ublk_drv: add device parameter UBLK_PARAM_TYPE_DEVT

Userspace side only knows device ID, but the associated path of ublkc* and
ublkb* could be changed by udev, and that depends on userspace's policy, so
add parameter of UBLK_PARAM_TYPE_DEVT for retrieving major/minor of the
ublkc* and ublkb*, then user may figure out major/minor of the ublk disks
he/she owns. With major/minor, it is easy to find the device node path.
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20230106041711.914434-5-ming.lei@redhat.comSigned-off-by: Jens Axboe <axboe@kernel.dk>

drivers/block/ublk_drv.c

@@ -54,7 +54,8 @@
 		| UBLK_F_USER_RECOVERY_REISSUE)
 
 /* All UBLK_PARAM_TYPE_* should be included here */
-#define UBLK_PARAM_TYPE_ALL (UBLK_PARAM_TYPE_BASIC | UBLK_PARAM_TYPE_DISCARD)
+#define UBLK_PARAM_TYPE_ALL (UBLK_PARAM_TYPE_BASIC | \
+		UBLK_PARAM_TYPE_DISCARD | UBLK_PARAM_TYPE_DEVT)
 
 struct ublk_rq_data {
 	struct llist_node node;
@@ -255,6 +256,10 @@ static int ublk_validate_params(const struct ublk_device *ub)
 			return -EINVAL;
 	}
 
+	/* dev_t is read-only */
+	if (ub->params.types & UBLK_PARAM_TYPE_DEVT)
+		return -EINVAL;
+
 	return 0;
 }
 
@@ -1777,6 +1782,22 @@ static int ublk_ctrl_get_dev_info(struct ublk_device *ub,
 	return 0;
 }
 
+/* TYPE_DEVT is readonly, so fill it up before returning to userspace */
+static void ublk_ctrl_fill_params_devt(struct ublk_device *ub)
+{
+	ub->params.devt.char_major = MAJOR(ub->cdev_dev.devt);
+	ub->params.devt.char_minor = MINOR(ub->cdev_dev.devt);
+
+	if (ub->ub_disk) {
+		ub->params.devt.disk_major = MAJOR(disk_devt(ub->ub_disk));
+		ub->params.devt.disk_minor = MINOR(disk_devt(ub->ub_disk));
+	} else {
+		ub->params.devt.disk_major = 0;
+		ub->params.devt.disk_minor = 0;
+	}
+	ub->params.types |= UBLK_PARAM_TYPE_DEVT;
+}
+
 static int ublk_ctrl_get_params(struct ublk_device *ub,
 		struct io_uring_cmd *cmd)
 {
@@ -1798,6 +1819,7 @@ static int ublk_ctrl_get_params(struct ublk_device *ub,
 		ph.len = sizeof(struct ublk_params);
 
 	mutex_lock(&ub->mutex);
+	ublk_ctrl_fill_params_devt(ub);
 	if (copy_to_user(argp, &ub->params, ph.len))
 		ret = -EFAULT;
 	else

include/uapi/linux/ublk_cmd.h

@@ -214,6 +214,17 @@ struct ublk_param_discard {
 	__u16	reserved0;
 };
 
+/*
+ * read-only, can't set via UBLK_CMD_SET_PARAMS, disk_devt is available
+ * after device is started
+ */
+struct ublk_param_devt {
+	__u32   char_major;
+	__u32   char_minor;
+	__u32   disk_major;
+	__u32   disk_minor;
+};
+
 struct ublk_params {
 	/*
 	 * Total length of parameters, userspace has to set 'len' for both
@@ -224,10 +235,12 @@ struct ublk_params {
 	__u32	len;
 #define UBLK_PARAM_TYPE_BASIC           (1 << 0)
 #define UBLK_PARAM_TYPE_DISCARD         (1 << 1)
+#define UBLK_PARAM_TYPE_DEVT            (1 << 2)
 	__u32	types;			/* types of parameter included */
 
 	struct ublk_param_basic		basic;
 	struct ublk_param_discard	discard;
+	struct ublk_param_devt		devt;
 };
 
 #endif