gcc-python-plugin-doc 0.15-1 / usr / share / doc / gcc-python-plugin-doc / html / _sources / 0.7.txt

0.7
~~~

This is a major update to the GCC Python plugin.

The main example script, cpychecker, has seen numerous improvements, and has
now detected many reference-counting bugs in real-world CPython extension code.
The usability and signal:noise ratio is greatly improved over previous releases.

Changes to the GCC Python Plugin
================================

It's now possible to create custom GCC attributes from Python, allowing you to
add custom high-level annotation to a C API, and to write scripts that will
verify these properties.  It's also possible to inject preprocessor macros
from Python.  Taken together, this allows code like this:

   .. literalinclude:: ../tests/examples/attributes-with-macros/input.c
      :lines: 22-45
      :language: c

Other improvements:

   * gcc's debug dump facilities are now exposed via a Python API

   * it's now possible to run Python commands in GCC (rather than scripts) using
     -fplugin-arg-python-command

   * improvements to the "source location" when reporting on an unhandled
     Python exception.  Amongst other tweaks, it's now possible for a script to
     override this, which the cpychecker uses, so that if it can't handle a
     particular line of C code, the GCC error report gives that location before
     reporting the Python traceback (making debugging much easier).

   * "switch" statements are now properly wrapped at the Python level
     (gcc.GimpleSwitch)

   * C bitfields are now wrapped at the Python level

   * gcc.Type instances now have a "sizeof" attribute, and an "attributes"
     attribute.

   * added a gcc.Gimple.walk_tree method, to make it easy to visit all nodes
     relating to a statement

   * added a new example: spell-checking all string literals in code

Improvements to "cpychecker"
============================

The "libcpychecker" Python code is a large example of using the plugin: it
extends GCC with code that tries to detect various bugs in CPython extension
modules.

The cpychecker analyzes the paths that can be followed through a C function,
and verifies various properties, including reference-count handling.

As of this release, the pass has found many reference-counting bugs in
real-world code.  You can see a list of the bugs that it has detected at:

http://gcc-python-plugin.readthedocs.org/en/latest/success.html

The checker is now *almost* capable of fully handling the C code within the
gcc python plugin itself.

The checker has also been reorganized to (I hope) make it easy to add checking
for other libraries and APIs.

Major rewrite of reference-count tracking
-----------------------------------------

I've rewritten the internals of how reference counts are tracked: the code now
makes a distinction betweeen all of the reference that can be analysed within a
single function, versus all of the other references that may exist in the rest
of the program.

This allows us to know for an object e.g. that the function doesn't directly
own any references, but that the reference count is still > 0 (a "borrowed
reference"), as compared to the case where the function owns a reference, but
we don't know of any in the rest of the program (this is typical when receiving
a "new reference" e.g. from a function call to a constructor).

Within the reference-count checker, we now look for memory locations that
store references to objects.   If those locations not on the stack, then the
references they store are now assumed to legally count towards the ob_refcnt
that the function "owns".  This is needed in order to correctly handle e.g.
the PyList_SET_ITEM() macro, which directly writes to the list's ob_item field,
"stealing" a reference: we can detect these references, and count them towards
the ob_refcnt value.

The checker can now detect types that look like PyObject subclasses at the C
level (by looking at the top-most fields), and uses this information in various
places.

The checker now exposes custom GCC attributes allowing you to mark APIs that
have non-standard reference-handling behavior:

.. code-block:: c

   PyObject *foo(void)
     CPYCHECKER_RETURNS_BORROWED_REF;

   extern void bar(int i, PyObject *obj, int j, PyObject *other)
     CPYCHECKER_STEALS_REFERENCE_TO_ARG(2)
     CPYCHECKER_STEALS_REFERENCE_TO_ARG(4);

It also exposes an attribute allowing you to the run-time and compile-time
type information for a Python extension class:
    
.. code-block:: c

  /* Define some PyObject subclass, as both a struct and a typedef */
  struct OurObjectStruct {
      PyObject_HEAD
      /* other fields */
  };
  typedef struct OurObjectStruct OurExtensionObject;

  /*
    Declare the PyTypeObject, using the custom attribute to associate it with
    the typedef above:
  */
  extern PyTypeObject UserDefinedExtension_Type
    CPYCHECKER_TYPE_OBJECT_FOR_TYPEDEF("OurExtensionObject");

Function calls with NULL-pointer arguments
------------------------------------------
The checker knows about various CPython API hooks that will crash on NULL
pointer arguments, and will emit warnings when it can determine a path through
the code that will lead to a definite call with a NULL value.


Dereferences of uninitialized pointers
--------------------------------------
The checker will now complain about paths through a function for which it can
prove that an uninitialized pointer will be dereferenced.


Error-reporting improvements
----------------------------

The error-reporting machinery can generate HTML reports: see e.g.:
http://readthedocs.org/docs/gcc-python-plugin/en/latest/cpychecker.html#reference-count-checking
and http://dmalcolm.livejournal.com/6560.html

The checker can now annotate its HTML (and textual) reports with information
showing how some pertinent aspect of the program's state changes during a
particular path through a function.

For example, when reporting on reference-counting errors, the HTML report
showing the flow through the function will now display all changes to an object's
ob_refcnt, together with all changes to what the value ought to be (e.g. due to
pointers being stored to persistent memory locations):

   .. figure:: sample-html-error-report.png
      :alt: screenshot of the HTML report

Similarly, when reporting on exception-handling errors, it now displays the
"history" of changes to the thread-local exception state.

There's also a debug mode which dumps _everything_ that changes within the
report, which is helpful for debugging the checker itself.

The error report will attempt to use the most representative name for a leaked
object, using a variable name or a C expression fragment as appropriate.

The checker will attempt to combine duplicate error reports, so that it will
only emit one error for all of the various traces of execution that exhibit a
particular reference-counting bug.

Finally, when writing out an HTML report, the path to the HTML is now noted
within gcc's regular stderr messages.


Signal:noise ratio improvements
-------------------------------

To suppress various false-positives that I commonly ran into on real code, the
checker now makes certain assumptions:
  
   * When encountering an unknown function that returns a PyObject*, the
     checker assumes that it will either return a new reference to a sane
     object (with a sane ob_type), or return NULL and set the thread-local
     exception state.

   * The checker assumes that a PyObject* argument to a function is non-NULL
     and has a >0 refcount, and has a sane ob_type (e.g. with a sane refcount
     and tp_dealloc)

   * When dereferencing a pointer that it has no knowledge about (e.g. a
     pointer field in a structure), the checker now assumes that it's
     non-NULL, unless it knows that NULL is a definite possibility i.e. it
     optimistically assumes that you know what you're doing (this could be
     turned into a command-line option).  Note that for the cases where we
     know that the pointer can _definitely_ be NULL, an error will still be
     reported (e.g. when considering the various possible return values for a
     function known to be able to return NULL).

Coverage of the CPython API
---------------------------

I've gone through much of the CPython API, "teaching" the checker about the
reference-count semantics of each API call (and which calls will crash if fed a
NULL pointer).  This involves writing a simple fragment of Python code for
each function, which describes the various different affects that the call can
have on the internal state within the callee.

This release adds support for calls to the following:
   * _PyObject_New
   * Py_{Initialize|Finalize}
   * Py_InitModule4
   * PyArg_ParseTuple[AndKeywords], and the PY_SSIZE_T_CLEAN variants (only
     partial coverage so far: "O", "O!" should work though)
   * PyArg_UnpackTuple
   * PyBool_FromLong
   * Py_BuildValue and the PY_SSIZE_T_CLEAN variant (only partial coverage so
     far)
   * PyDict_{GetItem,GetItemString,New,SetItem,SetItemString}
   * PyErr_{Format,NoMemory,Occurred,Print,PrintEx,SetFromErrno[WithFilename],
     SetObject,SetString}
   * PyEval_InitThreads
   * PyGILState_{Ensure,Release}
   * PyImport_{AppendInittab,ImportModule}
   * PyInt_{AsLong,FromLong}
   * PyList_Append
   * PyLong_{FromString,FromVoidPtr}
   * PyMem_{Malloc,Free}
   * PyModule_Add{IntConstant,Object,StringConstant}
   * PyObject_{Call,CallMethod,HasAttrString,IsTrue,Repr,Str}
   * PyRun_{SimpleFileExFlags,SimpleStringFlags}
   * PySequence_GetItem
   * PyString_{AsString,FromFormat,FromString,FromStringAndSize}
   * PyStructSequence_{InitType,New}
   * PySys_SetObject
   * PyTuple_{New,SetItem,Size}
   * PyType_{IsSubtype,Ready}

I've been targetting those API entrypoints that I use myself in the plugin;
this is one area which is particularly amenable to patching, for anyone who
wants to get involved.   I've also added a (disabled) hook that complains
about Python API entrypoints that weren't explicitly handled, to make it easy
to find gaps in our coverage of the CPython API.


Other user-visible improvments
------------------------------

   * There's now a "gcc-with-cpychecker" harness, to make it easier to invoke
     GCC with the cpychecker code from e.g. Makefiles

   * The checker now respects `__attribute((nonnull))` on function arguments
     when detecting NULL pointers

   * Handle functions that don't return (e.g. "exit")

   * Number the unknown heap regions, to clarify things when there's more than
     one

Internal improvements
---------------------

  * The cpychecker now has some protection against combinatorial explosion
    for functions that have very large numbers of possible routes through
    them.  For such functions, the checker will emit a note on stderr and
    not attempt to find reference-counting bugs in the function.

  * The cpychecker is now done as a custom pass (rather than by wiring up a
    callback associated with every pass)

  * I've tuned the logging within the checker, eliminating some CPU/memory
    consumption issues seen when analysing complicated C code.  In particular,
    the log message arguments are now only expanded when logging is enabled
    (previously this was happening all the time).

  * Lots of other internal improvements and bug fixes (e.g. handling of arrays
    vs pointers, static vs auto local variables, add missing handlers for
    various kinds of C expression, lots of work on improving the readability of
    error messages)