[dm-devel] [PATCH v2] dm-ioctl: return UUID in DM_LIST_DEVICES_CMD result

[Mikulas Patocka]

When LVM needs to find a device with a particular UUID it needs to ask for
UUID for each device. This patch returns UUID directly in the list of
devices, so that LVM doesn't have to query all the devices with an ioctl.
The UUID is returned if the flag DM_UUID_FLAG is set in the parameters.

Returning UUID is done in backward-compatible way. There's one unused
32-bit word value after the event number. This patch sets the bit
DM_NAME_LIST_FLAG_HAS_UUID if UUID is present and
DM_NAME_LIST_FLAG_DOESNT_HAVE_UUID if it isn't (if none of these bits is
set, then we have an old kernel that doesn't support returning UUIDs). The
UUID is stored after this word. The 'next' value is updated to point after
the UUID, so that old version of libdevmapper will skip the UUID without
attempting to interpret it.

Signed-off-by: Mikulas Patocka <mpatocka at redhat.com>

---
 drivers/md/dm-ioctl.c         |   20 +++++++++++++++++---
 include/uapi/linux/dm-ioctl.h |    7 +++++++
 2 files changed, 24 insertions(+), 3 deletions(-)

Index: linux-2.6/drivers/md/dm-ioctl.c
===================================================================
--- linux-2.6.orig/drivers/md/dm-ioctl.c	2021-03-09 21:04:07.000000000 +0100
+++ linux-2.6/drivers/md/dm-ioctl.c	2021-03-11 18:53:58.000000000 +0100
@@ -558,7 +558,9 @@ static int list_devices(struct file *fil
 	for (n = rb_first(&name_rb_tree); n; n = rb_next(n)) {
 		hc = container_of(n, struct hash_cell, name_node);
 		needed += align_val(offsetof(struct dm_name_list, name) + strlen(hc->name) + 1);
-		needed += align_val(sizeof(uint32_t));
+		needed += align_val(sizeof(uint32_t) * 2);
+		if (param->flags & DM_UUID_FLAG && hc->uuid)
+			needed += align_val(strlen(hc->uuid) + 1);
 	}
 
 	/*
@@ -577,6 +579,7 @@ static int list_devices(struct file *fil
 	 * Now loop through filling out the names.
 	 */
 	for (n = rb_first(&name_rb_tree); n; n = rb_next(n)) {
+		void *uuid_ptr;
 		hc = container_of(n, struct hash_cell, name_node);
 		if (old_nl)
 			old_nl->next = (uint32_t) ((void *) nl -
@@ -588,8 +591,19 @@ static int list_devices(struct file *fil
 
 		old_nl = nl;
 		event_nr = align_ptr(nl->name + strlen(hc->name) + 1);
-		*event_nr = dm_get_event_nr(hc->md);
-		nl = align_ptr(event_nr + 1);
+		event_nr[0] = dm_get_event_nr(hc->md);
+		event_nr[1] = 0;
+		uuid_ptr = align_ptr(event_nr + 2);
+		if (param->flags & DM_UUID_FLAG) {
+			if (hc->uuid) {
+				event_nr[1] |= DM_NAME_LIST_FLAG_HAS_UUID;
+				strcpy(uuid_ptr, hc->uuid);
+				uuid_ptr = align_ptr(uuid_ptr + strlen(hc->uuid) + 1);
+			} else {
+				event_nr[1] |= DM_NAME_LIST_FLAG_DOESNT_HAVE_UUID;
+			}
+		}
+		nl = uuid_ptr;
 	}
 	/*
 	 * If mismatch happens, security may be compromised due to buffer
Index: linux-2.6/include/uapi/linux/dm-ioctl.h
===================================================================
--- linux-2.6.orig/include/uapi/linux/dm-ioctl.h	2021-03-09 12:20:23.000000000 +0100
+++ linux-2.6/include/uapi/linux/dm-ioctl.h	2021-03-11 18:42:14.000000000 +0100
@@ -193,8 +193,15 @@ struct dm_name_list {
 	__u32 next;		/* offset to the next record from
 				   the _start_ of this */
 	char name[0];
+
+	/* uint32_t event_nr; */
+	/* uint32_t flags; */
+	/* char uuid[0]; */
 };
 
+#define DM_NAME_LIST_FLAG_HAS_UUID		1
+#define DM_NAME_LIST_FLAG_DOESNT_HAVE_UUID	2
+
 /*
  * Used to retrieve the target versions
  */