[RFC PATCH 0/3] Enable measuring the kernel's Source-based Code Coverage and MC/DC with Clang

Wentao Zhang
2024-08-24T23:06:53Z

This series adds support for building x86-64 kernels with Clang's Source-
based Code Coverage[1] in order to facilitate Modified Condition/Decision
Coverage (MC/DC)[2] that provably correlates to source code for all levels
of compiler optimization.

The newly added kernel/llvm-cov/ directory complements the existing gcov
implementation. Gcov works at the object code level which may better
reflect actual execution. However, Gcov lacks the necessary information to
correlate coverage measurement with source code location when compiler
optimization level is non-zero (which is the default when building the
kernel). In addition, gcov reports are occasionally ambiguous when
attempting to compare with source code level developer intent.

In the following gcov example from drivers/firmware/dmi_scan.c, an
expression with four conditions is reported to have six branch outcomes,
which is not ideally informative in many safety related use cases, such as
automotive, medical, and aerospace.

5: 1068: if (s == e || *e != '/' || !month || month > 12) {
branch 0 taken 5 (fallthrough)
branch 1 taken 0
branch 2 taken 5 (fallthrough)
branch 3 taken 0
branch 4 taken 0 (fallthrough)
branch 5 taken 5

On the other hand, Clang's Source-based Code Coverage instruments at the
compiler frontend which maintains an accurate mapping from coverage
measurement to source code location. Coverage reports reflect exactly how
the code is written regardless of optimization and can present advanced
metrics like branch coverage and MC/DC in a clearer way. Coverage report
for the same snippet by llvm-cov would look as follows:

1068| 5| if (s == e || *e != '/' || !month || month > 12) {
------------------
| Branch (1068:6): [True: 0, False: 5]
| Branch (1068:16): [True: 0, False: 5]
| Branch (1068:29): [True: 0, False: 5]
| Branch (1068:39): [True: 0, False: 5]
------------------

Clang has added MC/DC support as of its 18.1.0 release. MC/DC is a fine-
grained coverage metric required by many automotive and aviation industrial
standards for certifying mission-critical software [3].

In the following example from arch/x86/events/probe.c, llvm-cov gives the
MC/DC measurement for the compound logic decision at line 43.

43| 12| if (msr[bit].test && !msr[bit].test(bit, data))
------------------
|---> MC/DC Decision Region (43:8) to (43:50)
|
| Number of Conditions: 2
| Condition C1 --> (43:8)
| Condition C2 --> (43:25)
|
| Executed MC/DC Test Vectors:
|
| C1, C2 Result
| 1 { T, F = F }
| 2 { T, T = T }
|
| C1-Pair: not covered
| C2-Pair: covered: (1,2)
| MC/DC Coverage for Decision: 50.00%
|
------------------
44| 5| continue;

As the results suggest, during the span of measurement, only condition C2
(!msr[bit].test(bit, data)) is covered. That means C2 was evaluated to both
true and false, and in those test vectors C2 affected the decision outcome
independently. Therefore MC/DC for this decision is 1 out of 2 (50.00%).

To do a full kernel measurement, instrument the kernel with
LLVM_COV_KERNEL_MCDC enabled, and optionally set a
LLVM_COV_KERNEL_MCDC_MAX_CONDITIONS value (the default is six). Run the
testsuites, and collect the raw profile data under
/sys/kernel/debug/llvm-cov/profraw. Such raw profile data can be merged and
indexed, and used for generating coverage reports in various formats.

$ cp /sys/kernel/debug/llvm-cov/profraw vmlinux.profraw
$ llvm-profdata merge vmlinux.profraw -o vmlinux.profdata
$ llvm-cov show --show-mcdc --show-mcdc-summary \
--format=text --use-color=false -output-dir=coverage_reports \
-instr-profile vmlinux.profdata vmlinux

The first patch in this series enables Clang's Source-based Code Coverage
in the kernel. The second patch disables instrumentation in the same areas
that were disabled for kernel/gcov/. The third patch adds kernel
configuration directives to enable MC/DC instrumentation.

This work reuses code from a previous effort by Sami Tolvanen et al. [4].
Our aim is for source-based *code coverage* required for high assurance
(MC/DC) while [4] focused more on performance optimization.

This initial submission is restricted to x86-64. Support for other
architectures would need a bit more Makefile & linker script modification.
Informally we've confirmed that arm64 works and more are being tested.

Note that Source-based Code Coverage is Clang-specific and isn't compatible
with Clang's gcov support in kernel/gcov/. Currently, kernel/gcov/ is not
able to measure MC/DC without modifying CFLAGS_GCOV and it would face the
same issues in terms of source correlation as gcov in general does.

Some demo and results can be found in [5]. We will talk about this patch
series in the Refereed Track at LPC 2024 [6] and would appreciate the
community's feedback.

Known Limitations:

Kernel code with logical expressions exceeding
LVM_COV_KERNEL_MCDC_MAX_CONDITIONS will produce a compiler warning.
Expressions with up to 45 conditions are found in the Linux kernel source
tree, but 44 seems to be the max value before the build fails due to kernel
size. As of LLVM 19 the max number of conditions possible is 32767.

As of LLVM 19, certain expressions are still not covered, and will produce
build warnings when they are encountered:

"[...] if a boolean expression is embedded in the nest of another boolean
expression but separated by a non-logical operator, this is also not
supported. For example, in x = (a && b && c && func(d && f)), the d && f
case starts a new boolean expression that is separated from the other
conditions by the operator func(). When this is encountered, a warning
will be generated and the boolean expression will not be
instrumented." [7]

[1] https://clang.llvm.org/docs/SourceBasedCodeCoverage.html
[2] https://en.wikipedia.org/wiki/Modified_condition%2Fdecision_coverage
[3] https://digital-library.theiet.org/content/journals/10.1049/sej.1994.0025
[4] https://lore.kernel.org/lkml/20210407211704.367039-1-morbo@google.com/
[5] https://github.com/xlab-uiuc/linux-mcdc
[6] https://lpc.events/event/18/contributions/1718/
[7] https://clang.llvm.org/docs/SourceBasedCodeCoverage.html#mc-dc-instrumentation

Wentao Zhang (3):
[RFC PATCH 1/3] llvm-cov: add Clang's Source-based Code Coverage
support
[RFC PATCH 2/3] kbuild, llvm-cov: disable instrumentation in odd or
sensitive code
[RFC PATCH 3/3] llvm-cov: add Clang's MC/DC support

Makefile | 9 +
arch/Kconfig | 1 +
arch/x86/Kconfig | 1 +
arch/x86/boot/Makefile | 1 +
arch/x86/boot/compressed/Makefile | 1 +
arch/x86/entry/vdso/Makefile | 1 +
arch/x86/kernel/vmlinux.lds.S | 2 +
arch/x86/platform/efi/Makefile | 1 +
arch/x86/purgatory/Makefile | 1 +
arch/x86/realmode/rm/Makefile | 1 +
arch/x86/um/vdso/Makefile | 1 +
drivers/firmware/efi/libstub/Makefile | 2 +
include/asm-generic/vmlinux.lds.h | 38 ++++
kernel/Makefile | 1 +
kernel/llvm-cov/Kconfig | 65 +++++++
kernel/llvm-cov/Makefile | 5 +
kernel/llvm-cov/fs.c | 253 ++++++++++++++++++++++++++
kernel/llvm-cov/llvm-cov.h | 156 ++++++++++++++++
kernel/trace/Makefile | 1 +
scripts/Makefile.lib | 21 +++
scripts/Makefile.modfinal | 1 +
scripts/mod/modpost.c | 2 +
22 files changed, 565 insertions(+)
create mode 100644 kernel/llvm-cov/Kconfig
create mode 100644 kernel/llvm-cov/Makefile
create mode 100644 kernel/llvm-cov/fs.c
create mode 100644 kernel/llvm-cov/llvm-cov.h

--
2.45.2