.. _kmidiff_label:
===========
kmidiff
===========
``kmidiff`` compares the binary Kernel Module Interfaces of two Linux
Kernel trees. The binary KMI is the interface that the Linux Kernel
exposes to its modules. The trees we are interested in here are the
result of the build of the Linux Kernel source tree.
General approach
=================
And example of how to build your kernel if you want to compare it to
another one using kmidiff is: ::
git clone -b v4.5 git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git linux/v4.5
cd linux/v4.5
make allyesconfig all
Then install the modules into a directory, for instance, the
build/modules sub-directory of the your kernel source tree: ::
mkdir build/modules
make modules_install INSTALL_MOD_DIR=build/modules
Then construct a list of interfaces exported by the kernel, that you
want to compare: ::
cat > kmi-whitelist << EOF
[kernel_4.5_kmi_whitelist]
init_task
schedule
dev_queue_xmit
__kmalloc
printk
EOF
Suppose you've done something similar for the v4.6 branch of the Linux
kernel, you now have these two directories: ``linux/v4.5`` and ``linux/v4.6``.
Their modules are present under the directories
``linux/v4.5/build/modules`` and ``linux/v4.6/build/modules``.
To Comparing their KMI ``kmidiff`` needs to know where to find the
vmlinux binaries and their associated modules. Here would be what the
command line looks like: ::
kmidiff \
--kmi-whitelist linux/v4.6/kmi-whitelist \
--vmlinux1 linux/v4.5/vmlinux \
--vmlinux2 linux/v4.6/vmlinux \
linux/v4.5/build/modules \
linux/v4.6/build/modules
.. include:: tools-use-libabigail.txt
Invocation
==========
More generally, ``kmidiff`` is invoked under the form: ::
kmidiff [options] <first-modules-dir> <second-modules-dir>
Environment
===========
By default, ``kmidiff`` compares all the interfaces (exported
functions and variables) between the Kernel and its modules. In
practice, though, some users might want to compare a subset of the
those interfaces.
By default, ``kmidiff`` uses debug information in the `DWARF`_ debug
info format, if present, otherwise it compares interfaces using `CTF`_
or `BTF`_ debug info formats, if present. Finally, if no debug info
in these formats is found, it only considers `ELF`_ symbols and report
about their addition or removal.
Users can then define a "white list" of the interfaces to compare.
Such a white list is a just a file in the "INI" format that looks
like: ::
[kernel_version_x86_64_whitelist]
function1_name
function2_name
global_variable1_name
....
Note that the name of the section (the name that is between the two
brackets) of that INI file just has to end with the string
"whitelist". So you can define the name you want, for instance
``[kernel_46_x86_64_whitelist]``.
Then each line of that whitelist file is the name of an exported
function or variable. Only those interfaces along with the types
reachable from their signatures are going to be compared by
``kmidiff`` recursively.
Note that by default kmidiff analyzes the types reachable from the
interfaces associated with `ELF`_ symbols that are defined and
exported by the `Linux Kernel`_ as being the union of the ``vmlinux``
binary and all its compiled modules. It then compares those
interfaces (along with their types).
Options
=======
* ``--help | -h``
Display a short help about the command and exit.
* ``--version | -v``
Display the version of the program and exit.
* ``--verbose``
Display some verbose messages while executing.
* ``--debug-info-dir1 | --d1`` <*di-path1*>
For cases where the debug information for the binaries of the
first Linux kernel is split out into separate files, tells
``kmidiff`` where to find those separate debug information files.
Note that *di-path* must point to the root directory under which
the debug information is arranged in a tree-like manner. Under
Red Hat based systems, that directory is usually
``<root>/usr/lib/debug``.
* ``--debug-info-dir2 | --d2`` <*di-path2*>
Like ``--debug-info-dir1``, this options tells ``kmidiff`` where
to find the split debug information for the binaries of the second
Linux kernel.
* ``--vmlinux1 | --l1`` <*path-to-first-vmlinux*>
Sets the path to the first ``vmlinux`` binary to consider. This
has to be the uncompressed vmlinux binary compiled with debug
info.
* ``--vmlinux2 | --l2`` <*path-to-first-vmlinux*>
Sets the path to the second ``vmlinux`` binary to consider. This
has to be the uncompressed vmlinux binary compiled with debug
info.
* ``--kmi-whitelist | -w`` <*path-to-interface-whitelist*>
Set the path to the white list of interfaces to compare while
comparing the Kernel Module Interface of the first kernel against
the one of the second kernel.
If this option is not provided, *all* the exported interfaces of
the two kernels are compared. That takes a lot of times and is
not necessarily meaningful because many interface are probably
meant to see their reachable types change.
So please, make sure you always use this option unless you really
know what you are doing.
* ``--suppressions | --suppr`` <*path-to-suppressions*>
Use a :ref:`suppression specification <suppr_spec_label>` file
located at *path-to-suppressions*. Note that this option can
appear multiple times on the command line. In that case, all of
the provided suppression specification files are taken into
account.
Please note that, by default, if this option is not provided, then
the :ref:`default suppression specification files
<abidiff_default_supprs_label>` are loaded .
* ``--no-change-categorization | -x``
This option disables the categorization of changes into harmless
and harmful changes. Note that this categorization is a
pre-requisite for the filtering of changes so this option disables
that filtering. The goal of this option is to speed-up the
execution of the program for cases where the graph of changes is
huge and where the user is just interested in looking at, for
instance, leaf node changes without caring about their possible
impact on interfaces.
* ``--ctf``
Extract ABI information from `CTF`_ debug information, if present,
in the Kernel and Modules.
* ``--btf``
Extract ABI information from `BTF`_ debug information, if present,
in the Kernel and Modules.
* ``--impacted-interfaces | -i``
Tell what interfaces got impacted by each individual ABI change.
* ``--full-impact | -f``
Emit a change report that shows the full impact of each change on
exported interfaces. This is the default kind of report emitted
by tools like ``abidiff`` or ``abipkgdiff``.
* ``--exported-interfaces-only``
When using this option, this tool analyzes the descriptions of the
types reachable by the interfaces (functions and variables)
associated with `ELF`_ symbols that are defined and exported by
the `Linux Kernel`_.
Otherwise, the tool also has the ability to analyze the
descriptions of the types reachable by the interfaces associated
with `ELF`_ symbols that are visible outside their translation
unit. This later possibility is however much more resource
intensive and results in much slower operations.
That is why this option is enabled by default.
* ``--allow-non-exported-interfaces``
When using this option, this tool analyzes the descriptions of the
types reachable by the interfaces (functions and variables) that
are visible outside of their translation unit. Once that analysis
is done, an ABI Corpus is constructed by only considering the
subset of types reachable from interfaces associated to `ELF`_
symbols that are defined and exported by the binary. It's that
final ABI corpus which is compared against another one.
The problem with that approach however is that analyzing all the
interfaces that are visible from outside their translation unit
can amount to a lot of data, leading to very slow operations.
Note that this option is turned off by default.
* ``--show-bytes``
Show sizes and offsets in bytes, not bits. This option is
activated by default.
* ``--show-bits``
Show sizes and offsets in bits, not bytes. By default, sizes and
offsets are shown in bytes.
* ``--show-hex``
Show sizes and offsets in hexadecimal base. This option is
activated by default.
* ``--show-dec``
Show sizes and offsets in decimal base.
.. _ELF: http://en.wikipedia.org/wiki/Executable_and_Linkable_Format
.. _ksymtab: http://en.wikipedia.org/wiki/Executable_and_Linkable_Format
.. _Linux Kernel: https://kernel.org
.. _DWARF: http://www.dwarfstd.org
.. _CTF: https://raw.githubusercontent.com/wiki/oracle/binutils-gdb/files/ctf-spec.pdf
.. _BTF: https://docs.kernel.org/bpf/btf.html