[PATCH 2/2] st: skip buffer flush for information ioctls

[David Jeffery <djeffery@redhat.com>]

With commit 9604eea5bd3a ("scsi: st: Add third party poweron reset handling")
some customer tape applications fail from being unable to complete ioctls
to verify ID information for the device when there has been any type of
reset event to their tape devices.

The st driver currently will fail all standard scsi ioctls if a call to
flush_buffer fails in st_ioctl. This causes ioctls which otherwise have no
effect on tape state to succeed or fail based on events unrelated to the
requested ioctl.

This makes scsi information ioctls unreliable after a reset even
if no buffering is in use. With a reset setting the pos_unknown field,
flush_buffer will report failure and fail all ioctls. So any application
expecting to use ioctls to check the identify the device will be unable to
do so in such a state.

For scsi information ioctls, avoid the need for a buffer flush and allow
the ioctls to execute regardless of buffer state.

Signed-off-by: David Jeffery <djeffery@redhat.com>
Tested-by:     Laurence Oberman <loberman@redhat.com>
---

 drivers/scsi/st.c | 40 ++++++++++++++++++++++------------------
 1 file changed, 22 insertions(+), 18 deletions(-)

diff --git a/drivers/scsi/st.c b/drivers/scsi/st.c
index 87f0e303fdd6..d4d2c8e3f912 100644
--- a/drivers/scsi/st.c
+++ b/drivers/scsi/st.c
@@ -3542,30 +3542,34 @@ static long st_common_ioctl(struct scsi_tape *STp, struct st_modedef *STm,
 		goto out;
 	}
 
-	if ((i = flush_buffer(STp, 0)) < 0) {
-		retval = i;
-		goto out;
-	} else { /* flush_buffer succeeds */
-		if (STp->can_partitions) {
-			i = switch_partition(STp);
-			if (i < 0) {
-				retval = i;
-				goto out;
-			}
-		}
-	}
-	mutex_unlock(&STp->lock);
-
 	switch (cmd_in) {
+	case SCSI_IOCTL_GET_IDLUN:
+	case SCSI_IOCTL_GET_BUS_NUMBER:
+	case SCSI_IOCTL_GET_PCI:
+		break;
 	case SG_IO:
 	case SCSI_IOCTL_SEND_COMMAND:
 	case CDROM_SEND_PACKET:
-		if (!capable(CAP_SYS_RAWIO))
-			return -EPERM;
-		break;
+		if (!capable(CAP_SYS_RAWIO)) {
+			retval = -EPERM;
+			goto out;
+		}
+		fallthrough;
 	default:
-		break;
+		if ((i = flush_buffer(STp, 0)) < 0) {
+			retval = i;
+			goto out;
+		} else { /* flush_buffer succeeds */
+			if (STp->can_partitions) {
+				i = switch_partition(STp);
+				if (i < 0) {
+					retval = i;
+					goto out;
+				}
+			}
+		}
 	}
+	mutex_unlock(&STp->lock);
 
 	retval = scsi_ioctl(STp->device, file->f_mode & FMODE_WRITE,
 			    cmd_in, (void __user *)arg);
-- 
2.51.0