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# Both Smack and IMA/EVM rely on xattrs. Inheriting this class ensures
# that these xattrs get preserved in tar and jffs2 images.
#
# It also fixes the rootfs so that the content of directories with
# SMACK::TRANSMUTE is correctly labelled. This is because pseudo does
# not know the special semantic of SMACK::TRANSMUTE and omits the
# updating of the Smack label when creating entries inside such a directory,
# for example /etc (see base-files_%.bbappend). Without the fixup,
# files already installed during the image creation would have different (and
# wrong) Smack labels.
# xattr support is expected to be compiled into mtd-utils. We just need to
# use it.
EXTRA_IMAGECMD_jffs2_append = " --with-xattr"
# By default, OE-core uses tar from the host, which may or may not have the
# --xattrs parameter which was introduced in 1.27. For image building we
# use a recent enough tar instead.
#
# The GNU documentation does not specify whether --xattrs-include is necessary.
# In practice, it turned out to be not needed when creating archives and
# required when extracting, but it seems prudent to use it in both cases.
IMAGE_DEPENDS_tar_append = " tar-replacement-native"
EXTRANATIVEPATH += "tar-native"
IMAGE_CMD_TAR = "tar --xattrs --xattrs-include=*"
xattr_images_fix_transmute[dirs] = "${IMAGE_ROOTFS}"
python xattr_images_fix_transmute () {
# The recursive updating of the Smack label ensures that each entry
# has the label set for its parent directories if one of those was
# marked as transmuting.
#
# In addition, "_" is set explicitly on everything that would not
# have a label otherwise. This is a workaround for tools like swupd
# which transfers files from a rootfs onto a target device where Smack
# is active: on the target, each file gets assigned a label, typically
# the one from the process which creates it. swupd (or rather, the tools
# it is currently built on) knows how to set security.SMACK64="_" when
# it is set on the original files, but it does not know that it needs
# to remove that xattr when not set.
import os
import errno
if getattr(os, 'getxattr', None):
# Python 3: os has xattr support.
def lgetxattr(f, attr):
try:
value = os.getxattr(f, attr, follow_symlinks=False)
return value.decode('utf8')
except OSError as ex:
if ex.errno == errno.ENODATA:
return None
def lsetxattr(f, attr, value):
os.setxattr(f, attr.encode('utf8'), value.encode('utf8'), follow_symlinks=False)
else:
# Python 2: xattr support only in xattr module.
#
# Cannot use the 'xattr' module, it is not part of a standard Python
# installation. Instead re-implement using ctypes. Only has to be good
# enough for xattrs that are strings. Always operates on the symlinks themselves,
# not what they point to.
import ctypes
# We cannot look up the xattr functions inside libc. That bypasses
# pseudo, which overrides these functions via LD_PRELOAD. Instead we have to
# find the function address and then create a ctypes function from it.
libdl = ctypes.CDLL("libdl.so.2")
_dlsym = libdl.dlsym
_dlsym.restype = ctypes.c_void_p
RTLD_DEFAULT = ctypes.c_void_p(0)
_lgetxattr = ctypes.CFUNCTYPE(ctypes.c_ssize_t, ctypes.c_char_p, ctypes.c_char_p, ctypes.c_void_p, ctypes.c_size_t,
use_errno=True)(_dlsym(RTLD_DEFAULT, 'lgetxattr'))
_lsetxattr = ctypes.CFUNCTYPE(ctypes.c_int, ctypes.c_char_p, ctypes.c_char_p, ctypes.c_void_p, ctypes.c_size_t, ctypes.c_int,
use_errno=True)(_dlsym(RTLD_DEFAULT, 'lsetxattr'))
def lgetxattr(f, attr):
len = 32
while True:
buffer = ctypes.create_string_buffer('\000' * len)
res = _lgetxattr(f, attr, buffer, ctypes.c_size_t(len))
if res >= 0:
return buffer.value
else:
error = ctypes.get_errno()
if ctypes.get_errno() == errno.ERANGE:
len *= 2
elif error == errno.ENODATA:
return None
else:
raise IOError(error, 'lgetxattr(%s, %s): %d = %s = %s' %
(f, attr, error, errno.errorcode[error], os.strerror(error)))
def lsetxattr(f, attr, value):
res = _lsetxattr(f, attr, value, ctypes.c_size_t(len(value)), ctypes.c_int(0))
if res != 0:
error = ctypes.get_errno()
raise IOError(error, 'lsetxattr(%s, %s, %s): %d = %s = %s' %
(f, attr, value, error, errno.errorcode[error], os.strerror(error)))
def visit(path, deflabel, deftransmute):
isrealdir = os.path.isdir(path) and not os.path.islink(path)
curlabel = lgetxattr(path, 'security.SMACK64')
transmute = lgetxattr(path, 'security.SMACK64TRANSMUTE') == 'TRUE'
if not curlabel:
# Since swupd doesn't remove the label from an updated file assigned by
# the target device's kernel upon unpacking the file from an update,
# we have to set the floor label explicitly even though it is the default label
# and thus adding it would create additional overhead. Otherwise this
# would result in hash mismatches reported by `swupd verify`.
lsetxattr(path, 'security.SMACK64', deflabel)
if not transmute and deftransmute and isrealdir:
lsetxattr(path, 'security.SMACK64TRANSMUTE', 'TRUE')
# Identify transmuting directories and change the default Smack
# label inside them. In addition, directories themselves must become
# transmuting.
if isrealdir:
if transmute:
deflabel = lgetxattr(path, 'security.SMACK64')
deftransmute = True
if deflabel is None:
raise RuntimeError('%s: transmuting directory without Smack label' % path)
elif curlabel:
# Directory with explicit label set and not transmuting => do not
# change the content unless we run into another transmuting directory.
deflabel = '_'
deftransmute = False
for entry in os.listdir(path):
visit(os.path.join(path, entry), deflabel, deftransmute)
visit('.', '_', False)
}
# Same logic as in ima-evm-rootfs.bbclass: try to run as late as possible.
IMAGE_PREPROCESS_COMMAND_append_with-lsm-smack = " xattr_images_fix_transmute ; "
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