platform_build/tools/fs_config/README

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Generating the android_filesystem_config.h:

To generate the android_filesystem_config.h file, one can choose from
one of two methods. The first method, is to declare
TARGET_ANDROID_FILESYSTEM_CONFIG_H in the device BoardConfig.mk file. This
variable can only have one item in it, and it is used directly as the
android_filesystem_config.h header when building
fs_config_generate_$(TARGET_DEVICE) which is used to generate fs_config_files
and fs_config_dirs target executable.

The limitation with this, is that it can only be set once, thus if the device
has a make hierarchy, then each device needs its own file, and cannot share
from a common source or that common source needs to include everything from
both devices.

The other way is to set TARGET_FS_CONFIG_GEN, which can be a list of
intermediate fs configuration files. It is a build error on any one
these conditions:
 * Specify TARGET_FS_CONFIG_GEN and TARGET_ANDROID_FILESYSTEM_CONFIG_H
 * Specify TARGET_FS_CONFIG_GEN and provide
   $(TARGET_DEVICE_DIR)/android_filesystem_config.h

The parsing of the config file follows the Python ConfigParser specification,
with the sections and fields as defined below. There are two types of sections,
both sections require all options to be specified. The first section type is
the "caps" section.

The "caps" section follows the following syntax:

[path]
mode: Octal file mode
user: AID_<user>
group: AID_<group>
caps: cap*

Where:

[path]
  The filesystem path to configure. A path ending in / is considered a dir,
  else its a file.

mode:
  A valid octal file mode of at least 3 digits. If 3 is specified, it is
  prefixed with a 0, else mode is used as is.

user:
  Either the C define for a valid AID or the friendly name. For instance both
  AID_RADIO and radio are acceptable. Note custom AIDs can be defined in the
  AID section documented below.

group:
  Same as user.

caps:
  The name as declared in
  system/core/include/private/android_filesystem_capability.h without the
  leading CAP_. Mixed case is allowed. Caps can also be the raw:
   * binary (0b0101)
   * octal (0455)
   * int (42)
   * hex (0xFF)
  For multiple caps, just separate by whitespace.

It is an error to specify multiple sections with the same [path] in different
files. Note that the same file may contain sections that override the previous
section in Python versions <= 3.2. In Python 3.2 it's set to strict mode.


The next section type is the "AID" section, for specifying OEM specific AIDS.

The AID section follows the following syntax:

[AID_<name>]
value: <number>

Where:

[AID_<name>]
  The <name> can contain characters in the set uppercase, numbers
  and underscores.

value:
  A valid C style number string. Hex, octal, binary and decimal are supported.
  See "caps" above for more details on number formatting.

It is an error to specify multiple sections with the same [AID_<name>]. With
the same constraints as [path] described above. It is also an error to specify
multiple sections with the same value option. It is also an error to specify a
value that is outside of the inclusive OEM ranges:
 * AID_OEM_RESERVED_START(2900) - AID_OEM_RESERVED_END(2999)
 * AID_OEM_RESERVED_2_START(5000) - AID_OEM_RESERVED_2_END(5999)

as defined by system/core/include/private/android_filesystem_config.h.

Ordering within the TARGET_FS_CONFIG_GEN files is not relevant. The paths for files are sorted
like so within their respective array definition:
 * specified path before prefix match
 ** ie foo before f*
 * lexicographical less than before other
 ** ie boo before foo

Given these paths:

paths=['ac', 'a', 'acd', 'an', 'a*', 'aa', 'ac*']

The sort order would be:
paths=['a', 'aa', 'ac', 'acd', 'an', 'ac*', 'a*']

Thus the fs_config tools will match on specified paths before attempting prefix, and match on the
longest matching prefix.

The declared AIDS are sorted in ascending numerical order based on the option "value". The string
representation of value is preserved. Both choices were made for maximum readability of the generated
file and to line up files. Sync lines are placed with the source file as comments in the generated
header file.

For OEMs wishing to use the define AIDs in their native code, one can access the generated header
file like so:
  1. In your C code just #include "generated_oem_aid.h" and start using the declared identifiers.
  2. In your Makefile add this static library like so: LOCAL_HEADER_LIBRARIES := oemaids_headers

Unit Tests:

From within the fs_config directory, unit tests can be executed like so:
$ python -m unittest test_fs_config_generator.Tests
.............
----------------------------------------------------------------------
Ran 13 tests in 0.004s

OK

One could also use nose if they would like:
$ nose2

To add new tests, simply add a test_<xxx> method to the test class. It will automatically
get picked up and added to the test suite.

Using the android_filesystem_config.h:

The tool fs_config_generate is built as a dependency to fs_config_dirs and
fs_config_files host targets, and #includes the above supplied or generated
android_filesystem_config.h file, and can be instructed to generate the binary
data that lands in the device target locations /system/etc/fs_config_dirs and
/system/etc/fs_config_files and in the host's ${OUT} locations
${OUT}/target/product/<device>/system/etc/fs_config_dirs and
${OUT}/target/product/<device>/system/etc/fs_config_files. The binary files
are interpreted by the libcutils fs_conf() function, along with the built-in
defaults, to serve as overrides to complete the results. The Target files are
used by filesystem and adb tools to ensure that the file and directory
properties are preserved during runtime operations. The host files in the
${OUT} directory are used in the final stages when building the filesystem
images to set the file and directory properties.

For systems with separate partition images, such as vendor or oem,
fs_config_generate can be instructed to filter the specific file references
to land in each partition's etc/fs_config_dirs or etc/fs_config_files
locations. The filter can be instructed to blacklist a partition's data by
providing the comma separated minus sign prefixed partition names. The filter
can be instructed to whitelist partition data by providing the partition name.

For example:
- For system.img, but not vendor, oem or odm file references:
      -P -vendor,-oem,-odm
  This makes sure the results only contain content associated with the
  system, and not vendor, oem or odm, blacklisting their content.
- For vendor.img file references: -P vendor
- For oem.img file references: -P oem
- For odm.img file references: -P odm

fs_config_generate --help reports:

Generate binary content for fs_config_dirs (-D) and fs_config_files (-F)
from device-specific android_filesystem_config.h override. Filter based
on a comma separated partition list (-P) whitelist or prefixed by a
minus blacklist. Partitions are identified as path references to
<partition>/ or system/<partition>

Usage: fs_config_generate -D|-F [-P list] [-o output-file]