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 542f25a944715 ("HID: hyperv: Replace one-element array with flexible-array member")

Signed-off-by Arnd Bergmann <[email protected]>

---
 drivers/hid/hid-hyperv.c | 10 ++++------
 1 file changed, 4 insertions(+), 6 deletions(-)

drivers/hid/hid-hyperv.c +4 -7

index 49d4a26895e76..f33485d83d24f 100644
@@ -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;
 
--

2.39.2

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);
      |    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

[...]

From: Arnd Bergmann <[email protected]>

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

Quoted 4 lines · Arnd Bergmann

From: Arnd Bergmann <[email protected]>

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

JFYI: as of today I have run Linux Next with make W=1 allmodconfig on x86_64 and it seems there are still tons of similar issues which break the build.