xref: /linux/tools/objtool/Documentation/klp-write-tests.txt (revision 5b6dca02b3e2254d2fd5706d76d7ab4610a67296)

.. SPDX-License-Identifier: GPL-2.0

=====================================
Writing a test for objtool's klp code
=====================================

Instructions for adding a test to tools/objtool/tests/.  Read
klp-test-design.txt first if you need to know how the harness works; this
document is the procedure and the rules.

Two kinds of request bring you here:

* *"write a test for commit <sha>"* -- a fix went in without one.
* *"port the test at <location>, written against another harness"* -- a case
  exists elsewhere and should live in tree.

Both follow the same procedure.


The one rule that matters
=========================

**A test is not finished until you have watched it fail.**

Break the thing it guards -- revert the fix, or sabotage the exact line -- and
confirm the test fails.  Then restore and confirm it passes.  A test that has
never failed is not known to test anything, and this suite has produced
several that passed against deliberately broken code:

* an alternatives fixture whose empty entry pointed at its own end label rather
  than the neighbour's replacement, so the bug it guarded made no difference;
* a sympos fixture where symbol-table order and address order agreed, so
  counting and reading the linked image gave the same answer;
* a string fixture using a named ``char[]``, which never reached the
  contents-hashing path because that keys on ``SHF_STRINGS``;
* a static array the compiler proved constant, folded to zero, and emitted no
  relocation for -- so the two builds were byte-identical.

Every one looked correct.  Say in the commit message how you verified, and if
you could not isolate the behaviour to a single line, **say that too** rather
than implying otherwise.


Procedure
=========

1. **Read the fix.**  What input reaches the broken line?  What is observable
   in the output object when it misbehaves -- a missing section, a relocation
   naming the wrong symbol, an unchanged checksum, a rejected build?  If
   nothing is observable, stop and say so; see `When to give up`_.

2. **Decide where it lives.**  ``generic/`` unless the fixture needs
   architecture-specific assembly or the behaviour is architecture-specific,
   in which case ``x86/`` (or a new directory named for the architecture).

3. **Write the fixture** in the same directory's ``fixtures/``.  Reuse an
   existing one if it already produces the shape; add a ``-D`` knob rather
   than copying a fixture to change one line.

4. **Write the test.**  Assert the *premise* before the result -- see
   `State the premise`_.

5. **Verify by breaking the code.**  Then restore.

6. **Run the whole suite under both compilers**::

       make -C tools/objtool tests
       CC=clang make -C tools/objtool tests

7. **Commit** the test and its fixture together, alone.  One test per commit.


Writing the fixture
===================

Fixtures are freestanding C.  No kernel headers -- write out the kernel
structure by hand if you need one, as the existing special-section fixtures do.

Every fixture needs a ``.modinfo`` name, because klp diff reads the object's
module name from it::

    static const char __modinfo[]
        __attribute__((section(".modinfo"), used, aligned(1))) = "\0name=vmlinux";

Use ``MODNAME`` if the test needs to vary it.  Note that most fixtures hardcode
``vmlinux``: passing ``-DMODNAME`` to one that does silently does nothing and
the test quietly becomes a vmlinux test.

The patched build is selected with ``-DPATCHED``.  For a fixture with several
variants, gate each on both, so the original is always the baseline::

    #if defined(PATCHED) && defined(WHICH_CALL)
        r = callee_b(x);
    #else
        r = callee_a(x);
    #endif

and select one per build: ``build_pair foo.c -DWHICH_CALL``.  Without the
``defined(PATCHED)`` the flag applies to *both* builds and nothing differs.

Traps that have bitten before
-----------------------------

* **The compiler optimises your fixture away.**  A static never written is
  proved constant, its reads folded, and no relocation emitted.  Add a writer
  the compiler cannot see through.
* **String literals versus named arrays.**  The contents-hashing path keys on
  ``SHF_STRINGS``, which the compiler sets on the mergeable section a *literal*
  lands in, not on a ``char[]`` given a section of its own.
* **Per-function sections hide movement.**  With the default
  ``-ffunction-sections`` every function sits at offset 0 of its own section,
  so nothing ever moves.  A test about position needs
  ``build_pair foo.c -fno-function-sections``.
* **Special sections need boundaries.**  Either an entsize on the section or an
  ``ANNOTATE_DATA_SPECIAL`` annotation, or klp diff reports "missing special
  section entsize or annotations".  Their targets need real (global) symbols,
  or it reports "failed to convert reloc sym".
* **Prefer letting objtool generate what objtool generates.**
  ``.static_call_sites``, ``.mcount_loc``, ``.ibt_endbr_seal`` and ORC come
  from its check pass.  Call ``run_objtool_check --mcount`` and let it build
  them; a hand-written copy tests your reading of the format, not the format.

  Write one by hand only when the test needs a shape objtool will not produce,
  and say so in the fixture.  generic/fixtures/static_call.c does: objtool
  emits the site but not the ``ANNOTATE_DATA_SPECIAL`` that describes its
  boundaries -- those come from the kernel's macros -- so a fixture which has
  to vary whether the annotation is there writes both itself.


Writing the test
================

Start from the shortest existing test, generic/test-basic.sh.

State the premise
-----------------

A test which asserts only on the output passes when the compiler never emitted
the construct in the first place, and reads as coverage it does not have.  Say
what the input must contain::

    assert_input_section __jump_table     # the fixture must produce it -> fail
    require_input_section .kcfi_traps     # this compiler may not      -> skip

Prefer ``assert_*``.  Reach for ``require_*`` only where absence genuinely
depends on compiler version or flags, and follow it with something
unconditional so the test can never be entirely vacuous.

Where a test would otherwise duplicate a sibling, assert what makes it
different.  ``test-jump-label-module-static-key`` checks that the key really is
reached through its section symbol -- without that it is a second copy of
``test-jump-label-module-key``.

Assert both directions
----------------------

Check that the right thing happened *and* that the wrong thing did not.  A klp
diff which clones everything is as wrong as one which clones nothing::

    assert_patched changed
    assert_not_patched untouched

Skips
-----

* ``gcc_only``/``clang_only`` -- a settled fact about the compiler.  Declared,
  so it reads as expected forever.
* ``probe_skip`` -- this toolchain did not produce the construct.  Include what
  to do about it if there is anything::

      probe_skip "no matching clang/lld pair for a ThinLTO link; set THIN_CC and THIN_LD to one"

* A bare ``skip`` is **counted as a failure**.  Never use it.

Standing in for a kernel configuration
--------------------------------------

``FIXTURE_CFLAGS`` is what a fixture is built with.  Since objtool reads no
``.config``, changing these flags is how a test covers a configuration without
building a kernel.  Two ways:

* trailing arguments to ``build_pair``/``build_one``, which win, and cover
  anything expressible as a negation::

      build_pair foo.c -fno-function-sections

* otherwise assign ``FIXTURE_CFLAGS`` before building.

Either way **say in a comment which kernel configuration the change stands in
for**.  A flag with no stated motive is indistinguishable from a mistake.


What the test's comment must say
================================

The comment at the top is the test's justification.  It should let a reader
decide, without archaeology, whether a skip or a failure matters.  Include:

* **what breaks** in the running kernel if the behaviour regresses -- not the
  mechanism, the consequence;
* **why it is not caught otherwise**, which is usually "nothing fails at build
  time";
* **the fix commit** it guards, if there is one;
* **anything load-bearing about the fixture** that is not obvious, especially
  anything you got wrong first.

That last point is the one people skip.  If the fixture has to be built without
per-function sections, or the static must not be named ``__warned``, or the key
must be file-local -- write it down, or the next person will simplify it away.


Porting a test from another harness
===================================

Read the original's *case*, not its code.  The other harness probably builds a
real kernel module; here you write freestanding C.  A transliteration will
usually test something else.

* Work out which objtool behaviour the case exercises, then produce that shape
  the cheapest way here.
* Verify by breaking the code, exactly as for a new test -- a port is not
  correct because the original was.
* If the original names a fix commit, cite it.
* Credit the source in the commit message with the trailers the original
  carried, followed by your own.

Sometimes the port shows the case is already covered, and sometimes it shows
the case cannot be reproduced here.  Both are results; report them rather than
committing something that passes vacuously.


When to give up
===============

Some behaviour cannot be reached from a compiled fixture.  Say so, with what
you tried, instead of committing a test that passes either way.  Examples that
were genuinely abandoned:

* a memory leak -- needs valgrind, not an assertion on ELF;
* ``mkstemp`` with long paths, and other I/O edge cases;
* a NULL dereference reachable only through a debug path;
* changes made redundant by a fallback: removing the code changes no output
  because something else already handles the case;
* a fix whose code has since been rewritten, so there is nothing left to
  revert.

Also stop when the behaviour depends on something outside the fixture's
control -- an ELF library's handling of empty sections, or a compiler version's
naming of anonymous data.  A test which passes for you and skips for everyone
else is worse than none.


Checklist
=========

Before committing:

* the test fails with the code broken, and passes with it fixed
* the whole suite passes under **both** gcc and clang
* the premise is asserted, not assumed
* both directions are asserted where that applies
* no bare ``skip``
* the fixture is in the same directory as the test
* the comment names the consequence, the fix commit, and anything load-bearing
* the commit contains one test and its fixtures, and nothing else
* the commit message says how you verified it