dm ioctl: allow message to return data

This patch introduces enhanced message support that allows the
device-mapper core to recognise messages that are common to all devices,
and for messages to return data to userspace.

Core messages are processed by the function "message_for_md".  If the
device mapper doesn't support the message, it is passed to the target
driver.

If the message returns data, the kernel sets the flag
DM_MESSAGE_OUT_FLAG.
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>

drivers/md/dm-ioctl.c

@@ -1414,6 +1414,22 @@ static int table_status(struct dm_ioctl *param, size_t param_size)
 	return 0;
 }
 
+static bool buffer_test_overflow(char *result, unsigned maxlen)
+{
+	return !maxlen || strlen(result) + 1 >= maxlen;
+}
+
+/*
+ * Process device-mapper dependent messages.
+ * Returns a number <= 1 if message was processed by device mapper.
+ * Returns 2 if message should be delivered to the target.
+ */
+static int message_for_md(struct mapped_device *md, unsigned argc, char **argv,
+			  char *result, unsigned maxlen)
+{
+	return 2;
+}
+
 /*
  * Pass a message to the target that's at the supplied device offset.
  */
@@ -1425,6 +1441,8 @@ static int target_message(struct dm_ioctl *param, size_t param_size)
 	struct dm_table *table;
 	struct dm_target *ti;
 	struct dm_target_msg *tmsg = (void *) param + param->data_start;
+	size_t maxlen;
+	char *result = get_result_buffer(param, param_size, &maxlen);
 
 	md = find_device(param);
 	if (!md)
@@ -1448,6 +1466,10 @@ static int target_message(struct dm_ioctl *param, size_t param_size)
 		goto out_argv;
 	}
 
+	r = message_for_md(md, argc, argv, result, maxlen);
+	if (r <= 1)
+		goto out_argv;
+
 	table = dm_get_live_table(md);
 	if (!table)
 		goto out_argv;
@@ -1473,7 +1495,18 @@ static int target_message(struct dm_ioctl *param, size_t param_size)
  out_argv:
 	kfree(argv);
  out:
-	param->data_size = 0;
+	if (r >= 0)
+		__dev_status(md, param);
+
+	if (r == 1) {
+		param->flags |= DM_DATA_OUT_FLAG;
+		if (buffer_test_overflow(result, maxlen))
+			param->flags |= DM_BUFFER_FULL_FLAG;
+		else
+			param->data_size = param->data_start + strlen(result) + 1;
+		r = 0;
+	}
+
 	dm_put(md);
 	return r;
 }
@@ -1653,6 +1686,7 @@ static int validate_params(uint cmd, struct dm_ioctl *param)
 	param->flags &= ~DM_BUFFER_FULL_FLAG;
 	param->flags &= ~DM_UEVENT_GENERATED_FLAG;
 	param->flags &= ~DM_SECURE_DATA_FLAG;
+	param->flags &= ~DM_DATA_OUT_FLAG;
 
 	/* Ignores parameters */
 	if (cmd == DM_REMOVE_ALL_CMD ||

include/uapi/linux/dm-ioctl.h

@@ -336,4 +336,9 @@ enum {
  */
 #define DM_SECURE_DATA_FLAG		(1 << 15) /* In */
 
+/*
+ * If set, a message generated output data.
+ */
+#define DM_DATA_OUT_FLAG		(1 << 16) /* Out */
+
 #endif				/* _LINUX_DM_IOCTL_H */