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

[Heinrich Schuchardt]

On 3/26/23 12:45, Mark Kettenis wrote:
>> From: Heinrich Schuchardt <heinrich.schuchardt at canonical.com>
>> Date: Sun, 26 Mar 2023 12:25:54 +0200
>>
>> 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.
> 
> This sentence makes no sense.  Do they contain a serial number, a hard
> drive path *and* a partition UUID?  But then all *three* should be
> unique.

* UsbWWi() nodes contain a unique serial number
* HD() nodes contain a unique UUID.

Shortened device paths for files will only be created if the device path 
is not a block device. Currently neither the eficonfig nor the efidebug 
command allow to specify such a device path.

Best regards

Heinrich

>>    *
>> - * 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
>>
>>