Policy Generation:

Additional, per device, policy files can be added into the
policy build.

They can be configured through the use of three variables,
they are:
1. BOARD_SEPOLICY_REPLACE
2. BOARD_SEPOLICY_UNION
3. BOARD_SEPOLICY_DIRS

The variables should be set in the BoardConfig.mk file in
the device or vendor directories.

BOARD_SEPOLICY_UNION is a list of files that will be
"unioned", IE concatenated, at the END of their respective
file in external/sepolicy. Note, to add a unique file you
would use this variable.

BOARD_SEPOLICY_REPLACE is a list of files that will be
used instead of the corresponding file in external/sepolicy.

BOARD_SEPOLICY_DIRS contains a list of directories to search
for BOARD_SEPOLICY_UNION and BOARD_SEPOLICY_REPLACE files. Order
matters in this list.
eg.) If you have BOARD_SEPOLICY_UNION := widget.te and have 2
instances of widget.te files on BOARD_SEPOLICY_DIRS search path.
The first one found (at the first search dir containing the file)
gets processed first.
Reviewing out/target/product/<device>/etc/sepolicy_intermediates/policy.conf
will help sort out ordering issues.

It is an error to specify a BOARD_POLICY_REPLACE file that does
not exist in external/sepolicy.

It is an error to specify a BOARD_POLICY_REPLACE file that appears
multiple times on the policy search path defined by BOARD_SEPOLICY_DIRS.
eg.) if you specify shell.te in BOARD_SEPOLICY_REPLACE and
BOARD_SEPOLICY_DIRS is set to
"vendor/widget/common/sepolicy device/widget/x/sepolicy" and shell.te
appears in both locations, it is an error.

It is an error to specify the same file name in both
BOARD_POLICY_REPLACE and BOARD_POLICY_UNION.

It is an error to specify a BOARD_SEPOLICY_DIRS that has no entries when
specifying BOARD_SEPOLICY_REPLACE.

Example Usage:
From the Tuna device BoardConfig.mk, device/samsung/tuna/BoardConfig.mk

BOARD_SEPOLICY_DIRS := \
        device/samsung/tuna/sepolicy

BOARD_SEPOLICY_UNION := \
        genfs_contexts \
        file_contexts \
        sepolicy.te

SPECIFIC POLICY FILE INFORMATION

mac_permissions.xml:
  ABOUT:
    The mac_permissions.xml file is used for controlling the mmac solutions
    as well as mapping a public base16 signing key with an arbitrary seinfo
    string. Details of the files contents can be found in a comment at the
    top of that file. The seinfo string, previously mentioned, is the same string
    that is referenced in seapp_contexts.

    This file can be replaced through BOARD_SEPOLICY_REPLACE containing the
    value "mac_permissions.xml", however, appending (UNION) does NOT exist
    and will cause a build time failure. It is important to note the final
    processed version of this file is stripped of comments and whitespace.
    This is to preserve space on the system.img. If one wishes to view it in
    a more human friendly format, the "tidy" or "xmllint" command will assist
    you.

  TOOLING:
    insertkeys.py
      Is a helper script for mapping arbitrary tags in the signature stanzas of
      mac_permissions.xml to public keys found in pem files. This script takes
      a mac_permissions.xml file and configuration file in order to operate.
      Details of the configuration file (keys.conf) can be found in the subsection
      keys.conf. This script is also responsible for stipping the comments and
      whitespace from the xml file.

      keys.conf
        The keys.conf file is used for controlling the mapping of "tags" found in
        the mac_permissions.xml signature stanzas with actual public keys found in
        pem files. The configuration file can be used in BOARD_SEPOLICY_UNION and
        BOARD_SEPOLICY_REPLACE variables and is processed via m4.

        The script allows for mapping any string contained in TARGET_BUILD_VARIANT
        with specific path to a pem file. Typically TARGET_BUILD_VARIANT is either
        user, eng or userdebug. Additionally, one can specify "ALL" to map a path to
        any string specified in TARGET_BUILD_VARIANT. All tags are matched verbatim
        and all options are matched lowercase. The options are "tolowered" automatically
        for the user, it is convention to specify tags and options in all uppercase
        and tags start with @.

        NOTE: The pem files are base64 encoded and PackageManagerService, mac_permissions.xml
              and setool all use base16 encodings.