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When enabling Context Analysis (CONTEXT_ANALYSIS := y) in arch/x86/kvm code, Clang's Thread Safety Analysis failed to recognize that identical per_cpu() accesses refer to the same lock: | CC [M] arch/x86/kvm/vmx/posted_intr.o | arch/x86/kvm/vmx/posted_intr.c:186:2: error: releasing raw_spinlock '__ptr + __per_cpu_offset[vcpu->cpu]' that was not held [-Werror,-Wthread-safety-analysis] | 186 | raw_spin_unlock(&per_cpu(wakeup_vcpus_on_cpu_lock, vcpu->cpu)); | | ^ | ./include/linux/spinlock.h:276:32: note: expanded from macro 'raw_spin_unlock' | 276 | #define raw_spin_unlock(lock) _raw_spin_unlock(lock) | | ^ | arch/x86/kvm/vmx/posted_intr.c:207:1: error: raw_spinlock '__ptr + __per_cpu_offset[vcpu->cpu]' is still held at the end of function [-Werror,-Wthread-safety-analysis] | 207 | } | | ^ | arch/x86/kvm/vmx/posted_intr.c:182:2: note: raw_spinlock acquired here | 182 | raw_spin_lock_nested(&per_cpu(wakeup_vcpus_on_cpu_lock, vcpu->cpu), | | ^ | ./include/linux/spinlock.h:235:2: note: expanded from macro 'raw_spin_lock_nested' | 235 | _raw_spin_lock(((void)(subclass), (lock))) | | ^ | 2 errors generated. This occurred because the default RELOC_HIDE() implementation (used by the per-CPU macros) is a statement expression containing an intermediate 'unsigned long' variable (this version appears to predate Git history). While the analysis strips away inner casts when resolving pointer aliases, it stops when encountering intermediate non-pointer variables (this is Thread Safety Analysis specific and irrelevant for codegen). This prevents the analysis from concluding that the pointers passed to e.g. raw_spin_lock() and raw_spin_unlock() were identical when per-CPU accessors are used. Simplify RELOC_HIDE() to a single expression. This preserves the intent of obfuscating UB-introducing out-of-bounds pointer calculations from the compiler via the 'unsigned long' cast, but allows the alias analysis to successfully resolve the pointers. Using a recent Clang version, I observe that generated code remains the same for vmlinux; the intermediate variable was already being optimized away (for any respectable modern compiler, not doing so would be an optimizer bug). Note that GCC provides its own version of RELOC_HIDE(), so this change only affects Clang builds. Add a test case to lib/test_context-analysis.c to catch any regressions. Reported-by: Bart Van Assche <bvanassche@acm.org> Reported-by: Sean Christopherson <seanjc@google.com> Signed-off-by: Marco Elver <elver@google.com> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> Reviewed-by: Nathan Chancellor <nathan@kernel.org> Link: https://lore.kernel.org/all/e3946223-4543-4a76-a328-9c6865e95192@acm.org/ Link: https://patch.msgid.link/20260319135245.1420780-1-elver@google.com
Linux kernel ============ The Linux kernel is the core of any Linux operating system. It manages hardware, system resources, and provides the fundamental services for all other software. Quick Start ----------- * Report a bug: See Documentation/admin-guide/reporting-issues.rst * Get the latest kernel: https://kernel.org * Build the kernel: See Documentation/admin-guide/quickly-build-trimmed-linux.rst * Join the community: https://lore.kernel.org/ Essential Documentation ----------------------- All users should be familiar with: * Building requirements: Documentation/process/changes.rst * Code of Conduct: Documentation/process/code-of-conduct.rst * License: See COPYING Documentation can be built with make htmldocs or viewed online at: https://www.kernel.org/doc/html/latest/ Who Are You? ============ Find your role below: * New Kernel Developer - Getting started with kernel development * Academic Researcher - Studying kernel internals and architecture * Security Expert - Hardening and vulnerability analysis * Backport/Maintenance Engineer - Maintaining stable kernels * System Administrator - Configuring and troubleshooting * Maintainer - Leading subsystems and reviewing patches * Hardware Vendor - Writing drivers for new hardware * Distribution Maintainer - Packaging kernels for distros * AI Coding Assistant - LLMs and AI-powered development tools For Specific Users ================== New Kernel Developer -------------------- Welcome! Start your kernel development journey here: * Getting Started: Documentation/process/development-process.rst * Your First Patch: Documentation/process/submitting-patches.rst * Coding Style: Documentation/process/coding-style.rst * Build System: Documentation/kbuild/index.rst * Development Tools: Documentation/dev-tools/index.rst * Kernel Hacking Guide: Documentation/kernel-hacking/hacking.rst * Core APIs: Documentation/core-api/index.rst Academic Researcher ------------------- Explore the kernel's architecture and internals: * Researcher Guidelines: Documentation/process/researcher-guidelines.rst * Memory Management: Documentation/mm/index.rst * Scheduler: Documentation/scheduler/index.rst * Networking Stack: Documentation/networking/index.rst * Filesystems: Documentation/filesystems/index.rst * RCU (Read-Copy Update): Documentation/RCU/index.rst * Locking Primitives: Documentation/locking/index.rst * Power Management: Documentation/power/index.rst Security Expert --------------- Security documentation and hardening guides: * Security Documentation: Documentation/security/index.rst * LSM Development: Documentation/security/lsm-development.rst * Self Protection: Documentation/security/self-protection.rst * Reporting Vulnerabilities: Documentation/process/security-bugs.rst * CVE Procedures: Documentation/process/cve.rst * Embargoed Hardware Issues: Documentation/process/embargoed-hardware-issues.rst * Security Features: Documentation/userspace-api/seccomp_filter.rst Backport/Maintenance Engineer ----------------------------- Maintain and stabilize kernel versions: * Stable Kernel Rules: Documentation/process/stable-kernel-rules.rst * Backporting Guide: Documentation/process/backporting.rst * Applying Patches: Documentation/process/applying-patches.rst * Subsystem Profile: Documentation/maintainer/maintainer-entry-profile.rst * Git for Maintainers: Documentation/maintainer/configure-git.rst System Administrator -------------------- Configure, tune, and troubleshoot Linux systems: * Admin Guide: Documentation/admin-guide/index.rst * Kernel Parameters: Documentation/admin-guide/kernel-parameters.rst * Sysctl Tuning: Documentation/admin-guide/sysctl/index.rst * Tracing/Debugging: Documentation/trace/index.rst * Performance Security: Documentation/admin-guide/perf-security.rst * Hardware Monitoring: Documentation/hwmon/index.rst Maintainer ---------- Lead kernel subsystems and manage contributions: * Maintainer Handbook: Documentation/maintainer/index.rst * Pull Requests: Documentation/maintainer/pull-requests.rst * Managing Patches: Documentation/maintainer/modifying-patches.rst * Rebasing and Merging: Documentation/maintainer/rebasing-and-merging.rst * Development Process: Documentation/process/maintainer-handbooks.rst * Maintainer Entry Profile: Documentation/maintainer/maintainer-entry-profile.rst * Git Configuration: Documentation/maintainer/configure-git.rst Hardware Vendor --------------- Write drivers and support new hardware: * Driver API Guide: Documentation/driver-api/index.rst * Driver Model: Documentation/driver-api/driver-model/driver.rst * Device Drivers: Documentation/driver-api/infrastructure.rst * Bus Types: Documentation/driver-api/driver-model/bus.rst * Device Tree Bindings: Documentation/devicetree/bindings/ * Power Management: Documentation/driver-api/pm/index.rst * DMA API: Documentation/core-api/dma-api.rst Distribution Maintainer ----------------------- Package and distribute the kernel: * Stable Kernel Rules: Documentation/process/stable-kernel-rules.rst * ABI Documentation: Documentation/ABI/README * Kernel Configuration: Documentation/kbuild/kconfig.rst * Module Signing: Documentation/admin-guide/module-signing.rst * Kernel Parameters: Documentation/admin-guide/kernel-parameters.rst * Tainted Kernels: Documentation/admin-guide/tainted-kernels.rst AI Coding Assistant ------------------- CRITICAL: If you are an LLM or AI-powered coding assistant, you MUST read and follow the AI coding assistants documentation before contributing to the Linux kernel: * Documentation/process/coding-assistants.rst This documentation contains essential requirements about licensing, attribution, and the Developer Certificate of Origin that all AI tools must comply with. Communication and Support ========================= * Mailing Lists: https://lore.kernel.org/ * IRC: #kernelnewbies on irc.oftc.net * Bugzilla: https://bugzilla.kernel.org/ * MAINTAINERS file: Lists subsystem maintainers and mailing lists * Email Clients: Documentation/process/email-clients.rst