Re: [PATCH][next] virtchnl: Replace one-element array in struct virtchnl_vsi_queue_config_info

[Nguyen, Anthony L]

On Tue, 2021-05-25 at 18:16 -0500, Gustavo A. R. Silva wrote:
> There is a regular need in the kernel to provide a way to declare
> having a
> dynamically sized set of trailing elements in a structure. Kernel
> code
> should always use “flexible array members”[1] for these cases. The
> older
> style of one-element or zero-length arrays should no longer be
> used[2].
> 
> Refactor the code according to the use of a flexible-array member in
> struct
> virtchnl_vsi_queue_config_info instead of one-element array, and use
> the
> flex_array_size() helper.
> 
> [1] https://en.wikipedia.org/wiki/Flexible_array_member
> [2] 
> https://www.kernel.org/doc/html/v5.10/process/deprecated.html#zero-length-and-one-element-arrays
> 
> Link: https://github.com/KSPP/linux/issues/79
> Signed-off-by: Gustavo A. R. Silva <gustavoars@xxxxxxxxxx>
> ---
>  include/linux/avf/virtchnl.h | 9 ++++-----
>  1 file changed, 4 insertions(+), 5 deletions(-)
> 
> diff --git a/include/linux/avf/virtchnl.h
> b/include/linux/avf/virtchnl.h
> index b554913804bd..ed9c4998f8ac 100644
> --- a/include/linux/avf/virtchnl.h
> +++ b/include/linux/avf/virtchnl.h
> @@ -338,10 +338,10 @@ struct virtchnl_vsi_queue_config_info {
>  	u16 vsi_id;
>  	u16 num_queue_pairs;
>  	u32 pad;
> -	struct virtchnl_queue_pair_info qpair[1];
> +	struct virtchnl_queue_pair_info qpair[];
>  };
>  
> -VIRTCHNL_CHECK_STRUCT_LEN(72, virtchnl_vsi_queue_config_info);
> +VIRTCHNL_CHECK_STRUCT_LEN(8, virtchnl_vsi_queue_config_info);
>  
>  /* VIRTCHNL_OP_REQUEST_QUEUES
>   * VF sends this message to request the PF to allocate additional
> queues to
> @@ -997,9 +997,8 @@ virtchnl_vc_validate_vf_msg(struct
> virtchnl_version_info *ver, u32 v_opcode,
>  		if (msglen >= valid_len) {
>  			struct virtchnl_vsi_queue_config_info *vqc =
>  			    (struct virtchnl_vsi_queue_config_info
> *)msg;
> -			valid_len += (vqc->num_queue_pairs *
> -				      sizeof(struct
> -					     virtchnl_queue_pair_info))
> ;
> +			valid_len += flex_array_size(vqc, qpair,
> +						     vqc-
> >num_queue_pairs);

The virtchnl file acts as a binary interface between physical and
virtual functions. There's no guaruntee that the PF and VF will both
have the newest version. Thus changing this will break compatibility.
Specifically, the way the size was validated for this op code
incorrectly expects an extra queue pair structure. Some other
structures have similar length calculation flaws. We agree that fixing
this is important, but the fix needs to account that old drivers will
send an off by 1 size. 

To properly handle compatibility we need to introduce a feature flag to
indicate the new behavior. If the feature flag is not set, we acccept
messages with the old format (with the extra size). If both the PF and
VF support the feature flag, we'll use the correct size calculations.
We're looking to add this and would like to do all the virtchnl
structure fixes in one series.

>  			if (vqc->num_queue_pairs == 0)
>  				err_msg_format = true;
>  		}