hv_sock: Extract hvs_send_data() helper that takes only header

When building under -Warray-bounds, the compiler is especially
conservative when faced with casts from a smaller object to a larger
object. While this has found many real bugs, there are some cases that
are currently false positives (like here). With this as one of the last
few instances of the warning in the kernel before -Warray-bounds can be
enabled globally, rearrange the functions so that there is a header-only
version of hvs_send_data(). Silences this warning:

net/vmw_vsock/hyperv_transport.c: In function 'hvs_shutdown_lock_held.constprop':
net/vmw_vsock/hyperv_transport.c:231:32: warning: array subscript 'struct hvs_send_buf[0]' is partly outside array bounds of 'struct vmpipe_proto_header[1]' [-Warray-bounds]
  231 |         send_buf->hdr.pkt_type = 1;
      |         ~~~~~~~~~~~~~~~~~~~~~~~^~~
net/vmw_vsock/hyperv_transport.c:465:36: note: while referencing 'hdr'
  465 |         struct vmpipe_proto_header hdr;
      |                                    ^~~

This change results in no executable instruction differences.
Signed-off-by: Kees Cook <keescook@chromium.org>
Acked-by: Wei Liu <wei.liu@kernel.org>
Link: https://lore.kernel.org/r/20211207063217.2591451-1-keescook@chromium.orgSigned-off-by: Jakub Kicinski <kuba@kernel.org>

net/vmw_vsock/hyperv_transport.c

@@ -225,14 +225,20 @@ static size_t hvs_channel_writable_bytes(struct vmbus_channel *chan)
 	return round_down(ret, 8);
 }
 
+static int __hvs_send_data(struct vmbus_channel *chan,
+			   struct vmpipe_proto_header *hdr,
+			   size_t to_write)
+{
+	hdr->pkt_type = 1;
+	hdr->data_size = to_write;
+	return vmbus_sendpacket(chan, hdr, sizeof(*hdr) + to_write,
+				0, VM_PKT_DATA_INBAND, 0);
+}
+
 static int hvs_send_data(struct vmbus_channel *chan,
 			 struct hvs_send_buf *send_buf, size_t to_write)
 {
-	send_buf->hdr.pkt_type = 1;
-	send_buf->hdr.data_size = to_write;
-	return vmbus_sendpacket(chan, &send_buf->hdr,
-				sizeof(send_buf->hdr) + to_write,
-				0, VM_PKT_DATA_INBAND, 0);
+	return __hvs_send_data(chan, &send_buf->hdr, to_write);
 }
 
 static void hvs_channel_cb(void *ctx)
@@ -468,7 +474,7 @@ static void hvs_shutdown_lock_held(struct hvsock *hvs, int mode)
 		return;
 
 	/* It can't fail: see hvs_channel_writable_bytes(). */
-	(void)hvs_send_data(hvs->chan, (struct hvs_send_buf *)&hdr, 0);
+	(void)__hvs_send_data(hvs->chan, &hdr, 0);
 	hvs->fin_sent = true;
 }