ublk: add control command of UBLK_U_CMD_GET_FEATURES

Add control command of UBLK_U_CMD_GET_FEATURES for returning driver's
feature set or capability.

This way can simplify userspace for maintaining compatibility because
userspace doesn't need to send command to one device for querying driver
feature set any more. Such as, with the queried feature set, userspace
can choose to use:

- UBLK_CMD_GET_DEV_INFO2 or UBLK_CMD_GET_DEV_INFO,
- UBLK_U_CMD_* or UBLK_CMD_*

Userspace code:
	https://github.com/ming1/ubdsrv/commits/features-cmdSigned-off-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20230603040601.775227-1-ming.lei@redhat.comSigned-off-by: Jens Axboe <axboe@kernel.dk>

drivers/block/ublk_drv.c

@@ -2343,6 +2343,21 @@ static int ublk_ctrl_end_recovery(struct ublk_device *ub,
 	return ret;
 }
 
+static int ublk_ctrl_get_features(struct io_uring_cmd *cmd)
+{
+	const struct ublksrv_ctrl_cmd *header = io_uring_sqe_cmd(cmd->sqe);
+	void __user *argp = (void __user *)(unsigned long)header->addr;
+	u64 features = UBLK_F_ALL & ~UBLK_F_SUPPORT_ZERO_COPY;
+
+	if (header->len != UBLK_FEATURES_LEN || !header->addr)
+		return -EINVAL;
+
+	if (copy_to_user(argp, &features, UBLK_FEATURES_LEN))
+		return -EFAULT;
+
+	return 0;
+}
+
 /*
  * All control commands are sent via /dev/ublk-control, so we have to check
  * the destination device's permission
@@ -2423,6 +2438,7 @@ static int ublk_ctrl_uring_cmd_permission(struct ublk_device *ub,
 	case UBLK_CMD_GET_DEV_INFO2:
 	case UBLK_CMD_GET_QUEUE_AFFINITY:
 	case UBLK_CMD_GET_PARAMS:
+	case (_IOC_NR(UBLK_U_CMD_GET_FEATURES)):
 		mask = MAY_READ;
 		break;
 	case UBLK_CMD_START_DEV:
@@ -2472,6 +2488,11 @@ static int ublk_ctrl_uring_cmd(struct io_uring_cmd *cmd,
 	if (ret)
 		goto out;
 
+	if (cmd_op == UBLK_U_CMD_GET_FEATURES) {
+		ret = ublk_ctrl_get_features(cmd);
+		goto out;
+	}
+
 	if (_IOC_NR(cmd_op) != UBLK_CMD_ADD_DEV) {
 		ret = -ENODEV;
 		ub = ublk_get_device_from_id(header->dev_id);

include/uapi/linux/ublk_cmd.h

@@ -47,6 +47,14 @@
 	_IOWR('u', UBLK_CMD_END_USER_RECOVERY, struct ublksrv_ctrl_cmd)
 #define UBLK_U_CMD_GET_DEV_INFO2	\
 	_IOR('u', UBLK_CMD_GET_DEV_INFO2, struct ublksrv_ctrl_cmd)
+#define UBLK_U_CMD_GET_FEATURES	\
+	_IOR('u', 0x13, struct ublksrv_ctrl_cmd)
+
+/*
+ * 64bits are enough now, and it should be easy to extend in case of
+ * running out of feature flags
+ */
+#define UBLK_FEATURES_LEN  8
 
 /*
  * IO commands, issued by ublk server, and handled by ublk driver.