From 9e929d3db61462edd91cad8b56472ba510fc132b Mon Sep 17 00:00:00 2001
From: Christian Kellner <christian@kellner.me>
Date: Fri, 3 Jan 2020 13:56:35 +0100
Subject: [PATCH] assembler/qemu: refactor grub2 boot image patching

As noted in earlier commits the grub2 boot image needs to be patched
to contain the position of the grub2 core. By default, the location
in the boot image is hard-coded to be the mbr gap (sector 1) but for
GPT partition schemes a separate BIOS boot partition is used that is
located at a "random" location. Refactor the code to generalize the
boot image patching, where the default mbr gap location is just a
special case of the general.
---
 assemblers/org.osbuild.qemu | 46 ++++++++++++++++++++++---------------
 1 file changed, 28 insertions(+), 18 deletions(-)

diff --git a/assemblers/org.osbuild.qemu b/assemblers/org.osbuild.qemu
index 99e772f0..728a6890 100755
--- a/assemblers/org.osbuild.qemu
+++ b/assemblers/org.osbuild.qemu
@@ -360,15 +360,25 @@ def partition_table_from_options(options) -> PartitionTable:
 
 
 def grub2_write_boot_image(boot_f: BinaryIO,
-                           image_f: BinaryIO):
+                           image_f: BinaryIO,
+                           core_location: int):
     """Write the boot image (grub2 stage 1) to the MBR"""
 
     # The boot.img file is 512 bytes, but we must only copy the first 440
     # bytes, as these contain the bootstrapping code. The rest of the
     # first sector contains the partition table, and must not be
     # overwritten.
+    image_f.seek(0)
     image_f.write(boot_f.read(440))
 
+    # Additionally, write the location (in sectors) of
+    # the grub core image, into the boot image, so the
+    # latter can find the former. To exact location is
+    # taken from grub2's "boot.S":
+    #  GRUB_BOOT_MACHINE_KERNEL_SECTOR 0x5c (= 92)
+    image_f.seek(0x5c)
+    image_f.write(struct.pack("<Q", core_location))
+
 
 def grub2_write_core_mbrgap(core_f: BinaryIO,
                             image_f: BinaryIO,
@@ -384,6 +394,8 @@ def grub2_write_core_mbrgap(core_f: BinaryIO,
     image_f.seek(512)
     shutil.copyfileobj(core_f, image_f)
 
+    return 1  # the location of the core image in sectors
+
 
 def grub2_write_core_prep_part(core_f: BinaryIO,
                                image_f: BinaryIO,
@@ -402,6 +414,8 @@ def grub2_write_core_prep_part(core_f: BinaryIO,
     image_f.seek(prep_part.start_in_bytes)
     shutil.copyfileobj(core_f, image_f)
 
+    return prep_part.start
+
 
 def grub2_write_core_bios_boot(core_f: BinaryIO,
                                image_f: BinaryIO,
@@ -425,13 +439,7 @@ def grub2_write_core_bios_boot(core_f: BinaryIO,
     image_f.seek(bb.start_in_bytes + 500)
     image_f.write(struct.pack("<Q", bb.start + 1))
 
-    # Additionally, write the location (in sectors) of
-    # the grub core image, into the boot image, so the
-    # latter can find the former. To exact location is
-    # taken from grub2's "boot.S"":
-    #  GRUB_BOOT_MACHINE_KERNEL_SECTOR 0x5c (= 92)
-    image_f.seek(0x5c)
-    image_f.write(struct.pack("<Q", bb.start))
+    return bb.start
 
 
 def grub2_partition_id(pt: PartitionTable):
@@ -506,23 +514,25 @@ def install_grub2(image: str, pt: PartitionTable, options):
                    check=True)
 
     with open(image, "rb+") as image_f:
-        if platform == "i386-pc":
-            # On x86, install the level-1 bootloader into the start of the MBR
-            # The purpose of this is simply to jump into the stage-2 bootloader.
-            # On ppc64le & Open Firmware stage-2 is loaded by the firmware
-            with open(boot_path, "rb") as boot_f:
-                grub2_write_boot_image(boot_f, image_f)
-
+        # Write the newly created grub2 core to the image
         with open(core_path, "rb") as core_f:
             if platform == "powerpc-ieee1275":
                 # write the core to the PrEP partition
-                grub2_write_core_prep_part(core_f, image_f, pt)
+                core_loc = grub2_write_core_prep_part(core_f, image_f, pt)
             elif pt.label == "gpt":
                 # gpt requires a bios-boot partition
-                grub2_write_core_bios_boot(core_f, image_f, pt)
+                core_loc = grub2_write_core_bios_boot(core_f, image_f, pt)
             else:
                 # embed the core in the MBR gap
-                grub2_write_core_mbrgap(core_f, image_f, pt)
+                core_loc = grub2_write_core_mbrgap(core_f, image_f, pt)
+
+        # On certain platforms (x86) a level 1 boot loader is required
+        # to load to the core image (on ppc64le & Open Firmware this is
+        # done by the firmware itself)
+        if platform == "i386-pc":
+            # On x86, the boot image just jumps to core image
+            with open(boot_path, "rb") as boot_f:
+                grub2_write_boot_image(boot_f, image_f, core_loc)
 
 
 def main(tree, output_dir, options, loop_client):