RE: [PATCH][next] hv: hyperv.h: Replace one-element array with flexible-array in struct icmsg_negotiate

[Michael Kelley]

From: Gustavo A. R. Silva <gustavoars@xxxxxxxxxx> Sent: Monday, February 1, 2021 9:44 AM
> 
> 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 icmsg_negotiate, instead of a one-element array.
> 
> Also, this helps the ongoing efforts to enable -Warray-bounds and fix the
> following warnings:
> 
> drivers/hv/channel_mgmt.c:315:23: warning: array subscript 1 is above array bounds of 'struct ic_version[1]' [-Warray-bounds]
> drivers/hv/channel_mgmt.c:316:23: warning: array subscript 1 is above array bounds of 'struct ic_version[1]' [-Warray-bounds]
> 
> [1] https://en.wikipedia.org/wiki/Flexible_array_member
> [2] https://www.kernel.org/doc/html/v5.9/process/deprecated.html#zero-length-and-one-element-arrays
> 
> Link: https://github.com/KSPP/linux/issues/79
> Link: https://github.com/KSPP/linux/issues/109
> Signed-off-by: Gustavo A. R. Silva <gustavoars@xxxxxxxxxx>
> ---
>  include/linux/hyperv.h | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/include/linux/hyperv.h b/include/linux/hyperv.h
> index f0d48a368f13..7877746f1077 100644
> --- a/include/linux/hyperv.h
> +++ b/include/linux/hyperv.h
> @@ -1528,14 +1528,14 @@ struct icmsg_hdr {
>  #define IC_VERSION_NEGOTIATION_MAX_VER_COUNT 100
>  #define ICMSG_HDR (sizeof(struct vmbuspipe_hdr) + sizeof(struct icmsg_hdr))
>  #define ICMSG_NEGOTIATE_PKT_SIZE(icframe_vercnt, icmsg_vercnt) \
> -	(ICMSG_HDR + offsetof(struct icmsg_negotiate, icversion_data) + \
> +	(ICMSG_HDR + sizeof(struct icmsg_negotiate) + \
>  	 (((icframe_vercnt) + (icmsg_vercnt)) * sizeof(struct ic_version)))
> 
>  struct icmsg_negotiate {
>  	u16 icframe_vercnt;
>  	u16 icmsg_vercnt;
>  	u32 reserved;
> -	struct ic_version icversion_data[1]; /* any size array */
> +	struct ic_version icversion_data[]; /* any size array */
>  } __packed;
> 
>  struct shutdown_msg_data {
> --
> 2.27.0

Reviewed-by: Michael Kelley <mikelley@xxxxxxxxxxxxx>