wic: Add gpt-hybrid partition layout

Add support for formatting a disk with a hybrid MBR & GPT partition
scheme. In this scheme, the primary partitioning method is GPT, but a
valid MBR header is also written than can point to a subset of the GPT
partitions on the disk (any partitions marked with the `--mbr` flag will
be included in this MBR). The primary purpose of this method is to allow
for SoCs that can only find a bootloader in an MBR partition to use GPT
once the bootloader is running. As an example, older versions of the
Raspberry Pi firmware can only parse MBR partitions to find a kernel (or
other bootloader like u-boot), but once those have booted GPT partitions
can be used.

In addition to the partitions annotated with the `--mbr`, a "protective"
GPT partition of type 0xEE is added, as the existence of such a
partition is the indication to tooling that this a hybrid MBR and that
the GPT partition table should be parsed instead.

Signed-off-by: Joshua Watt <JPEWhacker@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

3 files changed, 69 insertions, 17 deletions

diff --git a/scripts/lib/wic/ksparser.py b/scripts/lib/wic/ksparser.py
index 667b2ff9c3..7ef3dc83dd 100644
--- a/scripts/lib/wic/ksparser.py
+++ b/scripts/lib/wic/ksparser.py
@@ -188,11 +188,12 @@ class KickStart():
         part.add_argument('--uuid')
         part.add_argument('--fsuuid')
         part.add_argument('--no-fstab-update', action='store_true')
+        part.add_argument('--mbr', action='store_true')
 
         bootloader = subparsers.add_parser('bootloader')
         bootloader.add_argument('--append')
         bootloader.add_argument('--configfile')
-        bootloader.add_argument('--ptable', choices=('msdos', 'gpt'),
+        bootloader.add_argument('--ptable', choices=('msdos', 'gpt', 'gpt-hybrid'),
                                 default='msdos')
         bootloader.add_argument('--timeout', type=int)
         bootloader.add_argument('--source')
diff --git a/scripts/lib/wic/partition.py b/scripts/lib/wic/partition.py
index f11c393df5..b1a2306dd1 100644
--- a/scripts/lib/wic/partition.py
+++ b/scripts/lib/wic/partition.py
@@ -60,6 +60,7 @@ class Partition():
         self.has_fstab = False
         self.update_fstab_in_rootfs = False
         self.hidden = args.hidden
+        self.mbr = args.mbr
 
         self.lineno = lineno
         self.source_file = ""
diff --git a/scripts/lib/wic/plugins/imager/direct.py b/scripts/lib/wic/plugins/imager/direct.py
index 55347f5480..9b619e41c1 100644
--- a/scripts/lib/wic/plugins/imager/direct.py
+++ b/scripts/lib/wic/plugins/imager/direct.py
@@ -342,7 +342,7 @@ class PartitionedImage():
         # generate parition and filesystem UUIDs
         for part in self.partitions:
             if not part.uuid and part.use_uuid:
-                if self.ptable_format == 'gpt':
+                if self.ptable_format in ('gpt', 'gpt-hybrid'):
                     part.uuid = str(uuid.uuid4())
                 else: # msdos partition table
                     part.uuid = '%08x-%02d' % (self.identifier, part.realnum)
@@ -398,6 +398,10 @@ class PartitionedImage():
                 raise WicError("setting custom partition type is not " \
                                "implemented for msdos partitions")
 
+            if part.mbr and self.ptable_format != 'gpt-hybrid':
+                raise WicError("Partition may only be included in MBR with " \
+                               "a gpt-hybrid partition table")
+
             # Get the disk where the partition is located
             self.numpart += 1
             if not part.no_table:
@@ -406,7 +410,7 @@ class PartitionedImage():
             if self.numpart == 1:
                 if self.ptable_format == "msdos":
                     overhead = MBR_OVERHEAD
-                elif self.ptable_format == "gpt":
+                elif self.ptable_format in ("gpt", "gpt-hybrid"):
                     overhead = GPT_OVERHEAD
 
                 # Skip one sector required for the partitioning scheme overhead
@@ -490,7 +494,7 @@ class PartitionedImage():
         # Once all the partitions have been layed out, we can calculate the
         # minumim disk size
         self.min_size = self.offset
-        if self.ptable_format == "gpt":
+        if self.ptable_format in ("gpt", "gpt-hybrid"):
             self.min_size += GPT_OVERHEAD
 
         self.min_size *= self.sector_size
@@ -511,22 +515,38 @@ class PartitionedImage():
 
         return exec_native_cmd(cmd, self.native_sysroot)
 
+    def _write_identifier(self, device, identifier):
+        logger.debug("Set disk identifier %x", identifier)
+        with open(device, 'r+b') as img:
+            img.seek(0x1B8)
+            img.write(identifier.to_bytes(4, 'little'))
+
+    def _make_disk(self, device, ptable_format, min_size):
+        logger.debug("Creating sparse file %s", device)
+        with open(device, 'w') as sparse:
+            os.ftruncate(sparse.fileno(), min_size)
+
+        logger.debug("Initializing partition table for %s", device)
+        exec_native_cmd("parted -s %s mklabel %s" % (device, ptable_format),
+                        self.native_sysroot)
+
+
     def create(self):
-        logger.debug("Creating sparse file %s", self.path)
-        with open(self.path, 'w') as sparse:
-            os.ftruncate(sparse.fileno(), self.min_size)
+        self._make_disk(self.path,
+                        "gpt" if self.ptable_format == "gpt-hybrid" else self.ptable_format,
+                        self.min_size)
 
-        logger.debug("Initializing partition table for %s", self.path)
-        exec_native_cmd("parted -s %s mklabel %s" %
-                        (self.path, self.ptable_format), self.native_sysroot)
+        self._write_identifier(self.path, self.identifier)
 
-        logger.debug("Set disk identifier %x", self.identifier)
-        with open(self.path, 'r+b') as img:
-            img.seek(0x1B8)
-            img.write(self.identifier.to_bytes(4, 'little'))
+        if self.ptable_format == "gpt-hybrid":
+            mbr_path = self.path + ".mbr"
+            self._make_disk(mbr_path, "msdos", self.min_size)
+            self._write_identifier(mbr_path, self.identifier)
 
         logger.debug("Creating partitions")
 
+        hybrid_mbr_part_num = 0
+
         for part in self.partitions:
             if part.num == 0:
                 continue
@@ -571,7 +591,14 @@ class PartitionedImage():
             self._create_partition(self.path, part.type,
                                    parted_fs_type, part.start, part.size_sec)
 
-            if self.ptable_format == "gpt" and (part.part_name or part.label):
+            if self.ptable_format == "gpt-hybrid" and part.mbr:
+                hybrid_mbr_part_num += 1
+                if hybrid_mbr_part_num > 4:
+                    raise WicError("Extended MBR partitions are not supported in hybrid MBR")
+                self._create_partition(mbr_path, "primary",
+                                       parted_fs_type, part.start, part.size_sec)
+
+            if self.ptable_format in ("gpt", "gpt-hybrid") and (part.part_name or part.label):
                 partition_label = part.part_name if part.part_name else part.label
                 logger.debug("partition %d: set name to %s",
                              part.num, partition_label)
@@ -586,7 +613,7 @@ class PartitionedImage():
                                          (part.num, part.part_type,
                                           self.path), self.native_sysroot)
 
-            if part.uuid and self.ptable_format == "gpt":
+            if part.uuid and self.ptable_format in ("gpt", "gpt-hybrid"):
                 logger.debug("partition %d: set UUID to %s",
                              part.num, part.uuid)
                 exec_native_cmd("sgdisk --partition-guid=%d:%s %s" % \
@@ -594,12 +621,16 @@ class PartitionedImage():
                                 self.native_sysroot)
 
             if part.active:
-                flag_name = "legacy_boot" if self.ptable_format == 'gpt' else "boot"
+                flag_name = "legacy_boot" if self.ptable_format in ('gpt', 'gpt-hybrid') else "boot"
                 logger.debug("Set '%s' flag for partition '%s' on disk '%s'",
                              flag_name, part.num, self.path)
                 exec_native_cmd("parted -s %s set %d %s on" % \
                                 (self.path, part.num, flag_name),
                                 self.native_sysroot)
+                if self.ptable_format == 'gpt-hybrid' and part.mbr:
+                    exec_native_cmd("parted -s %s set %d %s on" % \
+                                    (mbr_path, hybrid_mbr_part_num, "boot"),
+                                    self.native_sysroot)
             if part.system_id:
                 exec_native_cmd("sfdisk --part-type %s %s %s" % \
                                 (self.path, part.num, part.system_id),
@@ -612,6 +643,25 @@ class PartitionedImage():
                                 (self.path, part.num),
                                 self.native_sysroot)
 
+        if self.ptable_format == "gpt-hybrid":
+            # Write a protective GPT partition
+            hybrid_mbr_part_num += 1
+            if hybrid_mbr_part_num > 4:
+                raise WicError("Extended MBR partitions are not supported in hybrid MBR")
+
+            # parted cannot directly create a protective GPT partition, so
+            # create with an arbitrary type, then change it to the correct type
+            # with sfdisk
+            self._create_partition(mbr_path, "primary", "fat32", 1, GPT_OVERHEAD)
+            exec_native_cmd("sfdisk --part-type %s %d 0xee" % (mbr_path, hybrid_mbr_part_num),
+                            self.native_sysroot)
+
+            # Copy hybrid MBR
+            with open(mbr_path, "rb") as mbr_file:
+                with open(self.path, "r+b") as image_file:
+                    mbr = mbr_file.read(512)
+                    image_file.write(mbr)
+
     def cleanup(self):
         pass