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:
  The exact, C define for a valid AID. Note custom AIDs can be defined in the
  AID section documented below.

group:
  The exact, C define for a valid AID. Note custom AIDs can be defined in the
  AID section documented below.

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]. Per the ini
specifications enforced by Pythons ConfigParser.


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 be any valid character for a #define identifier in C.

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>]. Per the ini
specifications enforced by Pythons ConfigParser. 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.