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Wed, 04 Sep 2024 21:33:32 -0700 (PDT) From: Wentao Zhang To: wentaoz5@illinois.edu Cc: Matt.Kelly2@boeing.com, akpm@linux-foundation.org, andrew.j.oppelt@boeing.com, anton.ivanov@cambridgegreys.com, ardb@kernel.org, arnd@arndb.de, bhelgaas@google.com, bp@alien8.de, chuck.wolber@boeing.com, dave.hansen@linux.intel.com, dvyukov@google.com, hpa@zytor.com, jinghao7@illinois.edu, johannes@sipsolutions.net, jpoimboe@kernel.org, justinstitt@google.com, kees@kernel.org, kent.overstreet@linux.dev, linux-arch@vger.kernel.org, linux-efi@vger.kernel.org, linux-kbuild@vger.kernel.org, linux-kernel@vger.kernel.org, linux-trace-kernel@vger.kernel.org, linux-um@lists.infradead.org, llvm@lists.linux.dev, luto@kernel.org, marinov@illinois.edu, masahiroy@kernel.org, maskray@google.com, mathieu.desnoyers@efficios.com, matthew.l.weber3@boeing.com, mhiramat@kernel.org, mingo@redhat.com, morbo@google.com, nathan@kernel.org, ndesaulniers@google.com, oberpar@linux.ibm.com, paulmck@kernel.org, peterz@infradead.org, richard@nod.at, rostedt@goodmis.org, samitolvanen@google.com, samuel.sarkisian@boeing.com, steven.h.vanderleest@boeing.com, tglx@linutronix.de, tingxur@illinois.edu, tyxu@illinois.edu, x86@kernel.org Subject: [PATCH v2 0/4] Enable measuring the kernel's Source-based Code Coverage and MC/DC with Clang Date: Wed, 4 Sep 2024 23:32:41 -0500 Message-ID: <20240905043245.1389509-1-wentaoz5@illinois.edu> X-Mailer: git-send-email 2.45.2 In-Reply-To: <20240824230641.385839-1-wentaoz5@illinois.edu> References: <20240824230641.385839-1-wentaoz5@illinois.edu> Precedence: bulk X-Mailing-List: linux-efi@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 From: Wentao Zhang 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 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. 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 --- 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