Re: RFC - kernel test result specification (KTAP)

From: Kees Cook
Date: Sat Aug 28 2021 - 04:20:50 EST

[+kernelci, +Guillaume]


Please keep me in CC for these kinds of proposals. And thanks for looking
at this again! Please understand that while I may be coming across as
rather negative here, I would like to have a rational and documented
specification for the kernel's test output, too. My main objection here
is that we already _have_ a specification, and it's already being parsed
by machines, so making changes without strong justification is going to
be met with resistance. :) So, with that in mind, on to my reply...

On Tue, Aug 10, 2021 at 04:25:03PM -0700, Rae Moar wrote:
> We are looking to further standardise the output format used by kernel
> test frameworks like kselftest and KUnit. Thus far we have used the
> TAP (Test Anything Protocol) specification, but it has been extended
> in many different ways, so we would like to agree on a common "Kernel
> TAP" (KTAP) format to resolve these differences. Thus, below is a
> draft of a specification of KTAP. Note that this specification is
> largely based on the current format of test results for KUnit tests.

The kernel's largest producer of TAP is kselftest, and the largest
consumer is LAVA[1]. I would want buy-in from at least those responsible
for either side of those two things. (And I'm one of the people working
on both sides of it.)

The fundamental purpose of the kernel's TAP is to have many independent
tests runnable and parseable, specifically without any embedded framework
knowledge (or, even, any knowledge of TAP).

The tests run by kselftest come from 2 different TAP-producing harnesses
(kselftest.h for C, kselftest/ for TAP-agnostic tests) as well
as several hand-rolled instances in Shell, Python, and C. There used to
be more, but I have been steadily fixing their syntax[2] and merging[3]
separate implementations for a while now.


> Additionally, this specification was heavily inspired by the KTAP
> specification draft by Tim Bird
> (
> However, there are some notable differences to his specification. One
> such difference is the format of nested tests is more fully specified
> in the following specification. However, they are specified in a way
> which may not be compatible with many kselftest nested tests.

I commented extensively on that thread. :)

> =====================
> Specification of KTAP
> =====================
> TAP, or the Test Anything Protocol is a format for specifying test
> results used by a number of projects. It's website and specification
> are found at: The Linux Kernel uses TAP
> output for test results. However, KUnit (and other Kernel testing
> frameworks such as kselftest) have some special needs for test results
> which don't gel perfectly with the original TAP specification. Thus, a
> "Kernel TAP" (KTAP) format is specified to extend and alter TAP to
> support these use-cases.
> KTAP Output consists of 5 major elements (all line-based):
> - The version line
> - Plan lines
> - Test case result lines

These are required.

> - Diagnostic lines

This is optional.

> - A bail out line

Bail out should be optional, and possibly not included at all. (See

> An important component in this specification of KTAP is the
> specification of the format of nested tests. This can be found in the
> section on nested tests below.
> The version line
> ----------------
> The first line of KTAP output must be the version line. As this
> specification documents the first version of KTAP, the recommended
> version line is "KTAP version 1". However, since all kernel testing
> frameworks use TAP version lines, "TAP version 14" and "TAP version
> 13" are all acceptable version lines. Version lines with other
> versions of TAP or KTAP will not cause the parsing of the test results
> to fail but it will produce an error.

Maybe "TAP version Linux.1" ? I don't want to needlessly break existing

> Plan lines
> ----------
> Plan lines must follow the format of "1..N" where N is the number of
> subtests. The second line of KTAP output must be a plan line, which
> indicates the number of tests at the highest level, such that the
> tests do not have a parent. Also, in the instance of a test having
> subtests, the second line of the test after the subtest header must be
> a plan line which indicates the number of subtests within that test.

I do not want "subtests" as a specification concept, only "nested

> Test case result lines
> ----------------------
> Test case result lines must have the format:
> <result> <number> [-] [<description>] [<directive>] [<diagnostic data>]

"[<diagnostic data>]" is not defined below. I think what you mean is
"directive details".

I suggest:

<result> <number>[ [- ]<description>][# <directive> [<directive details>]]

> The result can be either "ok", which indicates the test case passed,
> or "not ok", which indicates that the test case failed.


> The number represents the number of the test case or suite being
> performed. The first test case or suite must have the number 1 and the
> number must increase by 1 for each additional test case or result at
> the same level and within the same testing suite.

Yes, though parsers should handle ordering failures and missed test
results (this is the purpose of the "plan" line).

> The "-" character is optional.
> The description is a description of the test, generally the name of
> the test, and can be any string of words (can't include #). The
> description is optional.

Yes, though the optional "- " is strictly part of the optional

> The directive is used to indicate if a test was skipped. The format

The directive is a single word -- currently only "SKIP" is recognized
by TAP 14(?), but kselftest uses also XFAIL, XPASS, TIMEOUT, and
error. (Though I would love to hear what "error" is intended to be used
for, if different from regular "not ok".)

(This could also be called "context" rather than "directive".)

> for the directive is: "# SKIP [<skip_description>]". The
> skip_description is optional and can be any string of words to
> describe why the test was skipped.

I would call this "directive details".

> The result of the test case result
> line can be either "ok" or "not ok" if the skip directive is used.
> Finally, note that TAP 14 specification includes TODO directives but
> these are not supported for KTAP.
> Examples of test case result lines:
> Test passed:
> ok 1 - test_case_name
> Test was skipped:
> not ok 1 - test_case_name # SKIP test_case_name should be skipped
> Test failed:
> not_ok 1 - test_case_name

This isn't valid. No "_" is recognized for "ok" vs "not ok".

> Diagnostic lines
> ----------------
> Diagnostic lines are used for description of testing operations.
> Diagnostic lines are generally formatted as "#
> <diagnostic_description>", where the description can be any string.
> However, in practice, diagnostic lines are all lines that don't follow
> the format of any other KTAP line format.

I still think there should be a distinction between "diagnostic lines"
and "unknown lines". For example, if kselftest is running on a console,
the dmesg output may be intermixed, and possibly temporally offset. Such
lines may not be useful, and may not be strictly correlated, where-as
the output from kselftest is at least self-consistent.

> Diagnostic lines can be
> anywhere in the test output after the first two lines.

I don't see a reason for this strictness. They can be anywhere.

> There are a few
> special diagnostic lines.

No; diagnostic lines must have no meaning: they are for humans and nothing
else. If other details are needed for machines, we should explicitly
create new format lines. I made a mistake when I used a diagnostic line
for embedding the test names. :( There is a need for parsers to discover
the name of a given test, though, so we do likely need something for this.

> Diagnostic lines of the format "# Subtest:
> <test_name>" indicate the start of a test with subtests. Also,
> diagnostic lines of the format "# <test_name>: <description>" refer to
> a specific test and tend to occur before the test result line of that
> test but are optional.

I don't think the above should be included in the spec -- diag lines
should have no parseable meaning.

> Bail out line
> -------------
> A bail out line can occur anywhere in the KTAP output and will
> indicate that a test has crashed. The format of a bail out line is
> "Bail out! [<description>]", where the description can give
> information on why the bail out occurred and can be any string.

I'm not a fan of the Bail out line. It's not a problem, exactly, but I
find it redundant. If we want an "end of test" line, let's make one.
"Bail out" is a special case of exit. If we want to handle test exit,
let's define it. E.g. make kselftest's test summary lines not
diagnostic lines:

# FAILED: 85 / 87 tests passed.
# Totals: pass:83 fail:2 xfail:0 xpass:0 skip:2 error:0

Also, parsers should treat a new "TAP version..." line at the same
nesting level as indication that the prior test has ended (and any tests
without result lines should be considered failed).

> Nested tests
> ------------
> The new specification for KTAP will support an arbitrary number of
> nested subtests. Thus, tests can now have subtests and those subtests
> can have subtests. This can be useful to further categorize tests and
> organize test results.
> The new required format for a test with subtests consists of: a
> subtest header line, a plan line, all subtests, and a final test
> result line.
> The first line of the test must be the subtest header line with the
> format: "# Subtest: <test_name>".

Please no. There is no reason to force a nested test to suddenly have
to know about its test execution depth/environment. A subtest is just a
nested TAP result. That it is nested is only meaningful to the parser, not
the executing test, and it must have the same output, nested or not. (e.g.
running the test alone or running the test under a full kselftest run,
the only difference is the indentation level.)

> The second line of the test must be the plan line, which is formatted
> as "1..N", where N is the number of subtests.
> Following the plan line, all lines pertaining to the subtests will follow.
> Finally, the last line of the test is a final test result line with
> the format: "(ok|not ok) <number> [-] [<description>] [<directive>]
> [<diagnostic data>]", which follows the same format as the general
> test result lines described in this section. The result line should
> indicate the result of the subtests. Thus, if one of the subtests
> fail, the test should fail. The description in the final test result
> line should match the name of the test in the subtest header.
> An example format:
> KTAP version 1
> 1..1
> # Subtest: test_suite
> 1..2
> ok 1 - test_1
> ok 2 - test_2
> ok 1 - test_suite

Again, I see only downsides to this. Nesting for the spec is simple
indentation-based recursion. Let's just keep what we already have:

TAP version 13
# TAP version 13
# 1..2
# ok 1 - test_1
# ok 2 - test_2
ok 1 - test_suite

> An example format with multiple levels of nested testing:
> KTAP version 1
> 1..1
> # Subtest: test_suite
> 1..2
> # Subtest: sub_test_suite
> 1..2
> ok 1 - test_1
> ok 2 test_2
> ok 1 - sub_test_suite
> ok 2 - test
> ok 1 - test_suite
> In the instance that the plan line is missing, the end of the test
> will be denoted by the final result line containing a description that
> matches the name of the test given in the subtest header. Note that
> thus, if the plan line is missing and one of the subtests have a
> matching name to the test suite this will cause errors.

A plan line is required. No special cases are needed. :)

If nesting level is lost, a parser will understand the nested test to
have ended, but probably so did the test runner:

TAP version 13
TAP version 13
TAP version 13
ok 1 - test_1
ok 2 test_2
not ok 1 - sub test unfinished plan
ok 2 - test
not ok 1 - test_suite

> Lastly, indentation is also recommended for improved readability.

Indentation is _required_. :)

Whether this is "# " or " " I don't really care, as long as the change
is coordinated. " " is easier for humans to read, but may make parsing of
"unknown" lines more difficult for machines.

> Major differences between TAP 14 and KTAP specification
> -------------------------------------------------------
> Note that the major differences between TAP 14 and KTAP specification:
> - yaml and json are not allowed in diagnostic messages

Agreed -- these are overkill (and very difficult to implement as they
_follow_ the test result line: anything generating them has already
finished running).

> - TODO directive not allowed

I would just say "unrecognized".

> - KTAP allows for an arbitrary number of tests to be nested with
> specified nested test format


> Example of KTAP
> ---------------
> KTAP version 1
> 1..1
> # Subtest: test_suite
> 1..1
> # Subtest: sub_test_suite
> 1..2
> ok 1 - test_1
> ok 2 test_2
> ok 1 - sub_test_suite
> ok 1 - test_suite

For a good example, please include all the possible combinations (SKIP,
not ok, diagnostics, etc etc)

> =========================================
> Note on incompatibilities with kselftests
> =========================================
> To my knowledge, the above specification seems to generally accept the
> TAP format of many non-nested test results of kselftests.
> An example of a common kselftests TAP format for non-nested test
> results that are accepted by the above specification:
> TAP version 13
> 1..2
> # selftests: vDSO: vdso_test_gettimeofday
> # The time is 1628024856.096879
> ok 1 selftests: vDSO: vdso_test_gettimeofday
> # selftests: vDSO: vdso_test_getcpu
> # Could not find __vdso_getcpu
> ok 2 selftests: vDSO: vdso_test_getcpu # SKIP
> However, one major difference noted with kselftests is the use of more
> directives than the "# SKIP" directive. kselftest also supports XPASS
> and XFAIL directives. Some additional examples found in kselftests:
> not ok 5 selftests: netfilter: # TIMEOUT 45 seconds
> not ok 45 selftests: kvm: kvm_binary_stats_test # exit=127
> Should the specification be expanded to include these directives?

Yes. (Though "exit=" is a mistake in -- this should probably
be reported without the '#')

> However, the general format for kselftests with nested test results
> seems to differ from the above specification. It seems that a general
> format for nested tests is as follows:
> TAP version 13
> 1..2
> # selftests: membarrier: membarrier_test_single_thread
> # TAP version 13
> # 1..2
> # ok 1 sys_membarrier available
> # ok 2 sys membarrier invalid command test: command = -1, flags = 0,
> errno = 22. Failed as expected
> ok 1 selftests: membarrier: membarrier_test_single_thread
> # selftests: membarrier: membarrier_test_multi_thread
> # TAP version 13
> # 1..2
> # ok 1 sys_membarrier available
> # ok 2 sys membarrier invalid command test: command = -1, flags = 0,
> errno = 22. Failed as expected
> ok 2 selftests: membarrier: membarrier_test_multi_thread
> The major differences here, that do not match the above specification,
> are use of "# " as an indentation and using a TAP version line to
> denote a new test with subtests rather than the subtest header line
> described above. If these are widely utilized formats in kselftests,
> should we include both versions in the specification or should we
> attempt to agree on a single format for nested tests? I personally
> believe we should try to agree on a single format for nested tests.
> This would allow for a cleaner specification of KTAP and would reduce
> possible confusion.

We already use "# " and the nested TAP lines to denote subtests. Without
a good reason to change it, we should avoid the churn with the existing

> ====
> So what do people think about the above specification?
> How should we handle the differences with kselftests?

I'm probably a broken record by now, but kselftests _is_ the
specification. ;) What about it needs changing, and why?

> If this specification is accepted, where should the specification be documented?

I imagine some .rst file under Documentation/dev-tools/, linked to from
kselftest.rst and kunit/...rst

Kees Cook