[Kimchi-devel] [PATCH] PowerPC bootable ISO detection code
Daniel Barboza
danielhb at linux.vnet.ibm.com
Mon Jul 28 10:27:07 UTC 2014
From: Daniel Henrique Barboza <danielhb at linux.vnet.ibm.com>
PowerPC ISO images does not follow the same rules as Intel's.
Intel ISOs uses ISO9660 extension called 'El Torito', which
describes the format of the Boot Record. PowerPC firmware, on
the other hand, looks for a file '/ppc/bootinfo.txt' which
describes how to boot the image.
This patch creates a function that opens the ISO file without
mounting it and, following ISO9660 standards, read the filesystem
trying to find the boot file that enables PowerPC booting.
Signed-off-by: Daniel Henrique Barboza <danielhb at linux.vnet.ibm.com>
---
src/kimchi/isoinfo.py | 166 +++++++++++++++++++++++++++++++++++++++++++++++++-
1 file changed, 165 insertions(+), 1 deletion(-)
diff --git a/src/kimchi/isoinfo.py b/src/kimchi/isoinfo.py
index c394a32..72b956d 100644
--- a/src/kimchi/isoinfo.py
+++ b/src/kimchi/isoinfo.py
@@ -18,6 +18,7 @@
# Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
import glob
+import platform
import os
import re
import struct
@@ -132,6 +133,12 @@ class IsoImage(object):
EL_TORITO_BOOT_RECORD = struct.Struct("=B5sB32s32sI")
EL_TORITO_VALIDATION_ENTRY = struct.Struct("=BBH24sHBB")
EL_TORITO_BOOT_ENTRY = struct.Struct("=BBHBBHL20x")
+ # Path table info starting in ISO9660 offset 132. We force little
+ # endian byte order (the '<' sign) because Power systems can run on
+ # both.
+ # First int is path table size, next 4 bytes are discarded (it is
+ # the same info but in big endian) and next int is the location.
+ PATH_TABLE_SIZE_LOC = struct.Struct("<I 4s I")
def __init__(self, path):
self.path = path
@@ -224,6 +231,160 @@ class IsoImage(object):
raise IsoFormatError("KCHISO0005E",
{'filename': self.path})
+ def _scan_ppc(self):
+ """
+ PowerPC firmware does not use the conventional El Torito boot
+ specification. Instead, it looks for a file '/ppc/bootinfo.txt'
+ which contains boot information. A PPC image is bootable if
+ this file exists in the filesystem [1].
+
+ To detect if a PPC ISO is bootable, we could simply mount the
+ ISO and search for the boot file as we would with any other
+ file in the filesystem. We can also look for the boot file
+ searching byte by byte the ISO image. This is possible because
+ the PPC ISO image follows the ISO9660 standard [2]. Mounting
+ the ISO requires extra resources and it takes longer than
+ searching the image data, thus we chose the latter approach
+ in this code.
+
+ To locate a file we must access the Path Table, which contains
+ the records of all the directories in the ISO. After locating
+ the directory/subdirectory that contains the file, we access
+ the Directory Record to find it.
+
+
+ .. [1] https://www.ibm.com/developerworks/community/wikis/home?\
+lang=en#!/wiki/W51a7ffcf4dfd_4b40_9d82_446ebc23c550/page/PowerLinux\
+%20Boot%20howto
+ .. [2] http://wiki.osdev.org/ISO_9660
+ """
+
+ # To locate any file we must access the Path Table, which
+ # contains the records of all the directories in the ISO.
+ # ISO9660 dictates that the Path Table location information
+ # is at offset 132, inside the Primary Volume Descriptor,
+ # after the SystemArea (16*SECTOR_SIZE).
+ #
+ # In the Path table info we're forcing little endian byte
+ # order (the '<' sign) because Power systems can run on
+ # both.
+ #
+ # First int is path table size, next 4 bytes are discarded (it is
+ # the same info but in big endian) and next int is the location.
+ PATH_TABLE_LOC_OFFSET = 16 * IsoImage.SECTOR_SIZE + 132
+ PATH_TABLE_SIZE_LOC = struct.Struct("<I 4s I")
+
+ path_table_loc_data = self._get_iso_data(PATH_TABLE_LOC_OFFSET,
+ PATH_TABLE_SIZE_LOC.size)
+ path_size, unused, path_loc = self._unpack(PATH_TABLE_SIZE_LOC,
+ path_table_loc_data)
+ # Fetch the Path Table using location and size found above
+ path_table_offset = path_loc * IsoImage.SECTOR_SIZE
+ path_table_data = self._get_iso_data(path_table_offset, path_size)
+
+ # Loop inside the path table to find the directory 'ppc'.
+ # The contents of the registers are:
+ # - length of the directory identifier (1 byte)
+ # - extended attribute record length (1 byte)
+ # - location of directory register (4 bytes)
+ # - directory number of parent dir (2 bytes)
+ # - directory name (size varies according to length)
+ # - padding field - 1 byte if the length is odd, not present if even
+ DIR_NAMELEN_LOCATION_PARENT = struct.Struct("<B B I H")
+ dir_struct_size = DIR_NAMELEN_LOCATION_PARENT.size
+ i = 0
+ while i < path_size:
+ dir_data = path_table_data[i: i+dir_struct_size]
+ i += dir_struct_size
+ # We won't use the Extended Attribute Record
+ dir_namelen, unused, dir_loc, dir_parent = \
+ self._unpack(DIR_NAMELEN_LOCATION_PARENT, dir_data)
+ if dir_parent == 1:
+ # read the dir name using the namelen
+ dir_name = path_table_data[i: i+dir_namelen].rstrip()
+ if dir_name == 'ppc':
+ # stop searching, dir was found
+ break
+ # Need to consider the optional padding field as well
+ i += dir_namelen + dir_namelen % 2
+
+ if i > path_size:
+ # Didn't find the '/ppc' directory. ISO is not bootable.
+ self.bootable = False
+ return
+
+ # Get the 'ppc' directory record using 'dir_loc'.
+ ppc_dir_offset = dir_loc * IsoImage.SECTOR_SIZE
+
+ # We need to find the sector size of this dir entry. The
+ # size of the File Section is located 10 bytes after
+ # the dir location.
+ DIR_SIZE_FMT = struct.Struct("<10sI")
+ data = self._get_iso_data(ppc_dir_offset, DIR_SIZE_FMT.size)
+ unused, dir_size = self._unpack(DIR_SIZE_FMT, data)
+ # If the dir is in the middle of a sector, the sector is
+ # padded zero and won't be utilized. We need to round up
+ # the result
+ dir_sectorsize = dir_size / IsoImage.SECTOR_SIZE
+ if dir_size % IsoImage.SECTOR_SIZE:
+ dir_sectorsize += 1
+
+ # Fixed-size directory record fields:
+ # - length of directory record (1 byte)
+ # - extended attr. record length (1 byte)
+ # - location of extend in both-endian format (8 bytes)
+ # - data length (size of extend) in both-endian (8 bytes)
+ # - recording date and time (7 bytes)
+ # - file flags (1 byte)
+ # - file unit size interleaved (1 byte)
+ # - interleave gap size (1 byte)
+ # - volume sequence number (4 bytes)
+ # - length of file identifier (1 byte)
+ #
+ # Of all these fields, we will use only 3 of them, 'ignoring'
+ # 30 bytes total.
+ STATIC_DIR_RECORD_FMT = struct.Struct("<B 24s B 6s B")
+ static_rec_size = STATIC_DIR_RECORD_FMT.size
+
+ # Maximum offset possible of all the records of this directory
+ DIR_REC_MAX = ppc_dir_offset + dir_sectorsize*IsoImage.SECTOR_SIZE
+ # Max size of a given directory record
+ MAX_DIR_SIZE = 255
+ # Name of the boot file
+ BOOT_FILE_NAME = "bootinfo.txt"
+
+ # Loop until one of the following happens:
+ # - boot file is found
+ # - end of directory record listing for the 'ppc' dir
+ while ppc_dir_offset < DIR_REC_MAX:
+ record_data = self._get_iso_data(ppc_dir_offset, MAX_DIR_SIZE)
+ dir_rec_len, unused, file_flags, unused2, file_name_len = \
+ self._unpack(STATIC_DIR_RECORD_FMT, record_data)
+
+ # if dir_rec_len = 0, increment offset (skip the
+ # dir_rec_len byte) and continue the loop
+ if dir_rec_len == 0:
+ ppc_dir_offset += 1
+ continue
+
+ # Get filename of the file/dir we're at.
+ filename = record_data[static_rec_size:
+ static_rec_size + file_name_len].rstrip()
+ # The second bit of the file_flags indicate if this record
+ # is a directory.
+ if filename == BOOT_FILE_NAME and (file_flags & 2) != 1:
+ self.bootable = True
+ return
+
+ # Update offset and keep looking. There is a padding here
+ # if the length of the file identifier is EVEN.
+ padding = 0
+ if not file_name_len % 2:
+ padding = 1
+ ppc_dir_offset += dir_rec_len + padding
+ # If reached this point the file wasn't found = not bootable
+ self.bootable = False
+
def _scan_primary_vol(self, data):
"""
Scan one sector for a Primary Volume Descriptor and extract the
@@ -269,7 +430,10 @@ class IsoImage(object):
return
self._scan_primary_vol(data)
- self._scan_el_torito(data)
+ if platform.machine().startswith('ppc'):
+ self._scan_ppc()
+ else:
+ self._scan_el_torito(data)
class Matcher(object):
--
1.8.3.1
More information about the Kimchi-devel
mailing list