s390/dasd: Avoid field over-reading memcpy()

In preparation for FORTIFY_SOURCE performing compile-time and run-time
field array bounds checking for memcpy(), memmove(), and memset(),
avoid intentionally reading across neighboring array fields.

Add a wrapping structure to serve as the memcpy() source, so the compiler
can do appropriate bounds checking, avoiding this future warning:

In function '__fortify_memcpy',
    inlined from 'create_uid' at drivers/s390/block/dasd_eckd.c:749:2:
./include/linux/fortify-string.h:246:4: error: call to '__read_overflow2_field' declared with attribute error: detected read beyond size of field (2nd parameter)

Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Stefan Haberland <sth@linux.ibm.com>
Link: https://lore.kernel.org/r/20210701142221.3408680-3-sth@linux.ibm.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

drivers/s390/block/dasd_eckd.c

@@ -746,7 +746,7 @@ static void create_uid(struct dasd_eckd_private *private)
 	memcpy(uid->vendor, private->ned->HDA_manufacturer,
 	       sizeof(uid->vendor) - 1);
 	EBCASC(uid->vendor, sizeof(uid->vendor) - 1);
-	memcpy(uid->serial, private->ned->HDA_location,
+	memcpy(uid->serial, &private->ned->serial,
 	       sizeof(uid->serial) - 1);
 	EBCASC(uid->serial, sizeof(uid->serial) - 1);
 	uid->ssid = private->gneq->subsystemID;

drivers/s390/block/dasd_eckd.h

@@ -332,8 +332,10 @@ struct dasd_ned {
 	__u8 dev_type[6];
 	__u8 dev_model[3];
 	__u8 HDA_manufacturer[3];
-	__u8 HDA_location[2];
-	__u8 HDA_seqno[12];
+	struct {
+		__u8 HDA_location[2];
+		__u8 HDA_seqno[12];
+	} serial;
 	__u8 ID;
 	__u8 unit_addr;
 } __attribute__ ((packed));