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assembler/qemu: clarify module usage for grub2

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Explain the concept and reason behind the grub2 core as well as the
details behind the selection of the core  modules that get included.
Also elaborate a bit on the MBR gap. For more details about this see
https://en.wikipedia.org/wiki/GNU_GRUB#Version_2_(GRUB_2)
NB: This commit also changes the order of the grub modules, which in
turn changes the layout of the core.img and thus the hash value used
in the test; adapt those value to reflect the changed core.img.
This commit is contained in:
Christian Kellner 2019-12-19 13:13:03 +01:00 • committed by Tom Gundersen
parent 828b568734
commit c57da5722f
2 changed files with 26 additions and 14 deletions
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38
assemblers/org.osbuild.qemu
View file

@ -332,33 +332,40 @@ def install_grub2(image: str, pt: PartitionTable):
"""Install grub2 to image"""
grub2_core = "/var/tmp/grub2-core.img"
# Create the level-2 & 3 stages of the bootloader, aka the core
# it consists of the kernel plus the core modules required to
# to locate and load the rest of the grub modules, specifically
# the "normal.mod" (Stage 4) module.
# The exact list of modules required to be built into the core
# depends on the system: it is the minimal set needed to find
# read the partition and its filesystem containing said modules
# and the grub configuration [NB: efi systems work differently]
# modules: access the disk and read the partition table
modules = ["biosdisk", "part_msdos"]
# modules: grubs needs to access the filesystems
root_part = pt.partition_containing_root()
root_fs_type = root_part.fs_type or "unknown"
if root_fs_type == "ext4":
fs_module = "ext2"
modules += ["ext2"]
elif root_fs_type == "xfs":
fs_module = "xfs"
modules += ["xfs"]
else:
raise ValueError(f"unknown root filesystem type: '{root_fs_type}'")
# Create the level-2 bootloader
# The purpose of this is to find the grub modules and configuration
# to be able to start the level-3 bootloader. It contains the modules
# necessary to do this, but nothing else.
# now created the core image
subprocess.run(["grub2-mkimage",
"--verbose",
"--directory", "/usr/lib/grub/i386-pc",
"--prefix", "(,msdos1)/boot/grub2",
"--format", "i386-pc",
"--compression", "auto",
"--output", grub2_core,
"part_msdos", fs_module, "biosdisk"],
"--output", grub2_core] +
modules,
check=True)
partition_offset = pt[0].start_in_bytes
assert os.path.getsize(grub2_core) < partition_offset - 512
with open(image, "rb+") as image_f:
# Install the level-1 bootloader into the start of the MBR
# The purpose of this is simply to jump into the level-2 bootloader.
@ -369,8 +376,13 @@ def install_grub2(image: str, pt: PartitionTable):
# overwritten.
image_f.write(boot_f.read(440))
# Install the level-2 bootloader into the space after the MBR, before
# the first partition.
# Embed Stage-2 in the MBR gap
# For historic and performance reasons the first partition
# is aligned to a specific sector number (used to be 64,
# now it is 2048), which leaves a gap between it and the MBR,
# where the core image can be embedded in; also check it fits
partition_offset = pt[0].start_in_bytes
assert os.path.getsize(grub2_core) < partition_offset - 512
with open(grub2_core, "rb") as core_f:
image_f.seek(512)
shutil.copyfileobj(core_f, image_f)

2
test/test_assemblers.py
View file

@ -98,7 +98,7 @@ class TestAssemblers(osbuildtest.TestCase):
self.assertPartitionTable(device, "dos", options["ptuuid"], 1, boot_partition=1)
self.assertGRUB2(device,
"26e3327c6b5ac9b5e21d8b86f19ff7cb4d12fb2d0406713f936997d9d89de3ee",
"1ebb35399388ba4007cee817a066570db948c450da911a3db67dba8083be247d",
"18031c9465e3f9ccb9aeb9c8e59dec6b82e91376e2628c8100b5461af62ad67c",
1024 * 1024)
self.assertFilesystem(device + "p1", options["root_fs_uuid"], "ext4", tree_id)