bli: Add a module for the Boot Loader Interface

Add a new module named bli. It implements a small but quite useful part
of the Boot Loader Interface [0]. This interface uses EFI variables for
communication between the boot loader and the operating system.

When loaded, this module sets two EFI variables under the vendor GUID
4a67b082-0a4c-41cf-b6c7-440b29bb8c4f:

- LoaderInfo: contains GRUB + <version number>.
  This allows the running operating system to identify the boot loader
  used during boot.

- LoaderDevicePartUUID: contains the partition UUID of the EFI System
  Partition (ESP). This is used by systemd-gpt-auto-generator [1] to
  find the root partitions (and others too), via partition type IDs [2].

This module is available on EFI platforms only. The bli module relies on
the part_gpt module which has to be loaded beforehand to make the GPT
partitions discoverable.

Update the documentation, add a new chapter "Modules" and describe the
bli module there.

[0] https://systemd.io/BOOT_LOADER_INTERFACE/
[1] https://www.freedesktop.org/software/systemd/man/systemd-gpt-auto-generator.html
[2] https://uapi-group.org/specifications/specs/discoverable_partitions_specification/

Signed-off-by: Oliver Steffen <osteffen@redhat.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>

docs/grub.texi

@@ -96,6 +96,7 @@ This edition documents version @value{VERSION}.
 * Filesystem::                  Filesystem syntax and semantics
 * Interface::                   The menu and the command-line
 * Environment::                 GRUB environment variables
+* Modules::                     Available modules
 * Commands::                    Available builtin commands
 * Internationalisation::        Topics relating to language support
 * Security::                    Authentication, authorisation, and signatures
@@ -3867,6 +3868,28 @@ using BIOS or EFI functions (no ATA, USB or IEEE1275).
 @command{grub-mkconfig} uses this facility to implement
 @samp{GRUB_SAVEDEFAULT} (@pxref{Simple configuration}).
 
+@node Modules
+@chapter Modules
+
+In this chapter, we list all modules that are available in GRUB (currently incomplete).
+
+Modules can be loaded via the @command{insmod} (@pxref{insmod}) command.
+
+@menu
+* bli::
+@end menu
+
+@node bli
+@section bli
+This module provides basic support for the Boot Loader Interface. The Boot
+Loader Interface specifies a set of EFI variables that are used to communicate
+boot-time information between the boot loader and the operating system.
+
+When loaded, the bli module publishes the GPT partition UUID of the EFI System
+Partition used during boot and a string identifying GRUB as the active boot
+loader, including the version number.
+
+This module is only available on UEFI platforms.
 
 @node Commands
 @chapter Available commands

grub-core/Makefile.core.def

@@ -2580,3 +2580,9 @@ module = {
   common = commands/memtools.c;
   condition = COND_MM_DEBUG;
 };
+
+module = {
+  name = bli;
+  efi = commands/bli.c;
+  enable = efi;
+};

grub-core/commands/bli.c

@@ -0,0 +1,120 @@
+/*
+ *  GRUB  --  GRand Unified Bootloader
+ *  Copyright (C) 2023  Free Software Foundation, Inc.
+ *
+ *  GRUB is free software: you can redistribute it and/or modify
+ *  it under the terms of the GNU General Public License as published by
+ *  the Free Software Foundation, either version 3 of the License, or
+ *  (at your option) any later version.
+ *
+ *  GRUB is distributed in the hope that it will be useful,
+ *  but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ *  GNU General Public License for more details.
+ *
+ *  You should have received a copy of the GNU General Public License
+ *  along with GRUB.  If not, see <http://www.gnu.org/licenses/>.
+ *
+ *  Implementation of the Boot Loader Interface.
+ */
+
+#include <grub/charset.h>
+#include <grub/efi/api.h>
+#include <grub/efi/disk.h>
+#include <grub/efi/efi.h>
+#include <grub/err.h>
+#include <grub/extcmd.h>
+#include <grub/gpt_partition.h>
+#include <grub/misc.h>
+#include <grub/mm.h>
+#include <grub/partition.h>
+#include <grub/types.h>
+
+GRUB_MOD_LICENSE ("GPLv3+");
+
+#define MODNAME "bli"
+
+static const grub_guid_t bli_vendor_guid = GRUB_EFI_VENDOR_BOOT_LOADER_INTERFACE_GUID;
+
+static grub_err_t
+get_part_uuid (const char *device_name, char **part_uuid)
+{
+  grub_device_t device;
+  grub_err_t status = GRUB_ERR_NONE;
+  grub_disk_t disk = NULL;
+  struct grub_gpt_partentry entry;
+
+  device = grub_device_open (device_name);
+  if (device == NULL)
+    return grub_error (grub_errno, N_("cannot open device: %s"), device_name);
+
+  disk = grub_disk_open (device->disk->name);
+  if (disk == NULL)
+    {
+      status = grub_error (grub_errno, N_("cannot open disk: %s"), device_name);
+      grub_device_close (device);
+      return status;
+    }
+
+  if (grub_strcmp (device->disk->partition->partmap->name, "gpt") != 0)
+    {
+      status = grub_error (GRUB_ERR_BAD_PART_TABLE,
+			   N_("this is not a GPT partition table: %s"), device_name);
+      goto fail;
+    }
+
+  if (grub_disk_read (disk, device->disk->partition->offset,
+		      device->disk->partition->index, sizeof (entry), &entry) != GRUB_ERR_NONE)
+    {
+      status = grub_error (grub_errno, N_("read error: %s"), device_name);
+      goto fail;
+    }
+
+  *part_uuid = grub_xasprintf ("%pG", &entry.guid);
+  if (*part_uuid == NULL)
+    status = grub_errno;
+
+ fail:
+  grub_disk_close (disk);
+  grub_device_close (device);
+
+  return status;
+}
+
+static grub_err_t
+set_loader_device_part_uuid (void)
+{
+  grub_efi_loaded_image_t *image;
+  char *device_name;
+  grub_err_t status = GRUB_ERR_NONE;
+  char *part_uuid = NULL;
+
+  image = grub_efi_get_loaded_image (grub_efi_image_handle);
+  if (image == NULL)
+    return grub_error (GRUB_ERR_BAD_DEVICE, N_("unable to find boot device"));
+
+  device_name = grub_efidisk_get_device_name (image->device_handle);
+  if (device_name == NULL)
+    return grub_error (GRUB_ERR_BAD_DEVICE, N_("unable to find boot device"));
+
+  status = get_part_uuid (device_name, &part_uuid);
+
+  if (status == GRUB_ERR_NONE)
+    status = grub_efi_set_variable_to_string ("LoaderDevicePartUUID", &bli_vendor_guid, part_uuid,
+					      GRUB_EFI_VARIABLE_BOOTSERVICE_ACCESS |
+					      GRUB_EFI_VARIABLE_RUNTIME_ACCESS);
+  else
+    grub_error (status, N_("unable to determine partition UUID of boot device"));
+
+  grub_free (part_uuid);
+  grub_free (device_name);
+  return status;
+}
+
+GRUB_MOD_INIT (bli)
+{
+  grub_efi_set_variable_to_string ("LoaderInfo", &bli_vendor_guid, PACKAGE_STRING,
+				   GRUB_EFI_VARIABLE_BOOTSERVICE_ACCESS |
+				   GRUB_EFI_VARIABLE_RUNTIME_ACCESS);
+  set_loader_device_part_uuid ();
+}

include/grub/efi/api.h

@@ -374,6 +374,11 @@
     { 0xac, 0x74, 0xca, 0x55, 0x52, 0x31, 0xcc, 0x68 } \
   }
 
+#define GRUB_EFI_VENDOR_BOOT_LOADER_INTERFACE_GUID \
+  { 0x4a67b082, 0x0a4c, 0x41cf, \
+    {0xb6, 0xc7, 0x44, 0x0b, 0x29, 0xbb, 0x8c, 0x4f } \
+  }
+
 struct grub_efi_sal_system_table
 {
   grub_uint32_t signature;