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.

Note

Linux and macOS development environments are supported, but not Windows.

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 Semantic Patch Language(SmPL) grammar documentation refer to:

http://coccinelle.lip6.fr/documentation.php

Using Coccinelle on Zephyr

coccicheck checker is the front-end to the Coccinelle infrastructure and has various modes:

Four basic modes are defined: patch, report, context, and org. The mode to use is specified by setting --mode=<mode> or -m=<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:

./scripts/coccicheck --mode=report

To produce patches, run:

./scripts/coccicheck --mode=patch

The coccicheck target applies every semantic patch available in the sub-directories of scripts/coccinelle to the entire source code tree.

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

To enable verbose messages set --verbose=1 option, for example:

./scripts/coccicheck --mode=report --verbose=1

Coccinelle parallelization

By default, coccicheck tries to run as parallel as possible. To change the parallelism, set the --jobs=<number> option. For example, to run across 4 CPUs:

./scripts/coccicheck --mode=report --jobs=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, the return value of the coccicheck command captures this return value.

Using Coccinelle with a single semantic patch

The option --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:

./scripts/coccicheck --mode=report --cocci=<example.cocci>

or:

./scripts/coccicheck --mode=report --cocci=./path/to/<example.cocci>

Controlling which files are processed by Coccinelle

By default the entire source tree is checked.

To apply Coccinelle to a specific directory, pass the path of specific directory as an argument.

For example, to check drivers/usb/ one may write:

./scripts/coccicheck --mode=patch drivers/usb/

The report mode is the default. You can select another one with the --mode=<mode> option 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 verbose option, 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.err option to coccicheck. For instance:

rm -f cocci.err
./scripts/coccicheck --mode=patch --debug=cocci.err
cat cocci.err

Debugging support is only supported when using Coccinelle >= 1.0.2.

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

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/.

The cocci script should have the following properties:

  • The script must have report mode.
  • The first few lines should state the purpose of the script using /// comments . Usually, this message would be used as the commit log when proposing a patch based on the script.

Example

/// Use ARRAY_SIZE instead of dividing sizeof array with sizeof an element
  • A more detailed information about the script with exceptional cases or false positives (if any) can be listed using //# comments.

Example

//# This makes an effort to find cases where ARRAY_SIZE can be used such as
//# where there is a division of sizeof the array by the sizeof its first
//# element or by any indexed element or the element type. It replaces the
//# division of the two sizeofs by ARRAY_SIZE.
  • Confidence: It is a property defined to specify the accuracy level of the script. It can be either High, Moderate or Low depending upon the number of false positives observed.

Example

// Confidence: High
  • Virtual rules: These are required to support the various modes framed in the script. The virtual rule specified in the script should have the corresponding mode handling rule.

Example

virtual context

@depends on context@
type T;
T[] E;
@@
(
* (sizeof(E)/sizeof(*E))
|
* (sizeof(E)/sizeof(E[...]))
|
* (sizeof(E)/sizeof(T))
)

Detailed description of the report mode

report generates a list in the following format:

file:line:column-column: message

Example

Running:

./scripts/coccicheck --mode=report --cocci=scripts/coccinelle/array_size.cocci

will execute the following part of the SmPL script:

<smpl>

@r depends on (org || report)@
type T;
T[] E;
position p;
@@
(
(sizeof(E)@p /sizeof(*E))
|
(sizeof(E)@p /sizeof(E[...]))
|
(sizeof(E)@p /sizeof(T))
)

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

msg="WARNING: Use ARRAY_SIZE"
coccilib.report.print_report(p[0], msg)

</smpl>

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

ext/hal/nxp/mcux/drivers/lpc/fsl_wwdt.c:66:49-50: WARNING: Use ARRAY_SIZE
ext/hal/nxp/mcux/drivers/lpc/fsl_ctimer.c:74:53-54: WARNING: Use ARRAY_SIZE
ext/hal/nxp/mcux/drivers/imx/fsl_dcp.c:944:45-46: WARNING: Use ARRAY_SIZE

Detailed description of the patch mode

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

Example

Running:

./scripts/coccicheck --mode=patch --cocci=scripts/coccinelle/misc/array_size.cocci

will execute the following part of the SmPL script:

<smpl>

@depends on patch@
type T;
T[] E;
@@
(
- (sizeof(E)/sizeof(*E))
+ ARRAY_SIZE(E)
|
- (sizeof(E)/sizeof(E[...]))
+ ARRAY_SIZE(E)
|
- (sizeof(E)/sizeof(T))
+ ARRAY_SIZE(E)
)

</smpl>

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

diff -u -p a/ext/lib/encoding/tinycbor/src/cborvalidation.c b/ext/lib/encoding/tinycbor/src/cborvalidation.c
--- a/ext/lib/encoding/tinycbor/src/cborvalidation.c
+++ b/ext/lib/encoding/tinycbor/src/cborvalidation.c
@@ -325,7 +325,7 @@ static inline CborError validate_number(
static inline CborError validate_tag(CborValue *it, CborTag tag, int flags, int recursionLeft)
{
  CborType type = cbor_value_get_type(it);
-    const size_t knownTagCount = sizeof(knownTagData) / sizeof(knownTagData[0]);
+    const size_t knownTagCount = ARRAY_SIZE(knownTagData);
   const struct KnownTagData *tagData = knownTagData;
   const struct KnownTagData * const knownTagDataEnd = knownTagData + knownTagCount;

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:

./scripts/coccicheck --mode=context --cocci=scripts/coccinelle/array_size.cocci

will execute the following part of the SmPL script:

<smpl>

@depends on context@
type T;
T[] E;
@@
(
* (sizeof(E)/sizeof(*E))
|
* (sizeof(E)/sizeof(E[...]))
|
* (sizeof(E)/sizeof(T))
)

</smpl>

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

diff -u -p ext/lib/encoding/tinycbor/src/cborvalidation.c /tmp/nothing/ext/lib/encoding/tinycbor/src/cborvalidation.c
--- ext/lib/encoding/tinycbor/src/cborvalidation.c
+++ /tmp/nothing/ext/lib/encoding/tinycbor/src/cborvalidation.c
@@ -325,7 +325,6 @@ static inline CborError validate_number(
static inline CborError validate_tag(CborValue *it, CborTag tag, int flags, int recursionLeft)
{
  CborType type = cbor_value_get_type(it);
-    const size_t knownTagCount = sizeof(knownTagData) / sizeof(knownTagData[0]);
   const struct KnownTagData *tagData = knownTagData;
   const struct KnownTagData * const knownTagDataEnd = knownTagData + knownTagCount;

Detailed description of the org mode

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

Example

Running:

./scripts/coccicheck --mode=org --cocci=scripts/coccinelle/misc/array_size.cocci

will execute the following part of the SmPL script:

<smpl>

@r depends on (org || report)@
type T;
T[] E;
position p;
@@
(
(sizeof(E)@p /sizeof(*E))
|
(sizeof(E)@p /sizeof(E[...]))
|
(sizeof(E)@p /sizeof(T))
)

@script:python depends on org@
p << r.p;
@@
coccilib.org.print_todo(p[0], "WARNING should use ARRAY_SIZE")

</smpl>

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

* TODO [[view:ext/lib/encoding/tinycbor/src/cborvalidation.c::face=ovl-face1::linb=328::colb=52::cole=53][WARNING should use ARRAY_SIZE]]

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