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: 542f25a94471 ("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>

diff --git a/drivers/hid/hid-hyperv.c b/drivers/hid/hid-hyperv.c
index 49d4a26895e76791be50237d37ccf7490197c7bd..f33485d83d24ff9ca5c80c36bab775f26ae8e68d 100644 (file)
--- a/drivers/hid/hid-hyperv.c
+++ b/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;