[PATCH] ublk: prevent invalid access with DEBUG

[Kevin Brodsky]

ublk_ch_uring_cmd_local() may jump to the out label before
initialising the io pointer. This will cause trouble if DEBUG is
defined, because the pr_devel() call dereferences io. Clang reports:

drivers/block/ublk_drv.c:2403:6: error: variable 'io' is used uninitialized whenever 'if' condition is true [-Werror,-Wsometimes-uninitialized]
 2403 |         if (tag >= ub->dev_info.queue_depth)
      |             ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
drivers/block/ublk_drv.c:2492:32: note: uninitialized use occurs here
 2492 |                         __func__, cmd_op, tag, ret, io->flags);
      |

Fix this by initialising io to NULL and checking it before
dereferencing it.

Signed-off-by: Kevin Brodsky <kevin.brodsky@xxxxxxx>
---
Cc: Ali Utku Selen <ali.utku.selen@xxxxxxx>
Cc: Ming Lei <ming.lei@xxxxxxxxxx>
Cc: Jens Axboe <axboe@xxxxxxxxx>
---
 drivers/block/ublk_drv.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/block/ublk_drv.c b/drivers/block/ublk_drv.c
index 0c74a41a6753..359564c40cb5 100644
--- a/drivers/block/ublk_drv.c
+++ b/drivers/block/ublk_drv.c
@@ -2367,7 +2367,7 @@ static int ublk_ch_uring_cmd_local(struct io_uring_cmd *cmd,
 	u16 buf_idx = UBLK_INVALID_BUF_IDX;
 	struct ublk_device *ub = cmd->file->private_data;
 	struct ublk_queue *ubq;
-	struct ublk_io *io;
+	struct ublk_io *io = NULL;
 	u32 cmd_op = cmd->cmd_op;
 	u16 q_id = READ_ONCE(ub_src->q_id);
 	u16 tag = READ_ONCE(ub_src->tag);
@@ -2488,7 +2488,7 @@ static int ublk_ch_uring_cmd_local(struct io_uring_cmd *cmd,
 
  out:
 	pr_devel("%s: complete: cmd op %d, tag %d ret %x io_flags %x\n",
-			__func__, cmd_op, tag, ret, io->flags);
+			__func__, cmd_op, tag, ret, io ? io->flags : 0);
 	return ret;
 }
 

base-commit: ac3fd01e4c1efce8f2c054cdeb2ddd2fc0fb150d
-- 
2.51.2