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Merge pull request #10 from chuckwolber/port-v0.6-to-linux-6.11
Port v0.6 patches to current upstream kernel and promote them to v1.0 on a development branch.
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| From 0f3be98299c1331dd6a8d1fb0ad7bf94f18f2ebe Mon Sep 17 00:00:00 2001 | ||
| From: Wentao Zhang <[email protected]> | ||
| Date: Thu, 18 Jul 2024 13:14:06 -0500 | ||
| Subject: [RFC PATCH 0/3] Enable measuring the kernel's Source-based Code Coverage and MC/DC with Clang | ||
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| This patch series adds support for building x86-64 kernels with Clang's Source- | ||
| based Code Coverage and measuring modified condition/decision coverage (MC/DC). | ||
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| The newly added kernel/llvm-cov/ directory complements the existing gcov | ||
| implementation. gcov works at a lower level and may better reflect the actual | ||
| execution of object code. However, it lacks the necessary information to connect | ||
| coverage measurement back to source code locations and gcov reports are | ||
| sometimes confusing. With a nonzero optimization level (which is the default | ||
| when building the kernel), it's even harder to rebuild the mapping between | ||
| coverage reports and source code. In the following example from | ||
| drivers/firmware/dmi_scan.c, an expression with four leaf conditions are | ||
| reported to have six branch outcomes, which is not ideally informative in many | ||
| use cases. | ||
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| 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 | ||
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| On the other hand, Clang's Source-based Code Coverage [1] does instrumentation | ||
| at the compiler frontend and maintains accurate mapping from coverage measure to | ||
| source code locations. The generated reports can reflect exactly how the code is | ||
| written regardless of optimization and can present advanced metrics like branch | ||
| coverage and MC/DC in a more human-friendly way. Coverage report for the same | ||
| snippet by llvm-cov would look like the below: | ||
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| 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] | ||
| ------------------ | ||
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||
| Clang has also added MC/DC support since its release 18.1.0. MC/DC is a fine- | ||
| grained coverage metric required by many automotive and aviation industrial | ||
| standards for certifying mission-critical software [2]. In the following example | ||
| from arch/x86/events/probe.c, llvm-cov gives the MC/DC measure for the 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; | ||
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| As the results suggest, during the given 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 the shown 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, 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. | ||
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||
| $ 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 patch in the series enables Clang's Source-based Code Coverage for the | ||
| kernel. The second patch disables instrumenting the same set of files that were | ||
| skipped by kernel/gcov/ as well. The third patch enables MC/DC instrumentation | ||
| which is built on top of Source-based Code Coverage. | ||
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| This work reuses a portion of code from a previous effort by Sami Tolvanen et | ||
| al. [3], but we aim for source-based *code coverage* required for high assurance | ||
| (MC/DC) while [3] focused more on performance optimization. | ||
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| 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. | ||
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| 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. | ||
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| Some demo and results can be found in [4]. We will talk about this patch series | ||
| in the Refereed Track of LPC 2024 [5] and would really like to hear the | ||
| community's feedback. | ||
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| [1] https://clang.llvm.org/docs/SourceBasedCodeCoverage.html | ||
| [2] https://digital-library.theiet.org/content/journals/10.1049/sej.1994.0025 | ||
| [3] https://lore.kernel.org/linux-doc/[email protected]/ | ||
| [4] https://github.com/xlab-uiuc/linux-mcdc | ||
| [5] https://lpc.events/event/18/contributions/1718/ | ||
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| 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 | ||
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| 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 | 51 ++++++ | ||
| 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, 551 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 | ||
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| -- | ||
| 2.34.1 | ||
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