[PATCH 1/1] efi_loader: correct shortening of device-paths

[Heinrich Schuchardt]

We use short device-paths in boot options so that a file on a block device
can be found independent of the port into which the device is plugged.

Usb() device-path nodes only contain port and interface information and
therefore cannot identify a block device.
UsbWwi() device-path nodes contain the serial number of USB devices.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt at canonical.com>
---
 include/efi_api.h                |  1 +
 lib/efi_loader/efi_device_path.c | 21 ++++++---------------
 2 files changed, 7 insertions(+), 15 deletions(-)

diff --git a/include/efi_api.h b/include/efi_api.h
index c4512eeb86..dc6e5ce236 100644
--- a/include/efi_api.h
+++ b/include/efi_api.h
@@ -604,6 +604,7 @@ struct efi_device_path_acpi_path {
 #  define DEVICE_PATH_SUB_TYPE_MSG_MAC_ADDR	0x0b
 #  define DEVICE_PATH_SUB_TYPE_MSG_UART		0x0e
 #  define DEVICE_PATH_SUB_TYPE_MSG_USB_CLASS	0x0f
+#  define DEVICE_PATH_SUB_TYPE_MSG_USB_WWI	0x10
 #  define DEVICE_PATH_SUB_TYPE_MSG_SATA		0x12
 #  define DEVICE_PATH_SUB_TYPE_MSG_NVME		0x17
 #  define DEVICE_PATH_SUB_TYPE_MSG_URI		0x18
diff --git a/lib/efi_loader/efi_device_path.c b/lib/efi_loader/efi_device_path.c
index 288baa1ca7..9ed5e6273d 100644
--- a/lib/efi_loader/efi_device_path.c
+++ b/lib/efi_loader/efi_device_path.c
@@ -92,17 +92,13 @@ int efi_dp_match(const struct efi_device_path *a,
 /**
  * efi_dp_shorten() - shorten device-path
  *
- * We can have device paths that start with a USB WWID or a USB Class node,
- * and a few other cases which don't encode the full device path with bus
- * hierarchy:
+ * When creating a short boot option we want to use a device-path that is
+ * independent of the location where the block device is plugged in.
  *
- * * MESSAGING:USB_WWID
- * * MESSAGING:USB_CLASS
- * * MEDIA:FILE_PATH
- * * MEDIA:HARD_DRIVE
- * * MESSAGING:URI
+ * UsbWwi() nodes contain a serial number, hard drive paths a partition
+ * UUID. Both should be unique.
  *
- * See UEFI spec (section 3.1.2, about short-form device-paths)
+ * See UEFI spec, section 3.1.2 for "short-form device path".
  *
  * @dp:		original device-path
  * @Return:	shortened device-path or NULL
@@ -110,12 +106,7 @@ int efi_dp_match(const struct efi_device_path *a,
 struct efi_device_path *efi_dp_shorten(struct efi_device_path *dp)
 {
 	while (dp) {
-		/*
-		 * TODO: Add MESSAGING:USB_WWID and MESSAGING:URI..
-		 * in practice fallback.efi just uses MEDIA:HARD_DRIVE
-		 * so not sure when we would see these other cases.
-		 */
-		if (EFI_DP_TYPE(dp, MESSAGING_DEVICE, MSG_USB) ||
+		if (EFI_DP_TYPE(dp, MESSAGING_DEVICE, MSG_USB_WWI) ||
 		    EFI_DP_TYPE(dp, MEDIA_DEVICE, HARD_DRIVE_PATH) ||
 		    EFI_DP_TYPE(dp, MEDIA_DEVICE, FILE_PATH))
 			return dp;
-- 
2.39.2