scsi: ibmvscsis: Silence -Warray-bounds warning

Instead of doing a cast to storage that is too small, add a union for the
high 64 bits. Silences the warnings under -Warray-bounds:

drivers/scsi/ibmvscsi_tgt/ibmvscsi_tgt.c: In function 'ibmvscsis_send_messages':
drivers/scsi/ibmvscsi_tgt/ibmvscsi_tgt.c:1934:44: error: array subscript 'struct viosrp_crq[0]' is partly outside array bounds of 'u64[1]' {aka 'long long unsigned int[1]'} [-Werror=array-bounds]
 1934 |                                         crq->valid = VALID_CMD_RESP_EL;
      |                                            ^~
drivers/scsi/ibmvscsi_tgt/ibmvscsi_tgt.c:1875:13: note: while referencing 'msg_hi'
 1875 |         u64 msg_hi = 0;
      |             ^~~~~~

There is no change to the resulting binary instructions.

Link: https://lore.kernel.org/lkml/20220125142430.75c3160e@canb.auug.org.au
Link: https://lore.kernel.org/r/20220208061231.3429486-1-keescook@chromium.org
Cc: Michael Cyr <mikecyr@linux.ibm.com>
Cc: "James E.J. Bottomley" <jejb@linux.ibm.com>
Cc: "Martin K. Petersen" <martin.petersen@oracle.com>
Cc: Tyrel Datwyler <tyreld@linux.ibm.com>
Cc: linux-scsi@vger.kernel.org
Cc: target-devel@vger.kernel.org
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Reviewed-by: Tyrel Datwyler <tyreld@linux.ibm.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

drivers/scsi/ibmvscsi_tgt/ibmvscsi_tgt.c

@@ -1872,11 +1872,8 @@ static void srp_snd_msg_failed(struct scsi_info *vscsi, long rc)
  */
 static void ibmvscsis_send_messages(struct scsi_info *vscsi)
 {
-	u64 msg_hi = 0;
-	/* note do not attempt to access the IU_data_ptr with this pointer
-	 * it is not valid
-	 */
-	struct viosrp_crq *crq = (struct viosrp_crq *)&msg_hi;
+	struct viosrp_crq empty_crq = { };
+	struct viosrp_crq *crq = &empty_crq;
 	struct ibmvscsis_cmd *cmd, *nxt;
 	long rc = ADAPT_SUCCESS;
 	bool retry = false;
@@ -1940,7 +1937,7 @@ static void ibmvscsis_send_messages(struct scsi_info *vscsi)
 					crq->IU_length = cpu_to_be16(cmd->rsp.len);
 
 					rc = h_send_crq(vscsi->dma_dev->unit_address,
-							be64_to_cpu(msg_hi),
+							be64_to_cpu(crq->high),
 							be64_to_cpu(cmd->rsp.tag));
 
 					dev_dbg(&vscsi->dev, "send_messages: cmd %p, tag 0x%llx, rc %ld\n",

include/scsi/viosrp.h

@@ -70,12 +70,17 @@ enum viosrp_crq_status {
 };
 
 struct viosrp_crq {
+	union {
+		__be64 high;			/* High 64 bits */
+		struct {
 			u8 valid;		/* used by RPA */
 			u8 format;		/* SCSI vs out-of-band */
 			u8 reserved;
 			u8 status;		/* non-scsi failure? (e.g. DMA failure) */
 			__be16 timeout;		/* in seconds */
 			__be16 IU_length;	/* in bytes */
+		};
+	};
 	__be64 IU_data_ptr;	/* the TCE for transferring data */
 };