Re: [Qemu-block] [PATCH] qcow2: Assert that L2 table offsets fit in the L1 table

[Alberto Garcia]

ping

On Fri 08 Feb 2019 04:44:53 PM CET, Alberto Garcia wrote:
> L1 table entries have a field to store the offset of an L2 table.
> The rest of the bits of the entry are currently reserved except from
> bit 63, which stores the COPIED flag.
>
> The offset is always taken from the entry using L1E_OFFSET_MASK to
> ensure that we only use the bits that belong to that field.
>
> While that mask is used every time we read from the L1 table, it is
> never used when we write to it. Due to the limits set elsewhere in the
> code QEMU can never produce L2 table offsets that don't fit in that
> field so any such offset when allocating an L2 table would indicate a
> bug in QEMU.
>
> Signed-off-by: Alberto Garcia <address@hidden>
> ---
>  block/qcow2-cluster.c | 3 +++
>  1 file changed, 3 insertions(+)
>
> diff --git a/block/qcow2-cluster.c b/block/qcow2-cluster.c
> index 30eca26c47..179aa2c728 100644
> --- a/block/qcow2-cluster.c
> +++ b/block/qcow2-cluster.c
> @@ -285,6 +285,9 @@ static int l2_allocate(BlockDriverState *bs, int l1_index)
>          goto fail;
>      }
>  
> +    /* The offset must fit in the offset field of the L1 table entry */
> +    assert((l2_offset & L1E_OFFSET_MASK) == l2_offset);
> +
>      /* If we're allocating the table at offset 0 then something is wrong */
>      if (l2_offset == 0) {
>          qcow2_signal_corruption(bs, true, -1, -1, "Preventing invalid "
> -- 
> 2.11.0