xref: /openbmc/linux/Documentation/dev-tools/coccinelle.rst (revision 9a87ffc99ec8eb8d35eed7c4f816d75f5cc9662e)

.. Copyright 2010 Nicolas Palix <npalix@diku.dk>
.. Copyright 2010 Julia Lawall <julia@diku.dk>
.. Copyright 2010 Gilles Muller <Gilles.Muller@lip6.fr>

.. highlight:: none

.. _devtools_coccinelle:

Coccinelle
==========

Coccinelle is a tool for pattern matching and text transformation that has
many uses in kernel development, including the application of complex,
tree-wide patches and detection of problematic programming patterns.

Getting Coccinelle
------------------

The semantic patches included in the kernel use features and options
which are provided by Coccinelle version 1.0.0-rc11 and above.
Using earlier versions will fail as the option names used by
the Coccinelle files and coccicheck have been updated.

Coccinelle is available through the package manager
of many distributions, e.g. :

 - Debian
 - Fedora
 - Ubuntu
 - OpenSUSE
 - Arch Linux
 - NetBSD
 - FreeBSD

Some distribution packages are obsolete and it is recommended
to use the latest version released from the Coccinelle homepage at
http://coccinelle.lip6.fr/

Or from Github at:

https://github.com/coccinelle/coccinelle

Once you have it, run the following commands::

        ./autogen
        ./configure
        make

as a regular user, and install it with::

        sudo make install

More detailed installation instructions to build from source can be
found at:

https://github.com/coccinelle/coccinelle/blob/master/install.txt

Supplemental documentation
--------------------------

For supplemental documentation refer to the wiki:

https://bottest.wiki.kernel.org/coccicheck

The wiki documentation always refers to the linux-next version of the script.

For Semantic Patch Language(SmPL) grammar documentation refer to:

https://coccinelle.gitlabpages.inria.fr/website/docs/main_grammar.html

Using Coccinelle on the Linux kernel
------------------------------------

A Coccinelle-specific target is defined in the top level
Makefile. This target is named ``coccicheck`` and calls the ``coccicheck``
front-end in the ``scripts`` directory.

Four basic modes are defined: ``patch``, ``report``, ``context``, and
``org``. The mode to use is specified by setting the MODE variable with
``MODE=<mode>``.

- ``patch`` proposes a fix, when possible.

- ``report`` generates a list in the following format:
  file:line:column-column: message

- ``context`` highlights lines of interest and their context in a
  diff-like style. Lines of interest are indicated with ``-``.

- ``org`` generates a report in the Org mode format of Emacs.

Note that not all semantic patches implement all modes. For easy use
of Coccinelle, the default mode is "report".

Two other modes provide some common combinations of these modes.

- ``chain`` tries the previous modes in the order above until one succeeds.

- ``rep+ctxt`` runs successively the report mode and the context mode.
  It should be used with the C option (described later)
  which checks the code on a file basis.

Examples
~~~~~~~~

To make a report for every semantic patch, run the following command::

		make coccicheck MODE=report

To produce patches, run::

		make coccicheck MODE=patch


The coccicheck target applies every semantic patch available in the
sub-directories of ``scripts/coccinelle`` to the entire Linux kernel.

For each semantic patch, a commit message is proposed.  It gives a
description of the problem being checked by the semantic patch, and
includes a reference to Coccinelle.

As with any static code analyzer, Coccinelle produces false
positives. Thus, reports must be carefully checked, and patches
reviewed.

To enable verbose messages set the V= variable, for example::

   make coccicheck MODE=report V=1

Coccinelle parallelization
--------------------------

By default, coccicheck tries to run as parallel as possible. To change
the parallelism, set the J= variable. For example, to run across 4 CPUs::

   make coccicheck MODE=report J=4

As of Coccinelle 1.0.2 Coccinelle uses Ocaml parmap for parallelization;
if support for this is detected you will benefit from parmap parallelization.

When parmap is enabled coccicheck will enable dynamic load balancing by using
``--chunksize 1`` argument. This ensures we keep feeding threads with work
one by one, so that we avoid the situation where most work gets done by only
a few threads. With dynamic load balancing, if a thread finishes early we keep
feeding it more work.

When parmap is enabled, if an error occurs in Coccinelle, this error
value is propagated back, and the return value of the ``make coccicheck``
command captures this return value.

Using Coccinelle with a single semantic patch
---------------------------------------------

The optional make variable COCCI can be used to check a single
semantic patch. In that case, the variable must be initialized with
the name of the semantic patch to apply.

For instance::

	make coccicheck COCCI=<my_SP.cocci> MODE=patch

or::

	make coccicheck COCCI=<my_SP.cocci> MODE=report


Controlling Which Files are Processed by Coccinelle
---------------------------------------------------

By default the entire kernel source tree is checked.

To apply Coccinelle to a specific directory, ``M=`` can be used.
For example, to check drivers/net/wireless/ one may write::

    make coccicheck M=drivers/net/wireless/

To apply Coccinelle on a file basis, instead of a directory basis, the
C variable is used by the makefile to select which files to work with.
This variable can be used to run scripts for the entire kernel, a
specific directory, or for a single file.

For example, to check drivers/bluetooth/bfusb.c, the value 1 is
passed to the C variable to check files that make considers
need to be compiled.::

    make C=1 CHECK=scripts/coccicheck drivers/bluetooth/bfusb.o

The value 2 is passed to the C variable to check files regardless of
whether they need to be compiled or not.::

    make C=2 CHECK=scripts/coccicheck drivers/bluetooth/bfusb.o

In these modes, which work on a file basis, there is no information
about semantic patches displayed, and no commit message proposed.

This runs every semantic patch in scripts/coccinelle by default. The
COCCI variable may additionally be used to only apply a single
semantic patch as shown in the previous section.

The "report" mode is the default. You can select another one with the
MODE variable explained above.

Debugging Coccinelle SmPL patches
---------------------------------

Using coccicheck is best as it provides in the spatch command line
include options matching the options used when we compile the kernel.
You can learn what these options are by using V=1; you could then
manually run Coccinelle with debug options added.

Alternatively you can debug running Coccinelle against SmPL patches
by asking for stderr to be redirected to stderr. By default stderr
is redirected to /dev/null; if you'd like to capture stderr you
can specify the ``DEBUG_FILE="file.txt"`` option to coccicheck. For
instance::

    rm -f cocci.err
    make coccicheck COCCI=scripts/coccinelle/free/kfree.cocci MODE=report DEBUG_FILE=cocci.err
    cat cocci.err

You can use SPFLAGS to add debugging flags; for instance you may want to
add both ``--profile --show-trying`` to SPFLAGS when debugging. For example
you may want to use::

    rm -f err.log
    export COCCI=scripts/coccinelle/misc/irqf_oneshot.cocci
    make coccicheck DEBUG_FILE="err.log" MODE=report SPFLAGS="--profile --show-trying" M=./drivers/mfd

err.log will now have the profiling information, while stdout will
provide some progress information as Coccinelle moves forward with
work.

NOTE:

DEBUG_FILE support is only supported when using coccinelle >= 1.0.2.

Currently, DEBUG_FILE support is only available to check folders, and
not single files. This is because checking a single file requires spatch
to be called twice leading to DEBUG_FILE being set both times to the same value,
giving rise to an error.

.cocciconfig support
--------------------

Coccinelle supports reading .cocciconfig for default Coccinelle options that
should be used every time spatch is spawned. The order of precedence for
variables for .cocciconfig is as follows:

- Your current user's home directory is processed first
- Your directory from which spatch is called is processed next
- The directory provided with the ``--dir`` option is processed last, if used

Since coccicheck runs through make, it naturally runs from the kernel
proper dir; as such the second rule above would be implied for picking up a
.cocciconfig when using ``make coccicheck``.

``make coccicheck`` also supports using M= targets. If you do not supply
any M= target, it is assumed you want to target the entire kernel.
The kernel coccicheck script has::

    if [ "$KBUILD_EXTMOD" = "" ] ; then
        OPTIONS="--dir $srctree $COCCIINCLUDE"
    else
        OPTIONS="--dir $KBUILD_EXTMOD $COCCIINCLUDE"
    fi

KBUILD_EXTMOD is set when an explicit target with M= is used. For both cases
the spatch ``--dir`` argument is used, as such third rule applies when whether
M= is used or not, and when M= is used the target directory can have its own
.cocciconfig file. When M= is not passed as an argument to coccicheck the
target directory is the same as the directory from where spatch was called.

If not using the kernel's coccicheck target, keep the above precedence
order logic of .cocciconfig reading. If using the kernel's coccicheck target,
override any of the kernel's .coccicheck's settings using SPFLAGS.

We help Coccinelle when used against Linux with a set of sensible default
options for Linux with our own Linux .cocciconfig. This hints to coccinelle
that git can be used for ``git grep`` queries over coccigrep. A timeout of 200
seconds should suffice for now.

The options picked up by coccinelle when reading a .cocciconfig do not appear
as arguments to spatch processes running on your system. To confirm what
options will be used by Coccinelle run::

      spatch --print-options-only

You can override with your own preferred index option by using SPFLAGS. Take
note that when there are conflicting options Coccinelle takes precedence for
the last options passed. Using .cocciconfig is possible to use idutils, however
given the order of precedence followed by Coccinelle, since the kernel now
carries its own .cocciconfig, you will need to use SPFLAGS to use idutils if
desired. See below section "Additional flags" for more details on how to use
idutils.

Additional flags
----------------

Additional flags can be passed to spatch through the SPFLAGS
variable. This works as Coccinelle respects the last flags
given to it when options are in conflict. ::

    make SPFLAGS=--use-glimpse coccicheck

Coccinelle supports idutils as well but requires coccinelle >= 1.0.6.
When no ID file is specified coccinelle assumes your ID database file
is in the file .id-utils.index on the top level of the kernel. Coccinelle
carries a script scripts/idutils_index.sh which creates the database with::

    mkid -i C --output .id-utils.index

If you have another database filename you can also just symlink with this
name. ::

    make SPFLAGS=--use-idutils coccicheck

Alternatively you can specify the database filename explicitly, for
instance::

    make SPFLAGS="--use-idutils /full-path/to/ID" coccicheck

See ``spatch --help`` to learn more about spatch options.

Note that the ``--use-glimpse`` and ``--use-idutils`` options
require external tools for indexing the code. None of them is
thus active by default. However, by indexing the code with
one of these tools, and according to the cocci file used,
spatch could proceed the entire code base more quickly.

SmPL patch specific options
---------------------------

SmPL patches can have their own requirements for options passed
to Coccinelle. SmPL patch-specific options can be provided by
providing them at the top of the SmPL patch, for instance::

	// Options: --no-includes --include-headers

SmPL patch Coccinelle requirements
----------------------------------

As Coccinelle features get added some more advanced SmPL patches
may require newer versions of Coccinelle. If an SmPL patch requires
a minimum version of Coccinelle, this can be specified as follows,
as an example if requiring at least Coccinelle >= 1.0.5::

	// Requires: 1.0.5

Proposing new semantic patches
------------------------------

New semantic patches can be proposed and submitted by kernel
developers. For sake of clarity, they should be organized in the
sub-directories of ``scripts/coccinelle/``.


Detailed description of the ``report`` mode
-------------------------------------------

``report`` generates a list in the following format::

  file:line:column-column: message

Example
~~~~~~~

Running::

	make coccicheck MODE=report COCCI=scripts/coccinelle/api/err_cast.cocci

will execute the following part of the SmPL script::

   <smpl>
   @r depends on !context && !patch && (org || report)@
   expression x;
   position p;
   @@

     ERR_PTR@p(PTR_ERR(x))

   @script:python depends on report@
   p << r.p;
   x << r.x;
   @@

   msg="ERR_CAST can be used with %s" % (x)
   coccilib.report.print_report(p[0], msg)
   </smpl>

This SmPL excerpt generates entries on the standard output, as
illustrated below::

    /home/user/linux/crypto/ctr.c:188:9-16: ERR_CAST can be used with alg
    /home/user/linux/crypto/authenc.c:619:9-16: ERR_CAST can be used with auth
    /home/user/linux/crypto/xts.c:227:9-16: ERR_CAST can be used with alg


Detailed description of the ``patch`` mode
------------------------------------------

When the ``patch`` mode is available, it proposes a fix for each problem
identified.

Example
~~~~~~~

Running::

	make coccicheck MODE=patch COCCI=scripts/coccinelle/api/err_cast.cocci

will execute the following part of the SmPL script::

    <smpl>
    @ depends on !context && patch && !org && !report @
    expression x;
    @@

    - ERR_PTR(PTR_ERR(x))
    + ERR_CAST(x)
    </smpl>

This SmPL excerpt generates patch hunks on the standard output, as
illustrated below::

    diff -u -p a/crypto/ctr.c b/crypto/ctr.c
    --- a/crypto/ctr.c 2010-05-26 10:49:38.000000000 +0200
    +++ b/crypto/ctr.c 2010-06-03 23:44:49.000000000 +0200
    @@ -185,7 +185,7 @@ static struct crypto_instance *crypto_ct
 	alg = crypto_attr_alg(tb[1], CRYPTO_ALG_TYPE_CIPHER,
 				  CRYPTO_ALG_TYPE_MASK);
 	if (IS_ERR(alg))
    -		return ERR_PTR(PTR_ERR(alg));
    +		return ERR_CAST(alg);

 	/* Block size must be >= 4 bytes. */
 	err = -EINVAL;

Detailed description of the ``context`` mode
--------------------------------------------

``context`` highlights lines of interest and their context
in a diff-like style.

      **NOTE**: The diff-like output generated is NOT an applicable patch. The
      intent of the ``context`` mode is to highlight the important lines
      (annotated with minus, ``-``) and gives some surrounding context
      lines around. This output can be used with the diff mode of
      Emacs to review the code.

Example
~~~~~~~

Running::

	make coccicheck MODE=context COCCI=scripts/coccinelle/api/err_cast.cocci

will execute the following part of the SmPL script::

    <smpl>
    @ depends on context && !patch && !org && !report@
    expression x;
    @@

    * ERR_PTR(PTR_ERR(x))
    </smpl>

This SmPL excerpt generates diff hunks on the standard output, as
illustrated below::

    diff -u -p /home/user/linux/crypto/ctr.c /tmp/nothing
    --- /home/user/linux/crypto/ctr.c	2010-05-26 10:49:38.000000000 +0200
    +++ /tmp/nothing
    @@ -185,7 +185,6 @@ static struct crypto_instance *crypto_ct
 	alg = crypto_attr_alg(tb[1], CRYPTO_ALG_TYPE_CIPHER,
 				  CRYPTO_ALG_TYPE_MASK);
 	if (IS_ERR(alg))
    -		return ERR_PTR(PTR_ERR(alg));

 	/* Block size must be >= 4 bytes. */
 	err = -EINVAL;

Detailed description of the ``org`` mode
----------------------------------------

``org`` generates a report in the Org mode format of Emacs.

Example
~~~~~~~

Running::

	make coccicheck MODE=org COCCI=scripts/coccinelle/api/err_cast.cocci

will execute the following part of the SmPL script::

    <smpl>
    @r depends on !context && !patch && (org || report)@
    expression x;
    position p;
    @@

      ERR_PTR@p(PTR_ERR(x))

    @script:python depends on org@
    p << r.p;
    x << r.x;
    @@

    msg="ERR_CAST can be used with %s" % (x)
    msg_safe=msg.replace("[","@(").replace("]",")")
    coccilib.org.print_todo(p[0], msg_safe)
    </smpl>

This SmPL excerpt generates Org entries on the standard output, as
illustrated below::

    * TODO [[view:/home/user/linux/crypto/ctr.c::face=ovl-face1::linb=188::colb=9::cole=16][ERR_CAST can be used with alg]]
    * TODO [[view:/home/user/linux/crypto/authenc.c::face=ovl-face1::linb=619::colb=9::cole=16][ERR_CAST can be used with auth]]
    * TODO [[view:/home/user/linux/crypto/xts.c::face=ovl-face1::linb=227::colb=9::cole=16][ERR_CAST can be used with alg]]