Implement a 'uuid' parameter to export a device UUID in a sysfs
attribute 'uuid'. This allows udev to create persistent device
links for ublk devices:

KERNEL=="ublkb*", ATTRS{uuid}=="?*", SYMLINK+="disk/by-id/ublk-$attr{uuid}"

Signed-off-by: Hannes Reinecke <hare@kernel.org>
---
 drivers/block/ublk_drv.c      | 57 +++++++++++++++++++++++++++++++++--
 include/uapi/linux/ublk_cmd.h |  6 ++++
 2 files changed, 61 insertions(+), 2 deletions(-)

diff --git a/drivers/block/ublk_drv.c b/drivers/block/ublk_drv.c
index aaf94d2fb789..859414065be0 100644
--- a/drivers/block/ublk_drv.c
+++ b/drivers/block/ublk_drv.c
@@ -89,7 +89,7 @@
 	(UBLK_PARAM_TYPE_BASIC | UBLK_PARAM_TYPE_DISCARD | \
 	 UBLK_PARAM_TYPE_DEVT | UBLK_PARAM_TYPE_ZONED |    \
 	 UBLK_PARAM_TYPE_DMA_ALIGN | UBLK_PARAM_TYPE_SEGMENT | \
-	 UBLK_PARAM_TYPE_INTEGRITY)
+	 UBLK_PARAM_TYPE_INTEGRITY | UBLK_PARAM_TYPE_UUID)
 
 struct ublk_uring_cmd_pdu {
 	/*
@@ -761,6 +761,16 @@ static int ublk_validate_params(const struct ublk_device *ub)
 			return -EINVAL;
 	}
 
+	if (ub->params.types & UBLK_PARAM_TYPE_UUID) {
+		const struct ublk_param_uuid *p = &ub->params.uuid;
+		uuid_t uuid;
+
+		import_uuid(&uuid, p->uuid);
+		if (uuid_is_null(&uuid)) {
+			printk("uuid is NULL\n");
+			return -EINVAL;
+		}
+	}
 	return 0;
 }
 
@@ -2971,6 +2981,49 @@ static int ublk_add_chdev(struct ublk_device *ub)
 	return ret;
 }
 
+static ssize_t uuid_show(struct device *dev,
+		struct device_attribute *attr, char *buf)
+{
+	struct gendisk *disk = dev_to_disk(dev);
+	struct ublk_device *ub = disk->private_data;
+	const struct ublk_param_uuid *p = &ub->params.uuid;
+	uuid_t uuid;
+
+	import_uuid(&uuid, p->uuid);
+	return sprintf(buf, "%pU\n", &uuid);
+}
+
+static DEVICE_ATTR_RO(uuid);
+
+static struct attribute *ublk_attrs[] = {
+	&dev_attr_uuid.attr,
+	NULL,
+};
+
+static umode_t ublk_attrs_are_visible(struct kobject *kobj,
+		struct attribute *a, int n)
+{
+	struct device *dev = kobj_to_dev(kobj);
+	struct gendisk *disk = dev_to_disk(dev);
+	struct ublk_device *ub = disk->private_data;
+
+	if (a == &dev_attr_uuid.attr &&
+	    (ub->params.types & UBLK_PARAM_TYPE_UUID))
+		return S_IRUGO;
+
+	return a->mode;
+}
+
+static const struct attribute_group ublk_attr_group = {
+	.attrs = ublk_attrs,
+	.is_visible = ublk_attrs_are_visible,
+};
+
+static const struct attribute_group *ublk_attr_groups[] = {
+	&ublk_attr_group,
+	NULL,
+};
+
 /* align max io buffer size with PAGE_SIZE */
 static void ublk_align_max_io_size(struct ublk_device *ub)
 {
@@ -3149,7 +3202,7 @@ static int ublk_ctrl_start_dev(struct ublk_device *ub,
 			goto out_put_cdev;
 	}
 
-	ret = add_disk(disk);
+	ret = device_add_disk(NULL, disk, ublk_attr_groups);
 	if (ret)
 		goto out_put_cdev;
 
diff --git a/include/uapi/linux/ublk_cmd.h b/include/uapi/linux/ublk_cmd.h
index 90f47da4f435..b30a0de44b85 100644
--- a/include/uapi/linux/ublk_cmd.h
+++ b/include/uapi/linux/ublk_cmd.h
@@ -630,6 +630,10 @@ struct ublk_param_integrity {
 	__u8	pad[5];
 };
 
+struct ublk_param_uuid {
+	__u8	uuid[16];
+};
+
 struct ublk_params {
 	/*
 	 * Total length of parameters, userspace has to set 'len' for both
@@ -645,6 +649,7 @@ struct ublk_params {
 #define UBLK_PARAM_TYPE_DMA_ALIGN       (1 << 4)
 #define UBLK_PARAM_TYPE_SEGMENT         (1 << 5)
 #define UBLK_PARAM_TYPE_INTEGRITY       (1 << 6) /* requires UBLK_F_INTEGRITY */
+#define UBLK_PARAM_TYPE_UUID            (1 << 7)
 	__u32	types;			/* types of parameter included */
 
 	struct ublk_param_basic		basic;
@@ -654,6 +659,7 @@ struct ublk_params {
 	struct ublk_param_dma_align	dma;
 	struct ublk_param_segment	seg;
 	struct ublk_param_integrity	integrity;
+	struct ublk_param_uuid		uuid;
 };
 
 #endif
-- 
2.43.0