============================
Taint Analysis Configuration
============================

The Clang Static Analyzer uses taint analysis to detect security-related issues in code.
The backbone of taint analysis in the Clang SA is the `GenericTaintChecker`, which the user can access via the :ref:`alpha-security-taint-TaintPropagation` checker alias and this checker has a default taint-related configuration.
The built-in default settings are defined in code, and they are always in effect once the checker is enabled, either directly or via the alias.
The checker also provides a configuration interface for extending the default settings by providing a configuration file in `YAML <http://llvm.org/docs/YamlIO.html#introduction-to-yaml>`_ format.
This documentation describes the syntax of the configuration file and gives the informal semantics of the configuration options.

.. contents::
   :local:

.. _clangsa-taint-configuration-overview:

Overview
________

Taint analysis works by checking for the occurrence of special operations during the symbolic execution of the program.
Taint analysis defines sources, sinks, and propagation rules. It identifies errors by detecting a flow of information that originates from a taint source, reaches a taint sink, and propagates through the program paths via propagation rules.
A source, sink, or an operation that propagates taint is mainly domain-specific knowledge, but there are some built-in defaults provided by :ref:`alpha-security-taint-TaintPropagation`.
It is possible to express that a statement sanitizes tainted values by providing a ``Filters`` section in the external configuration (see :ref:`clangsa-taint-configuration-example` and :ref:`clangsa-taint-filter-details`).
There are no default filters defined in the built-in settings.
The checker's documentation also specifies how to provide a custom taint configuration with command-line options.

.. _clangsa-taint-configuration-example:

Example configuration file
__________________________

.. code-block:: yaml

  # The entries that specify arguments use 0-based indexing when specifying
  # input arguments, and -1 is used to denote the return value.

  Filters:
    # Filter functions
    # Taint is sanitized when tainted variables are pass arguments to filters.

    # Filter function
    #   void cleanse_first_arg(int* arg)
    #
    # Result example:
    #   int x; // x is tainted
    #   cleanse_first_arg(&x); // x is not tainted after the call
    - Name: cleanse_first_arg
      Args: [0]

  Propagations:
    # Source functions
    # The omission of SrcArgs key indicates unconditional taint propagation,
    # which is conceptually what a source does.

    # Source function
    #   size_t fread(void *ptr, size_t size, size_t nmemb, FILE * stream)
    #
    # Result example:
    #   FILE* f = fopen("file.txt");
    #   char buf[1024];
    #   size_t read = fread(buf, sizeof(buf[0]), sizeof(buf)/sizeof(buf[0]), f);
    #   // both read and buf are tainted
    - Name: fread
      DstArgs: [0, -1]

    # Propagation functions
    # The presence of SrcArgs key indicates conditional taint propagation,
    # which is conceptually what a propagator does.

    # Propagation function
    #   char *dirname(char *path)
    #
    # Result example:
    #   char* path = read_path();
    #   char* dir = dirname(path);
    #   // dir is tainted if path was tainted
    - Name: dirname
      SrcArgs: [0]
      DstArgs: [-1]

  Sinks:
    # Sink functions
    # If taint reaches any of the arguments specified, a warning is emitted.

    # Sink function
    #   int system(const char* command)
    #
    # Result example:
    #   const char* command = read_command();
    #   system(command); // emit diagnostic if command is tainted
    - Name: system
      Args: [0]

In the example file above, the entries under the `Propagation` key implement the conceptual sources and propagations, and sinks have their dedicated `Sinks` key.
The user can define operations (function calls) where the tainted values should be cleansed by listing entries under the `Filters` key.
Filters model the sanitization of values done by the programmer, and providing these is key to avoiding false-positive findings.

Configuration file syntax and semantics
_______________________________________

The configuration file should have valid `YAML <http://llvm.org/docs/YamlIO.html#introduction-to-yaml>`_ syntax.

The configuration file can have the following top-level keys:
 - Filters
 - Propagations
 - Sinks

Under the `Filters` key, the user can specify a list of operations that remove taint (see :ref:`clangsa-taint-filter-details` for details).

Under the `Propagations` key, the user can specify a list of operations that introduce and propagate taint (see :ref:`clangsa-taint-propagation-details` for details).
The user can mark taint sources with a `SrcArgs` key in the `Propagation` key, while propagations have none.
The lack of the `SrcArgs` key means unconditional propagation, which is how sources are modeled.
The semantics of propagations are such, that if any of the source arguments are tainted (specified by indexes in `SrcArgs`) then all of the destination arguments (specified by indexes in `DstArgs`) also become tainted.

Under the `Sinks` key, the user can specify a list of operations where the checker should emit a bug report if tainted data reaches it (see :ref:`clangsa-taint-sink-details` for details).

.. _clangsa-taint-filter-details:

Filter syntax and semantics
###########################

An entry under `Filters` is a `YAML <http://llvm.org/docs/YamlIO.html#introduction-to-yaml>`_ object with the following mandatory keys:
 - `Name` is a string that specifies the name of a function.
   Encountering this function during symbolic execution the checker will sanitize taint from the memory region referred to by the given arguments or return a sanitized value.
 - `Args` is a list of numbers in the range of ``[-1..int_max]``.
   It indicates the indexes of arguments in the function call.
   The number ``-1`` signifies the return value; other numbers identify call arguments.
   The values of these arguments are considered clean after the function call.

The following keys are optional:
 - `Scope` is a string that specifies the prefix of the function's name in its fully qualified name. This option restricts the set of matching function calls. It can encode not only namespaces but struct/class names as well to match member functions.

 .. _clangsa-taint-propagation-details:

Propagation syntax and semantics
################################

An entry under `Propagation` is a `YAML <http://llvm.org/docs/YamlIO.html#introduction-to-yaml>`_ object with the following mandatory keys:
 - `Name` is a string that specifies the name of a function.
   Encountering this function during symbolic execution propagate taint from one or more arguments to other arguments and possibly the return value.
   It helps model the taint-related behavior of functions that are not analyzable otherwise.

The following keys are optional:
 - `Scope` is a string that specifies the prefix of the function's name in its fully qualified name. This option restricts the set of matching function calls.
 - `SrcArgs` is a list of numbers in the range of ``[0..int_max]`` that indicates the indexes of arguments in the function call.
   Taint-propagation considers the values of these arguments during the evaluation of the function call.
   If any `SrcArgs` arguments are tainted, the checker will consider all `DstArgs` arguments tainted after the call.
 - `DstArgs` is a list of numbers in the range of ``[-1..int_max]`` that indicates the indexes of arguments in the function call.
   The number ``-1`` specifies the return value of the function.
   If any `SrcArgs` arguments are tainted, the checker will consider all `DstArgs` arguments tainted after the call.
 - `VariadicType` is a string that can be one of ``None``, ``Dst``, ``Src``.
   It is used in conjunction with `VariadicIndex` to specify arguments inside a variadic argument.
   The value of ``Src`` will treat every call site argument that is part of a variadic argument list as a source concerning propagation rules (as if specified by `SrcArg`).
   The value of ``Dst`` will treat every call site argument that is part of a variadic argument list a destination concerning propagation rules.
   The value of ``None`` will not consider the arguments that are part of a variadic argument list (this option is redundant but can be used to temporarily switch off handling of a particular variadic argument option without removing the VariadicIndex key).
 - `VariadicIndex` is a number in the range of ``[0..int_max]``. It indicates the starting index of the variadic argument in the signature of the function.


.. _clangsa-taint-sink-details:

Sink syntax and semantics
#########################

An entry under `Sinks` is a `YAML <http://llvm.org/docs/YamlIO.html#introduction-to-yaml>`_ object with the following mandatory keys:
 - `Name` is a string that specifies the name of a function.
   Encountering this function during symbolic execution will emit a taint-related diagnostic if any of the arguments specified with `Args` are tainted at the call site.
 - `Args` is a list of numbers in the range of ``[0..int_max]`` that indicates the indexes of arguments in the function call.
   The checker reports an error if any of the specified arguments are tainted.

The following keys are optional:
 - `Scope` is a string that specifies the prefix of the function's name in its fully qualified name. This option restricts the set of matching function calls.