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| v2.0/ |
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| From 7c60851daae39815fa696fc2265e838b1913f7ac Mon Sep 17 00:00:00 2001 | ||
| From: Wentao Zhang <[email protected]> | ||
| Date: Wed, 4 Sep 2024 18:13:07 -0500 | ||
| Subject: [RFC PATCH 0/4] Enable measuring the kernel's Source-based Code Coverage and MC/DC with Clang | ||
|
|
||
| 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. | ||
|
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||
| 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%). | ||
|
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||
| 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 | ||
|
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||
| The first two patches enable the llvm-cov infrastructure, where the first | ||
| enables source based code coverage and the second adds MC/DC support. The | ||
| next patch disables instrumentation for curve25519-x86_64.c for the same | ||
| reason as gcov. The final patch enables coverage for x86-64. | ||
|
|
||
| The choice to use a new Makefile variable LLVM_COV_PROFILE, instead of | ||
| reusing GCOV_PROFILE, was deliberate. More work needs to be done to | ||
| determine if it is appropriate to reuse the same flag. In addition, given | ||
| the fundamentally different approaches to instrumentation and the resulting | ||
| variation in coverage reports, there is a strong likelihood that coverage | ||
| type will need to be managed separately. | ||
|
|
||
| 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]. | ||
|
|
||
| Known Limitations: | ||
|
|
||
| Kernel code with logical expressions exceeding | ||
| LVM_COV_KERNEL_MCDC_MAX_CONDITIONS will produce a compiler warning. | ||
| Expressions with up to 47 conditions are found in the Linux kernel source | ||
| tree (as of v6.11), but 46 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. | ||
|
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||
| As of LLVM 19, certain expressions are still not covered, and will produce | ||
| build warnings when they are encountered: | ||
|
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||
| "[...] 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] | ||
|
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||
|
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||
| [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/[email protected]/ | ||
| [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 | ||
|
|
||
| --- | ||
| v1 -> v2: | ||
|
|
||
| * Rebased onto v6.11-rc6 from v6.11-rc4. | ||
| * llvm-cov/Kconfig: add CONFIG_LLVM_COV_PROFILE_ALL and be consistent with | ||
| GCOV conventions. | ||
| * Makefile.lib: let the default of LLVM_COV_PROFILE depend on | ||
| CONFIG_LLVM_COV_PROFILE_ALL instead of always being "y", and use | ||
| $(is-kernel-object) following 9c2d1328f88a "kbuild: provide reasonable | ||
| defaults for tool coverage". | ||
| * asm-generic/vmlinux.lds.h: use macro BOUNDED_SECTION_POST_LABEL following | ||
| 9b351be25360 "vmlinux.lds.h: add BOUNDED_SECTION* macros". | ||
| * Makefile.lib: "basetarget" -> "target-stem" following bf48d9b756b9 | ||
| "kbuild: change tool coverage variables to take the path relative to | ||
| $(obj)". | ||
| * kernel/trace: remove "LLVM_COV_PROFILE = :n" and let user manually enable | ||
| it with LLVM_COV_PROFILE_ALL, if they are sure of the performance impact. | ||
| * x86, kbuild: remove "LLVM_COV_PROFILE = :n" instances that have been | ||
| taken care of by $is-kernel-object. Also remove the one in | ||
| arch/x86/platform/efi/ as llvm-cov doesn't have const propagation issues | ||
| as gcov did (ac6119e7f25b "efi/x86: Disable instrumentation in the EFI | ||
| runtime handling code"). | ||
| * modfinal, kbuild: remove "LLVM_COV_PROFILE = :n" as llvm-cov doesn't use | ||
| function pointers so it doesn't have CFI-related issues like gcov did | ||
| (25a21fbb934a "kbuild: Disable GCOV for *.mod.o"). | ||
|
|
||
| --- | ||
|
|
||
| Wentao Zhang (4): | ||
| llvm-cov: add Clang's Source-based Code Coverage support | ||
| llvm-cov: add Clang's MC/DC support | ||
| x86: disable llvm-cov instrumentation | ||
| x86: enable llvm-cov support | ||
|
|
||
| Makefile | 9 ++ | ||
| arch/Kconfig | 1 + | ||
| arch/x86/Kconfig | 2 + | ||
| arch/x86/crypto/Makefile | 3 +- | ||
| arch/x86/kernel/vmlinux.lds.S | 2 + | ||
| include/asm-generic/vmlinux.lds.h | 36 +++++ | ||
| kernel/Makefile | 1 + | ||
| kernel/llvm-cov/Kconfig | 100 ++++++++++++ | ||
| kernel/llvm-cov/Makefile | 5 + | ||
| kernel/llvm-cov/fs.c | 253 ++++++++++++++++++++++++++++++ | ||
| kernel/llvm-cov/llvm-cov.h | 156 ++++++++++++++++++ | ||
| scripts/Makefile.lib | 23 +++ | ||
| scripts/mod/modpost.c | 2 + | ||
| 13 files changed, 592 insertions(+), 1 deletion(-) | ||
| 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 | ||
|
|
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