hv: hyperv.h: Replace one-element array with flexible-array in struct icmsg_negotiate

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@kernel.org>
Reviewed-by: Michael Kelley <mikelley@microsoft.com>
Link: https://lore.kernel.org/r/20210201174334.GA171933@embeddedor
Signed-off-by: Wei Liu <wei.liu@kernel.org>

include/linux/hyperv.h

@@ -1529,14 +1529,14 @@ 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 {