HID: hyperv: avoid struct memcpy overrun warning

A previous patch addressed the fortified memcpy warning for most
builds, but I still see this one with gcc-9:

In file included from include/linux/string.h:254,
                 from drivers/hid/hid-hyperv.c:8:
In function 'fortify_memcpy_chk',
    inlined from 'mousevsc_on_receive' at drivers/hid/hid-hyperv.c:272:3:
include/linux/fortify-string.h:583:4: error: call to '__write_overflow_field' declared with attribute warning: detected write beyond size of field (1st parameter); maybe use struct_group()? [-Werror=attribute-warning]
  583 |    __write_overflow_field(p_size_field, size);
      |    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

My guess is that the WARN_ON() itself is what confuses gcc, so it no
longer sees that there is a correct range check. Rework the code in a
way that helps readability and avoids the warning.

Fixes: 542f25a ("HID: hyperv: Replace one-element array with flexible-array member")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Michael Kelley <mikelley@microsoft.com>
Link: https://lore.kernel.org/r/20230705140242.844167-1-arnd@kernel.org
Signed-off-by: Benjamin Tissoires <bentiss@kernel.org>

Author: arndb

drivers/hid/hid-hyperv.c

@@ -258,19 +258,17 @@ static void mousevsc_on_receive(struct hv_device *device,
 
 	switch (hid_msg_hdr->type) {
 	case SYNTH_HID_PROTOCOL_RESPONSE:
+		len = struct_size(pipe_msg, data, pipe_msg->size);
+
 		/*
 		 * While it will be impossible for us to protect against
 		 * malicious/buggy hypervisor/host, add a check here to
 		 * ensure we don't corrupt memory.
 		 */
-		if (struct_size(pipe_msg, data, pipe_msg->size)
-			> sizeof(struct mousevsc_prt_msg)) {
-			WARN_ON(1);
+		if (WARN_ON(len > sizeof(struct mousevsc_prt_msg)))
 			break;
-		}
 
-		memcpy(&input_dev->protocol_resp, pipe_msg,
-				struct_size(pipe_msg, data, pipe_msg->size));
+		memcpy(&input_dev->protocol_resp, pipe_msg, len);
 		complete(&input_dev->wait_event);
 		break;