Re: [PATCH v3] efi/libstub: add initial Boot Loader Interface support

[Ard Biesheuvel]

On Tue, 8 Sep 2026, at 20:48, Vincent Mailhol wrote:
> On 06/09/2026 at 23:52, Vincent Mailhol wrote:
>> The Boot Loader Interface (BLI) [1] defines EFI variables that expose
>> boot loader state to the running OS. LoaderInfo identifies the boot
>> loader, while LoaderDevicePartUUID records the GPT partition UUID of
>> the partition containing it.
>> 
>> LoaderDevicePartUUID is used, for example, by systemd-gpt-auto-generator
>> [2] to identify the disk the boot loader was launched from and
>> automatically detect and mount partitions on it.
>> 
>> GRUB [3] and systemd-boot [4] populate these variables, but when the
>> kernel is started directly by EFI firmware, there is no conventional
>> external boot loader to provide them. In that case, because the EFI stub
>> performs the boot loader role, it should provide the variables itself.
>> 
>> Use LoaderInfo as a sentinel: if it is already set by an earlier boot
>> stage or cannot be set, bail out. Otherwise, populate the other BLI
>> variables.
>> 
>> Parse the loaded image device path, extract the GUID signature from its
>> GPT HD() node and publish it under the Linux loader entry vendor GUID as
>> the volatile LoaderDevicePartUUID EFI variable.
>> 
>> Install the efi_bli_set_variables() hook in both the generic efi-stub.c
>> path and the x86-specific x86-stub.c path.
>> 
>> [1] The Boot Loader Interface
>> Link: https://systemd.io/BOOT_LOADER_INTERFACE/
>> 
>> [2] systemd-gpt-auto-generator
>> Link: https://www.freedesktop.org/software/systemd/man/latest/systemd-gpt-auto-generator.html
>> 
>> [3] GRUB -- §16.2 bli
>> Link: https://www.gnu.org/software/grub/manual/grub/html_node/bli_005fmodule.html
>> 
>> [4] systemd -- systemd-boot UEFI Boot Manager
>> Link: https://github.com/systemd/systemd/blob/main/docs/BOOT.md?plain=1#L102
>> 
>> Signed-off-by: Vincent Mailhol <mailhol@xxxxxxxxxx>
>> ---
>
> (...)
>
>> +static void efi_bli_populate_loader_part_uuid(efi_loaded_image_t *image)
>> +{
>> +	static efi_guid_t device_path_guid = EFI_DEVICE_PATH_PROTOCOL_GUID;
>> +	efi_char16_t partuuid[UUID_STRING_LEN + 1];
>> +	const struct efi_hd_dev_path *hd_node;
>> +	const struct efi_dev_path *path;
>> +
>> +	if (efi_bs_call(handle_protocol, efi_table_attr(image, device_handle),
>> +			&device_path_guid, (void **)&path) != EFI_SUCCESS)
>> +		return;
>> +
>> +	hd_node = efi_bli_find_hd_node(path);
>> +	if (!hd_node)
>> +		return;
>> +
>> +	if (efi_snprintf(partuuid, ARRAY_SIZE(partuuid), "%pUl",
>> +			 hd_node->signature.b) != UUID_STRING_LEN)
>                          ^^^^^^^^^^^^^^^^^^^^
> I just realize that there is a small mistake here. Conceptually
> speaking, %pUl expects a pointer to a efi_guid_t. Of course, because of
> the function being variadic, no type enforcement is done and the
> compiler is happy with hd_node->signature.b which is an u8 array.
>
> But the clean approach is definitely:
>
> 	if (efi_snprintf(partuuid, ARRAY_SIZE(partuuid), "%pUl",
> 			 &hd_node->signature) != UUID_STRING_LEN)
>
> @Ard, do you want me to send a v4, or can you just fix while applying?
>

I'll fix that up.


>> +		return;
>> +
>> +	set_efi_var(L"LoaderDevicePartUUID", &loader_entry_guid,
>> +		    EFI_VARIABLE_BOOTSERVICE_ACCESS | EFI_VARIABLE_RUNTIME_ACCESS,
>> +		    sizeof(partuuid), partuuid);
>> +}
>
> (...)
>
>
> Yours sincerely,
> Vincent Mailhol