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>

2 files changed, 40 insertions, 1 deletions

diff --git a/drivers/md/dm-ioctl.c b/drivers/md/dm-ioctl.c
index 7eb0682d574f..aa04f0224642 100644
--- a/drivers/md/dm-ioctl.c
+++ b/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 ||
diff --git a/include/uapi/linux/dm-ioctl.h b/include/uapi/linux/dm-ioctl.h
index b8a6bddf0727..7e75b6fd8d45 100644
--- a/include/uapi/linux/dm-ioctl.h
+++ b/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 */