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@xxxxxxxxxx> --- 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 */ -- dm-devel mailing list dm-devel@xxxxxxxxxx https://listman.redhat.com/mailman/listinfo/dm-devel