block: null_blk: Fix null_zone_write()

The bio and rq fields of struct nullb_cmd are now overlapping in a
union. So we cannot use a test on ->bio being non-NULL to detect the
NULL_Q_BIO queue mode. null_zone_write() use such broken test to set the
sector position of a zone append write in the command bio or request.
When the null_blk device uses the NULL_Q_MQ queue mode,
null_zone_write() wrongly end up setting the bio sector position,
resulting in the command request to be broken and random crashes
following.

Fix this by testing the device queue mode directly.

Fixes: 8ba816b2 ("null-blk: save memory footprint for struct nullb_cmd")
Signed-off-by: Damien Le Moal <damien.lemoal@opensource.wdc.com>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Link: https://lore.kernel.org/r/20220602120344.1365329-1-damien.lemoal@opensource.wdc.comSigned-off-by: Jens Axboe <axboe@kernel.dk>

drivers/block/null_blk/main.c

@@ -77,12 +77,6 @@ enum {
 	NULL_IRQ_TIMER		= 2,
 };
 
-enum {
-	NULL_Q_BIO		= 0,
-	NULL_Q_RQ		= 1,
-	NULL_Q_MQ		= 2,
-};
-
 static bool g_virt_boundary = false;
 module_param_named(virt_boundary, g_virt_boundary, bool, 0444);
 MODULE_PARM_DESC(virt_boundary, "Require a virtual boundary for the device. Default: False");

drivers/block/null_blk/null_blk.h

@@ -60,6 +60,13 @@ struct nullb_zone {
 	unsigned int capacity;
 };
 
+/* Queue modes */
+enum {
+	NULL_Q_BIO	= 0,
+	NULL_Q_RQ	= 1,
+	NULL_Q_MQ	= 2,
+};
+
 struct nullb_device {
 	struct nullb *nullb;
 	struct config_item item;

drivers/block/null_blk/zoned.c

@@ -398,10 +398,10 @@ static blk_status_t null_zone_write(struct nullb_cmd *cmd, sector_t sector,
 	 */
 	if (append) {
 		sector = zone->wp;
-		if (cmd->bio)
-			cmd->bio->bi_iter.bi_sector = sector;
-		else
+		if (dev->queue_mode == NULL_Q_MQ)
 			cmd->rq->__sector = sector;
+		else
+			cmd->bio->bi_iter.bi_sector = sector;
 	} else if (sector != zone->wp) {
 		ret = BLK_STS_IOERR;
 		goto unlock;