arm-trusted-firmware/docs/coding-guidelines.rst

Trusted Firmware-A Coding Guidelines
====================================

.. section-numbering::
    :suffix: .

.. contents::

The following sections contain TF coding guidelines. They are continually
evolving and should not be considered "set in stone". Feel free to question them
and provide feedback.

Some of the guidelines may also apply to other codebases.

**Note:** the existing TF codebase does not necessarily comply with all the
below guidelines but the intent is for it to do so eventually.

Checkpatch overrides
--------------------

Some checkpatch warnings in the TF codebase are deliberately ignored. These
include:

- ``**WARNING: line over 80 characters**``: Although the codebase should
  generally conform to the 80 character limit this is overly restrictive in some
  cases.

- ``**WARNING: Use of volatile is usually wrong``: see
  `Why the “volatile” type class should not be used`_ . Although this document
  contains some very useful information, there are several legimate uses of the
  volatile keyword within the TF codebase.

Headers and inclusion
---------------------

Header guards
^^^^^^^^^^^^^

For a header file called "some_driver.h" the style used by the Trusted Firmware
is:

.. code:: c

  #ifndef SOME_DRIVER_H
  #define SOME_DRIVER_H

  <header content>

  #endif /* SOME_DRIVER_H */

Include statement ordering
^^^^^^^^^^^^^^^^^^^^^^^^^^

All header files that are included by a source file must use the following,
grouped ordering. This is to improve readability (by making it easier to quickly
read through the list of headers) and maintainability.

#. *System* includes: Header files from the standard *C* library, such as
   ``stddef.h`` and ``string.h``.

#. *Project* includes: Header files under the ``include/`` directory within TF
   are *project* includes.

#. *Platform* includes: Header files relating to a single, specific platform,
   and which are located under the ``plat/<platform_name>`` directory within TF,
   are *platform* includes.

Within each group, ``#include`` statements must be in alphabetical order,
taking both the file and directory names into account.

Groups must be separated by a single blank line for clarity.

The example below illustrates the ordering rules using some contrived header
file names; this type of name reuse should be otherwise avoided.

.. code:: c

  #include <string.h>

  #include <a_dir/example/a_header.h>
  #include <a_dir/example/b_header.h>
  #include <a_dir/test/a_header.h>
  #include <b_dir/example/a_header.h>

  #include "./a_header.h"

Include statement variants
^^^^^^^^^^^^^^^^^^^^^^^^^^

Two variants of the ``#include`` directive are acceptable in the TF codebase.
Correct use of the two styles improves readability by suggesting the location
of the included header and reducing ambiguity in cases where generic and
platform-specific headers share a name.

For header files that are in the same directory as the source file that is
including them, use the ``"..."`` variant.

For header files that are **not** in the same directory as the source file that
is including them, use the ``<...>`` variant.

Example (bl1_fwu.c):

.. code:: c

  #include <assert.h>
  #include <errno.h>
  #include <string.h>

  #include "bl1_private.h"

Platform include paths
^^^^^^^^^^^^^^^^^^^^^^

Platforms are allowed to add more include paths to be passed to the compiler.
The ``PLAT_INCLUDES`` variable is used for this purpose. This is needed in
particular for the file ``platform_def.h``.

Example:

.. code:: c

  PLAT_INCLUDES  += -Iinclude/plat/myplat/include

Types and typedefs
------------------

Use of built-in *C* and *libc* data types
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

The TF codebase should be kept as portable as possible, especially since both
64-bit and 32-bit platforms are supported. To help with this, the following data
type usage guidelines should be followed:

- Where possible, use the built-in *C* data types for variable storage (for
  example, ``char``, ``int``, ``long long``, etc) instead of the standard *C99*
  types. Most code is typically only concerned with the minimum size of the
  data stored, which the built-in *C* types guarantee.

- Avoid using the exact-size standard *C99* types in general (for example,
  ``uint16_t``, ``uint32_t``, ``uint64_t``, etc) since they can prevent the
  compiler from making optimizations. There are legitimate uses for them,
  for example to represent data of a known structure. When using them in struct
  definitions, consider how padding in the struct will work across architectures.
  For example, extra padding may be introduced in AArch32 systems if a struct
  member crosses a 32-bit boundary.

- Use ``int`` as the default integer type - it's likely to be the fastest on all
  systems. Also this can be assumed to be 32-bit as a consequence of the
  `Procedure Call Standard for the Arm Architecture`_ and the `Procedure Call
  Standard for the Arm 64-bit Architecture`_ .

- Avoid use of ``short`` as this may end up being slower than ``int`` in some
  systems. If a variable must be exactly 16-bit, use ``int16_t`` or
  ``uint16_t``.

- Avoid use of ``long``. This is guaranteed to be at least 32-bit but, given
  that `int` is 32-bit on Arm platforms, there is no use for it. For integers of
  at least 64-bit, use ``long long``.

- Use ``char`` for storing text. Use ``uint8_t`` for storing other 8-bit data.

- Use ``unsigned`` for integers that can never be negative (counts,
  indices, sizes, etc). TF intends to comply with MISRA "essential type" coding
  rules (10.X), where signed and unsigned types are considered different
  essential types. Choosing the correct type will aid this. MISRA static
  analysers will pick up any implicit signed/unsigned conversions that may lead
  to unexpected behaviour.

- For pointer types:

  - If an argument in a function declaration is pointing to a known type then
    simply use a pointer to that type (for example: ``struct my_struct *``).

  - If a variable (including an argument in a function declaration) is pointing
    to a general, memory-mapped address, an array of pointers or another
    structure that is likely to require pointer arithmetic then use
    ``uintptr_t``. This will reduce the amount of casting required in the code.
    Avoid using ``unsigned long`` or ``unsigned long long`` for this purpose; it
    may work but is less portable.

  - For other pointer arguments in a function declaration, use ``void *``. This
    includes pointers to types that are abstracted away from the known API and
    pointers to arbitrary data. This allows the calling function to pass a
    pointer argument to the function without any explicit casting (the cast to
    ``void *`` is implicit). The function implementation can then do the
    appropriate casting to a specific type.

  - Use ``ptrdiff_t`` to compare the difference between 2 pointers.

- Use ``size_t`` when storing the ``sizeof()`` something.

- Use ``ssize_t`` when returning the ``sizeof()`` something from a function that
  can also return an error code; the signed type allows for a negative return
  code in case of error. This practice should be used sparingly.

- Use ``u_register_t`` when it's important to store the contents of a register
  in its native size (32-bit in AArch32 and 64-bit in AArch64). This is not a
  standard *C99* type but is widely available in libc implementations,
  including the FreeBSD version included with the TF codebase. Where possible,
  cast the variable to a more appropriate type before interpreting the data. For
  example, the following struct in ``ep_info.h`` could use this type to minimize
  the storage required for the set of registers:

.. code:: c

    typedef struct aapcs64_params {
            u_register_t arg0;
            u_register_t arg1;
            u_register_t arg2;
            u_register_t arg3;
            u_register_t arg4;
            u_register_t arg5;
            u_register_t arg6;
            u_register_t arg7;
    } aapcs64_params_t;

If some code wants to operate on ``arg0`` and knows that it represents a 32-bit
unsigned integer on all systems, cast it to ``unsigned int``.

These guidelines should be updated if additional types are needed.

Avoid anonymous typedefs of structs/enums in headers
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

For example, the following definition:

.. code:: c

  typedef struct {
          int arg1;
          int arg2;
  } my_struct_t;


is better written as:

.. code:: c

  struct my_struct {
          int arg1;
          int arg2;
  };

This allows function declarations in other header files that depend on the
struct/enum to forward declare the struct/enum instead of including the
entire header:

.. code:: c

  #include <my_struct.h>
  void my_func(my_struct_t *arg);

instead of:

.. code:: c

  struct my_struct;
  void my_func(struct my_struct *arg);

Some TF definitions use both a struct/enum name **and** a typedef name. This
is discouraged for new definitions as it makes it difficult for TF to comply
with MISRA rule 8.3, which states that "All declarations of an object or
function shall use the same names and type qualifiers".

The Linux coding standards also discourage new typedefs and checkpatch emits
a warning for this.

Existing typedefs will be retained for compatibility.

Error handling and robustness
-----------------------------

Using CASSERT to check for compile time data errors
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Where possible, use the ``CASSERT`` macro to check the validity of data known at
compile time instead of checking validity at runtime, to avoid unnecessary
runtime code.

For example, this can be used to check that the assembler's and compiler's views
of the size of an array is the same.

.. code:: c

  #include <cassert.h>

  define MY_STRUCT_SIZE 8 /* Used by assembler source files */

  struct my_struct {
      uint32_t arg1;
      uint32_t arg2;
  };

  CASSERT(MY_STRUCT_SIZE == sizeof(struct my_struct), assert_my_struct_size_mismatch);


If ``MY_STRUCT_SIZE`` in the above example were wrong then the compiler would
emit an error like this:

.. code:: c

  my_struct.h:10:1: error: size of array ‘assert_my_struct_size_mismatch’ is negative


Using assert() to check for programming errors
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

In general, each secure world TF image (BL1, BL2, BL31 and BL32) should be
treated as a tightly integrated package; the image builder should be aware of
and responsible for all functionality within the image, even if code within that
image is provided by multiple entities. This allows us to be more aggressive in
interpreting invalid state or bad function arguments as programming errors using
``assert()``, including arguments passed across platform porting interfaces.
This is in contrast to code in a Linux environment, which is less tightly
integrated and may attempt to be more defensive by passing the error back up the
call stack.

Where possible, badly written TF code should fail early using ``assert()``. This
helps reduce the amount of untested conditional code. By default these
statements are not compiled into release builds, although this can be overridden
using the ``ENABLE_ASSERTIONS`` build flag.

Examples:

- Bad argument supplied to library function
- Bad argument provided by platform porting function
- Internal secure world image state is inconsistent


Handling integration errors
^^^^^^^^^^^^^^^^^^^^^^^^^^^

Each secure world image may be provided by a different entity (for example, a
Trusted Boot vendor may provide the BL2 image, a TEE vendor may provide the BL32
image and the OEM/SoC vendor may provide the other images).

An image may contain bugs that are only visible when the images are integrated.
The system integrator may not even have access to the debug variants of all the
images in order to check if asserts are firing. For example, the release variant
of BL1 may have already been burnt into the SoC. Therefore, TF code that detects
an integration error should _not_ consider this a programming error, and should
always take action, even in release builds.

If an integration error is considered non-critical it should be treated as a
recoverable error. If the error is considered critical it should be treated as
an unexpected unrecoverable error.

Handling recoverable errors
^^^^^^^^^^^^^^^^^^^^^^^^^^^

The secure world **must not** crash when supplied with bad data from an external
source. For example, data from the normal world or a hardware device. Similarly,
the secure world **must not** crash if it detects a non-critical problem within
itself or the system. It must make every effort to recover from the problem by
emitting a ``WARN`` message, performing any necessary error handling and
continuing.

Examples:

- Secure world receives SMC from normal world with bad arguments.
- Secure world receives SMC from normal world at an unexpected time.
- BL31 receives SMC from BL32 with bad arguments.
- BL31 receives SMC from BL32 at unexpected time.
- Secure world receives recoverable error from hardware device. Retrying the
  operation may help here.
- Non-critical secure world service is not functioning correctly.
- BL31 SPD discovers minor configuration problem with corresponding SP.

Handling unrecoverable errors
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

In some cases it may not be possible for the secure world to recover from an
error. This situation should be handled in one of the following ways:

1. If the unrecoverable error is unexpected then emit an ``ERROR`` message and
   call ``panic()``. This will end up calling the platform-specific function
   ``plat_panic_handler()``.
2. If the unrecoverable error is expected to occur in certain circumstances,
   then emit an ``ERROR`` message and call the platform-specific function
   ``plat_error_handler()``.

Cases 1 and 2 are subtly different. A platform may implement ``plat_panic_handler``
and ``plat_error_handler`` in the same way (for example, by waiting for a secure
watchdog to time-out or by invoking an interface on the platform's power
controller to reset the platform). However, ``plat_error_handler`` may take
additional action for some errors (for example, it may set a flag so the
platform resets into a different mode). Also, ``plat_panic_handler()`` may
implement additional debug functionality (for example, invoking a hardware
breakpoint).

Examples of unexpected unrecoverable errors:

- BL32 receives an unexpected SMC response from BL31 that it is unable to
  recover from.
- BL31 Trusted OS SPD code discovers that BL2 has not loaded the corresponding
  Trusted OS, which is critical for platform operation.
- Secure world discovers that a critical hardware device is an unexpected and
  unrecoverable state.
- Secure world receives an unexpected and unrecoverable error from a critical
  hardware device.
- Secure world discovers that it is running on unsupported hardware.

Examples of expected unrecoverable errors:

- BL1/BL2 fails to load the next image due to missing/corrupt firmware on disk.
- BL1/BL2 fails to authenticate the next image due to an invalid certificate.
- Secure world continuously receives recoverable errors from a hardware device
  but is unable to proceed without a valid response.

Handling critical unresponsiveness
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

If the secure world is waiting for a response from an external source (for
example, the normal world or a hardware device) which is critical for continued
operation, it must not wait indefinitely. It must have a mechanism (for example,
a secure watchdog) for resetting itself and/or the external source to prevent
the system from executing in this state indefinitely.

Examples:

- BL1 is waiting for the normal world to raise an SMC to proceed to the next
  stage of the secure firmware update process.
- A Trusted OS is waiting for a response from a proxy in the normal world that
  is critical for continued operation.
- Secure world is waiting for a hardware response that is critical for continued
  operation.

Security considerations
-----------------------

Part of the security of a platform is handling errors correctly, as described in
the previous section. There are several other security considerations covered in
this section.

Do not leak secrets to the normal world
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

The secure world **must not** leak secrets to the normal world, for example in
response to an SMC.

Handling Denial of Service attacks
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

The secure world **should never** crash or become unusable due to receiving too
many normal world requests (a *Denial of Service* or *DoS* attack). It should
have a mechanism for throttling or ignoring normal world requests.

Performance considerations
--------------------------

Avoid printf and use logging macros
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

``debug.h`` provides logging macros (for example, ``WARN`` and ``ERROR``)
which wrap ``tf_log`` and which allow the logging call to be compiled-out
depending on the ``make`` command. Use these macros to avoid print statements
being compiled unconditionally into the binary.

Each logging macro has a numerical log level:

.. code:: c

  #define LOG_LEVEL_NONE    0
  #define LOG_LEVEL_ERROR   10
  #define LOG_LEVEL_NOTICE  20
  #define LOG_LEVEL_WARNING 30
  #define LOG_LEVEL_INFO    40
  #define LOG_LEVEL_VERBOSE 50


By default, all logging statements with a log level ``<= LOG_LEVEL_INFO`` will
be compiled into debug builds and all statements with a log level
``<= LOG_LEVEL_NOTICE`` will be compiled into release builds. This can be
overridden from the command line or by the platform makefile (although it may be
necessary to clean the build directory first). For example, to enable
``VERBOSE`` logging on FVP:

``make PLAT=fvp LOG_LEVEL=50 all``

Use const data where possible
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

For example, the following code:

.. code:: c

  struct my_struct {
          int arg1;
          int arg2;
  };

  void init(struct my_struct *ptr);

  void main(void)
  {
          struct my_struct x;
          x.arg1 = 1;
          x.arg2 = 2;
          init(&x);
  }

is better written as:

.. code:: c

  struct my_struct {
          int arg1;
          int arg2;
  };

  void init(const struct my_struct *ptr);

  void main(void)
  {
          const struct my_struct x = { 1, 2 };
          init(&x);
  }

This allows the linker to put the data in a read-only data section instead of a
writeable data section, which may result in a smaller and faster binary. Note
that this may require dependent functions (``init()`` in the above example) to
have ``const`` arguments, assuming they don't need to modify the data.

Library and driver code
-----------------------

TF library code (under ``lib/`` and ``include/lib``) is any code that provides a
reusable interface to other code, potentially even to code outside of TF.

In some systems drivers must conform to a specific driver framework to provide
services to the rest of the system. TF has no driver framework and the
distinction between a driver and library is somewhat subjective.

A driver (under ``drivers/`` and ``include/drivers/``) is defined as code that
interfaces with hardware via a memory mapped interface.

Some drivers (for example, the Arm CCI driver in ``include/drivers/arm/cci.h``)
provide a general purpose API to that specific hardware. Other drivers (for
example, the Arm PL011 console driver in ``drivers/arm/pl011/pl011_console.S``)
provide a specific hardware implementation of a more abstract library API. In
the latter case there may potentially be multiple drivers for the same hardware
device.

Neither libraries nor drivers should depend on platform-specific code. If they
require platform-specific data (for example, a base address) to operate then
they should provide an initialization function that takes the platform-specific
data as arguments.

TF common code (under ``common/`` and ``include/common/``) is code that is re-used
by other generic (non-platform-specific) TF code. It is effectively internal
library code.

.. _`Why the “volatile” type class should not be used`: https://www.kernel.org/doc/html/latest/process/volatile-considered-harmful.html
.. _`Procedure Call Standard for the Arm Architecture`: http://infocenter.arm.com/help/topic/com.arm.doc.ihi0042f/IHI0042F_aapcs.pdf
.. _`Procedure Call Standard for the Arm 64-bit Architecture`: http://infocenter.arm.com/help/topic/com.arm.doc.ihi0055b/IHI0055B_aapcs64.pdf