efi_loader: bootmgr: fix a problem in loading an image from a short-path

Booting from a short-form device path which starts with the first element
being a File Path Media Device Path failed because it doesn't contain
any valid device with simple file system protocol and efi_dp_find_obj()
in efi_load_image_from_path() will return NULL.
For instance,
/VenHw(e61d73b9-a384-4acc-aeab-82e828f3628b)/Scsi(0,0)/\helloworld.efi
-> shortened version: /\helloworld.efi

With this patch applied, all the media devices with simple file system
protocol are enumerated and the boot manager attempts to boot temporarily
generated device paths one-by-one.

This new implementation is still a bit incompatible with the UEFI
specification in terms of:
* not creating real boot options
* not try
  "If a device does not support the EFI_SIMPLE_FILE_SYSTEM_PROTOCOL, but
  supports the EFI_BLOCK_IO_PROTOCOL protocol, then the EFI Boot Service
  ConnectController must be called for this device with DriverImageHandle
  and RemainingDevicePath set to NULL and the Recursive flag is set to TRUE."
(See section 3.1.2 "Load Option Processing".)

But it still gives us a closer and better solution than the current.

Fixes: commit 9cdf4702 ("efi_loader: support booting via short-form device-path")
Signed-off-by: NAKASHI Takahiro <takahiro.akashi@linaro.org>
Reviewed-by: NHeinrich Schuchardt <heinrich.schuchardt@canonical.com>

lib/efi_loader/efi_bootmgr.c

@@ -76,6 +76,100 @@ struct efi_device_path *expand_media_path(struct efi_device_path *device_path)
 	return full_path;
 }
 
+/**
+ * try_load_from_file_path() - try to load a file
+ *
+ * Given a file media path iterate through a list of handles and try to
+ * to load the file from each of them until the first success.
+ *
+ * @fs_handles: array of handles with the simple file protocol
+ * @num:	number of handles in fs_handles
+ * @fp:		file path to open
+ * @handle:	on return pointer to handle for loaded image
+ * @removable:	if true only consider removable media, else only non-removable
+ */
+static efi_status_t try_load_from_file_path(efi_handle_t *fs_handles,
+					    efi_uintn_t num,
+					    struct efi_device_path *fp,
+					    efi_handle_t *handle,
+					    bool removable)
+{
+	struct efi_handler *handler;
+	struct efi_device_path *dp;
+	int i;
+	efi_status_t ret;
+
+	for (i = 0; i < num; i++) {
+		if (removable != efi_disk_is_removable(fs_handles[i]))
+			continue;
+
+		ret = efi_search_protocol(fs_handles[i], &efi_guid_device_path,
+					  &handler);
+		if (ret != EFI_SUCCESS)
+			continue;
+
+		dp = handler->protocol_interface;
+		if (!dp)
+			continue;
+
+		dp = efi_dp_append(dp, fp);
+		if (!dp)
+			continue;
+
+		ret = EFI_CALL(efi_load_image(true, efi_root, dp, NULL, 0,
+					      handle));
+		efi_free_pool(dp);
+		if (ret == EFI_SUCCESS)
+			return ret;
+	}
+
+	return EFI_NOT_FOUND;
+}
+
+/**
+ * try_load_from_short_path
+ * @fp:		file path
+ * @handle:	pointer to handle for newly installed image
+ *
+ * Enumerate all the devices which support file system operations,
+ * prepend its media device path to the file path, @fp, and
+ * try to load the file.
+ * This function should be called when handling a short-form path
+ * which is starting with a file device path.
+ *
+ * Return:	status code
+ */
+static efi_status_t try_load_from_short_path(struct efi_device_path *fp,
+					     efi_handle_t *handle)
+{
+	efi_handle_t *fs_handles;
+	efi_uintn_t num;
+	efi_status_t ret;
+
+	ret = EFI_CALL(efi_locate_handle_buffer(
+					BY_PROTOCOL,
+					&efi_simple_file_system_protocol_guid,
+					NULL,
+					&num, &fs_handles));
+	if (ret != EFI_SUCCESS)
+		return ret;
+	if (!num)
+		return EFI_NOT_FOUND;
+
+	/* removable media first */
+	ret = try_load_from_file_path(fs_handles, num, fp, handle, true);
+	if (ret == EFI_SUCCESS)
+		goto out;
+
+	/* fixed media */
+	ret = try_load_from_file_path(fs_handles, num, fp, handle, false);
+	if (ret == EFI_SUCCESS)
+		goto out;
+
+out:
+	return ret;
+}
+
 /**
  * try_load_entry() - try to load image for boot option
  *
@@ -116,10 +210,15 @@ static efi_status_t try_load_entry(u16 n, efi_handle_t *handle,
 		log_debug("trying to load \"%ls\" from %pD\n", lo.label,
 			  lo.file_path);
 
-		file_path = expand_media_path(lo.file_path);
-		ret = EFI_CALL(efi_load_image(true, efi_root, file_path,
-					      NULL, 0, handle));
-		efi_free_pool(file_path);
+		if (EFI_DP_TYPE(lo.file_path, MEDIA_DEVICE, FILE_PATH)) {
+			/* file_path doesn't contain a device path */
+			ret = try_load_from_short_path(lo.file_path, handle);
+		} else {
+			file_path = expand_media_path(lo.file_path);
+			ret = EFI_CALL(efi_load_image(true, efi_root, file_path,
+						      NULL, 0, handle));
+			efi_free_pool(file_path);
+		}
 		if (ret != EFI_SUCCESS) {
 			log_warning("Loading %ls '%ls' failed\n",
 				    varname, lo.label);