platform_build/tools/fs_config

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The fs_config_generator.py tool uses the platform android_filesystem_config.h and the
TARGET_FS_CONFIG_GEN files to generate the fs_config_dirs and fs_config_files files for each
partition, as well as passwd and group files, and the generated_oem_aid.h header.

The fs_config_dirs and fs_config_files binary files are interpreted by the libcutils fs_config()
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.

See ./fs_config_generator.py fsconfig --help for how these files are generated.

The passwd and group files are formatted as documented in man pages passwd(5) and group(5) and used
by bionic for implementing getpwnam() and related functions.

See ./fs_config_generator.py passwd --help and ./fs_config_generator.py group --help for how these
files are generated.

The generated_oem_aid.h creates identifiers for non-platform AIDs for developers wishing to use them
in their native code.  To do so, include the oemaids_headers header library in the corresponding
makefile and #include "generated_oem_aid.h" in the code wishing to use these identifiers.

See ./fs_config_generator.py oemaid --help for how this file is generated.

The parsing of the TARGET_FS_CONFIG_GEN files 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.

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.