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

[Vincent Mailhol]

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?

> +		return;
> +
> +	set_efi_var(L"LoaderDevicePartUUID", &loader_entry_guid,
> +		    EFI_VARIABLE_BOOTSERVICE_ACCESS | EFI_VARIABLE_RUNTIME_ACCESS,
> +		    sizeof(partuuid), partuuid);
> +}

(...)


Yours sincerely,
Vincent Mailhol