Re: [PATCH v2] efi/libstub: populate LoaderDevicePartUUID

["Ard Biesheuvel" <ardb@kernel.org>]

Hello Vincent,

On Thu, 3 Sep 2026, at 23:19, Vincent Mailhol wrote:
> The Boot Loader Interface [1] defines LoaderDevicePartUUID. That EFI
> variable records the GPT partition UUID of the partition containing the
> boot loader.
>
> This 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 it, but when the kernel is
> started directly by EFI firmware, there is no conventional external boot
> loader to provide the variable. In that case, because the EFI stub
> performs the boot loader role, it should provide the variable itself.
>

Fair enough.

But shouldn't it set LoaderInfo as well then?

> 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. Do not overwrite an
> existing variable, so a value supplied by an earlier boot stage keeps
> precedence.
>
> Install the efi_bli_set_variables() hook in both the generic efi-stub.c
> path and the x86-specific x86-stub.c path.
>
> Add CONFIG_EFI_STUB_BLI to make this new feature configurable.
>

I don't think this is needed tbh. Better to enable this unconditionally
so we can rely on this being present in the longer term.

> For an x86_64 build using gcc 15.3.0, bloat-o-meter reports the
> following difference between builds without and with CONFIG_EFI_STUB_BLI:
>
>   add/remove: 5/0 grow/shrink: 1/0 up/down: 639/0 (639)
>   Function                                     old     new   delta
>   efi_bli_set_variables                          -     563    +563
>   loader_entry_guid                              -      16     +16
>   hex                                            -      16     +16
>   guid_index                                     -      16     +16
>   device_path_guid                               -      16     +16
>   efi_stub_entry                              4180    4192     +12
>   Total: Before=29223, After=29862, chg +2.19%
>
> [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#L111
>
> Signed-off-by: Vincent Mailhol <mailhol@kernel.org>
> ---
> Changes in v2:
>
>   - Add CONFIG_EFI_STUB_BLI to make the BLI feature optional.
>
>   - Use static storage for GUID initializers. This reduces the size by
>     about 10% compared to v1.
>
>   - Remove the redundant NULL check on image.
>
>   - Move the nibble-to-ASCII-hex conversion out of efi_bli_guid_to_str()
>     into the new efi_bli_nibble_to_hex().
>
>   - Rename variables to be closer to the EFI specification.
>
> Link to v1: 
> https://lore.kernel.org/r/20260725-efi_stub_bli-v1-1-966e4748077f@kernel.org
> ---
>  drivers/firmware/efi/Kconfig            |  15 +++++
>  drivers/firmware/efi/libstub/Makefile   |   2 +
>  drivers/firmware/efi/libstub/bli.c      | 114 
> ++++++++++++++++++++++++++++++++
>  drivers/firmware/efi/libstub/efi-stub.c |   1 +
>  drivers/firmware/efi/libstub/efistub.h  |   6 ++
>  drivers/firmware/efi/libstub/x86-stub.c |   1 +
>  6 files changed, 139 insertions(+)
>
> diff --git a/drivers/firmware/efi/Kconfig b/drivers/firmware/efi/Kconfig
> index 29e0729299f5..eff7c6164096 100644
> --- a/drivers/firmware/efi/Kconfig
> +++ b/drivers/firmware/efi/Kconfig
> @@ -100,6 +100,21 @@ config EFI_ARMSTUB_DTB_LOADER
>  	  functionality for bootloaders that do not have such support
>  	  this option is necessary.
> 
> +config EFI_STUB_BLI
> +	bool "EFI stub Boot Loader Interface support"
> +	depends on EFI_STUB
> +	default y
> +	help
> +	  The Boot Loader Interface defines EFI variables that expose
> +	  information about the boot loader to the running OS.
> +
> +	  Enable this option to let the EFI stub populate the volatile
> +	  LoaderDevicePartUUID EFI variable when it is missing. This allows
> +	  user space services such as systemd-gpt-auto-generator to discover
> +	  partitions on the disk from which the kernel was loaded.
> +
> +	  If unsure, say Y.
> +
>  config EFI_BOOTLOADER_CONTROL
>  	tristate "EFI Bootloader Control"
>  	select UCS2_STRING
> diff --git a/drivers/firmware/efi/libstub/Makefile 
> b/drivers/firmware/efi/libstub/Makefile
> index 77a2b2d74f3f..000a73a4bc52 100644
> --- a/drivers/firmware/efi/libstub/Makefile
> +++ b/drivers/firmware/efi/libstub/Makefile
> @@ -75,6 +75,8 @@ libfdt-deps			:= fdt_rw.c fdt_ro.c fdt_wip.c fdt.c \
>  lib-$(CONFIG_EFI_PARAMS_FROM_FDT) += fdt.o \
>  				     $(patsubst %.c,lib-%.o,$(libfdt-deps))
> 
> +lib-$(CONFIG_EFI_STUB_BLI)	+= bli.o
> +
>  $(obj)/lib-%.o: $(srctree)/lib/%.c FORCE
>  	$(call if_changed_rule,cc_o_c)
> 
> diff --git a/drivers/firmware/efi/libstub/bli.c 
> b/drivers/firmware/efi/libstub/bli.c
> new file mode 100644
> index 000000000000..d54eb42729a7
> --- /dev/null
> +++ b/drivers/firmware/efi/libstub/bli.c
> @@ -0,0 +1,114 @@
> +// SPDX-License-Identifier: GPL-2.0
> +
> +#include <linux/efi.h>
> +#include <linux/errno.h>
> +#include <linux/unaligned.h>
> +
> +#include "efistub.h"
> +
> +struct efi_hd_dev_path {
> +	struct efi_generic_dev_path header;
> +	u32 partition_number;
> +	u64 partition_start;
> +	u64 partition_size;
> +	efi_guid_t signature;
> +	u8 partition_format;
> +	u8 signature_type;
> +} __packed;
> +

Please add this to linux/efi.h and add it to the union
in struct efi_dev_path as well.

> +#define EFI_HD_PARTITION_FORMAT_GPT	2
> +#define EFI_HD_SIGNATURE_TYPE_GUID	2
> +

These too

> +static efi_char16_t efi_bli_nibble_to_hex(u8 nibble)
> +{
> +	static const char hex[16] __nonstring = "0123456789abcdef";
> +
> +	return hex[nibble];
> +}
> +
> +static void efi_bli_guid_to_str(const efi_guid_t *guid, efi_char16_t 
> *out)
> +{
> +	static const u8 guid_index[UUID_SIZE] = {
> +		3, 2, 1, 0, 5, 4, 7, 6, 8, 9, 10, 11, 12, 13, 14, 15,
> +	};
> +
> +	for (int i = 0; i < ARRAY_SIZE(guid_index); i++) {
> +		u8 byte = guid->b[guid_index[i]];
> +
> +		*out++ = efi_bli_nibble_to_hex(byte >> 4);
> +		*out++ = efi_bli_nibble_to_hex(byte & 0xf);
> +
> +		switch (i) {
> +		case 3:
> +		case 5:
> +		case 7:
> +		case 9:
> +			*out++ = L'-';
> +		}
> +	}
> +
> +	*out = L'\0';
> +}
> +

I'm reluctant to add this kind of code as a special one-off, so I got a
bit carried away and took this code and put it in the stub's printf
layer.

Could you please check whether the first two patches at [0] are
sufficient for efi_bli_guid_to_str() to be replaced by a simple
efi_snprintf("%pUl", ...) call here?


-- 
Ard.


[0] https://git.kernel.org/pub/scm/linux/kernel/git/ardb/linux.git/log/?h=efi-libstub-native-utf16