static noinline int check_leaf(struct btrfs_root *root,
>  	return 0;
>  }
>  
> +static noinline int check_node(struct btrfs_root *root,
> +			       struct extent_buffer *node)
> +{
> +	unsigned long nr = btrfs_header_nritems(node);
> +
> +	if (nr <= 0 || nr >= BTRFS_NODEPTRS_PER_BLOCK(root)) {

nr is unsigned, so it's just  "== 0"

and the BTRFS_NODEPTRS_PER_BLOCK value is inclusive, which should
explain Holger's findings.

493 * sizeof (btrfs_key_ptr) + sizeof (btrfs_header) + slack = nodesize

493 * 33 + 101 + slack = 16k (the closest value)

gives slack = 14 (smaller than sizeof (btrfs_key_ptr))