Using gcov with the Linux kernel
================================

gcov profiling kernel support enables the use of GCC's coverage testing
tool gcov_ with the Linux kernel. Coverage data of a running kernel
is exported in gcov-compatible format via the "gcov" debugfs directory.
To get coverage data for a specific file, change to the kernel build
directory and use gcov with the ``-o`` option as follows (requires root)::

    # cd /tmp/linux-out
    # gcov -o /sys/kernel/debug/gcov/tmp/linux-out/kernel spinlock.c

This will create source code files annotated with execution counts
in the current directory. In addition, graphical gcov front-ends such
as lcov_ can be used to automate the process of collecting data
for the entire kernel and provide coverage overviews in HTML format.

Possible uses:

* debugging (has this line been reached at all?)
* test improvement (how do I change my test to cover these lines?)
* minimizing kernel configurations (do I need this option if the
  associated code is never run?)

.. _gcov: https://gcc.gnu.org/onlinedocs/gcc/Gcov.html
.. _lcov: http://ltp.sourceforge.net/coverage/lcov.php


Preparation
-----------

Configure the kernel with::

        CONFIG_DEBUG_FS=y
        CONFIG_GCOV_KERNEL=y

and to get coverage data for the entire kernel::

        CONFIG_GCOV_PROFILE_ALL=y

Note that kernels compiled with profiling flags will be significantly
larger and run slower. Also CONFIG_GCOV_PROFILE_ALL may not be supported
on all architectures.

Profiling data will only become accessible once debugfs has been
mounted::

        mount -t debugfs none /sys/kernel/debug


Customization
-------------

To enable profiling for specific files or directories, add a line
similar to the following to the respective kernel Makefile:

- For a single file (e.g. main.o)::

	GCOV_PROFILE_main.o := y

- For all files in one directory::

	GCOV_PROFILE := y

To exclude files from being profiled even when CONFIG_GCOV_PROFILE_ALL
is specified, use::

	GCOV_PROFILE_main.o := n

and::

	GCOV_PROFILE := n

Only files which are linked to the main kernel image or are compiled as
kernel modules are supported by this mechanism.


Files
-----

The gcov kernel support creates the following files in debugfs:

``/sys/kernel/debug/gcov``
	Parent directory for all gcov-related files.

``/sys/kernel/debug/gcov/reset``
	Global reset file: resets all coverage data to zero when
        written to.

``/sys/kernel/debug/gcov/path/to/compile/dir/file.gcda``
	The actual gcov data file as understood by the gcov
        tool. Resets file coverage data to zero when written to.

``/sys/kernel/debug/gcov/path/to/compile/dir/file.gcno``
	Symbolic link to a static data file required by the gcov
        tool. This file is generated by gcc when compiling with
        option ``-ftest-coverage``.


Modules
-------

Kernel modules may contain cleanup code which is only run during
module unload time. The gcov mechanism provides a means to collect
coverage data for such code by keeping a copy of the data associated
with the unloaded module. This data remains available through debugfs.
Once the module is loaded again, the associated coverage counters are
initialized with the data from its previous instantiation.

This behavior can be deactivated by specifying the gcov_persist kernel
parameter::

        gcov_persist=0

At run-time, a user can also choose to discard data for an unloaded
module by writing to its data file or the global reset file.


Separated build and test machines
---------------------------------

The gcov kernel profiling infrastructure is designed to work out-of-the
box for setups where kernels are built and run on the same machine. In
cases where the kernel runs on a separate machine, special preparations
must be made, depending on where the gcov tool is used:

a) gcov is run on the TEST machine

    The gcov tool version on the test machine must be compatible with the
    gcc version used for kernel build. Also the following files need to be
    copied from build to test machine:

    from the source tree:
      - all C source files + headers

    from the build tree:
      - all C source files + headers
      - all .gcda and .gcno files
      - all links to directories

    It is important to note that these files need to be placed into the
    exact same file system location on the test machine as on the build
    machine. If any of the path components is symbolic link, the actual
    directory needs to be used instead (due to make's CURDIR handling).

b) gcov is run on the BUILD machine

    The following files need to be copied after each test case from test
    to build machine:

    from the gcov directory in sysfs:
      - all .gcda files
      - all links to .gcno files

    These files can be copied to any location on the build machine. gcov
    must then be called with the -o option pointing to that directory.

    Example directory setup on the build machine::

      /tmp/linux:    kernel source tree
      /tmp/out:      kernel build directory as specified by make O=
      /tmp/coverage: location of the files copied from the test machine

      [user@build] cd /tmp/out
      [user@build] gcov -o /tmp/coverage/tmp/out/init main.c


Note on compilers
-----------------

GCC and LLVM gcov tools are not necessarily compatible. Use gcov_ to work with
GCC-generated .gcno and .gcda files, and use llvm-cov_ for Clang.

.. _gcov: https://gcc.gnu.org/onlinedocs/gcc/Gcov.html
.. _llvm-cov: https://llvm.org/docs/CommandGuide/llvm-cov.html

Build differences between GCC and Clang gcov are handled by Kconfig. It
automatically selects the appropriate gcov format depending on the detected
toolchain.


Troubleshooting
---------------

Problem
    Compilation aborts during linker step.

Cause
    Profiling flags are specified for source files which are not
    linked to the main kernel or which are linked by a custom
    linker procedure.

Solution
    Exclude affected source files from profiling by specifying
    ``GCOV_PROFILE := n`` or ``GCOV_PROFILE_basename.o := n`` in the
    corresponding Makefile.

Problem
    Files copied from sysfs appear empty or incomplete.

Cause
    Due to the way seq_file works, some tools such as cp or tar
    may not correctly copy files from sysfs.

Solution
    Use ``cat`` to read ``.gcda`` files and ``cp -d`` to copy links.
    Alternatively use the mechanism shown in Appendix B.


Appendix A: gather_on_build.sh
------------------------------

Sample script to gather coverage meta files on the build machine
(see 6a):

.. code-block:: sh

    #!/bin/bash

    KSRC=$1
    KOBJ=$2
    DEST=$3

    if [ -z "$KSRC" ] || [ -z "$KOBJ" ] || [ -z "$DEST" ]; then
      echo "Usage: $0 <ksrc directory> <kobj directory> <output.tar.gz>" >&2
      exit 1
    fi

    KSRC=$(cd $KSRC; printf "all:\n\t@echo \${CURDIR}\n" | make -f -)
    KOBJ=$(cd $KOBJ; printf "all:\n\t@echo \${CURDIR}\n" | make -f -)

    find $KSRC $KOBJ \( -name '*.gcno' -o -name '*.[ch]' -o -type l \) -a \
                     -perm /u+r,g+r | tar cfz $DEST -P -T -

    if [ $? -eq 0 ] ; then
      echo "$DEST successfully created, copy to test system and unpack with:"
      echo "  tar xfz $DEST -P"
    else
      echo "Could not create file $DEST"
    fi


Appendix B: gather_on_test.sh
-----------------------------

Sample script to gather coverage data files on the test machine
(see 6b):

.. code-block:: sh

    #!/bin/bash -e

    DEST=$1
    GCDA=/sys/kernel/debug/gcov

    if [ -z "$DEST" ] ; then
      echo "Usage: $0 <output.tar.gz>" >&2
      exit 1
    fi

    TEMPDIR=$(mktemp -d)
    echo Collecting data..
    find $GCDA -type d -exec mkdir -p $TEMPDIR/\{\} \;
    find $GCDA -name '*.gcda' -exec sh -c 'cat < $0 > '$TEMPDIR'/$0' {} \;
    find $GCDA -name '*.gcno' -exec sh -c 'cp -d $0 '$TEMPDIR'/$0' {} \;
    tar czf $DEST -C $TEMPDIR sys
    rm -rf $TEMPDIR

    echo "$DEST successfully created, copy to build system and unpack with:"
    echo "  tar xfz $DEST"
