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assembler/qemu: refactor grub2 boot image patching

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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.
This commit is contained in:
Christian Kellner 2020-01-03 13:56:35 +01:00 • committed by Lars Karlitski
parent 814cb4eb80
commit 9e929d3db6
1 changed files with 28 additions and 18 deletions
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46
assemblers/org.osbuild.qemu
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@ -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):