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On EV6 and later the hardware records exception status bits in the FPCR before delivering a software completion trap, and those bits can be wrong for the instruction that trapped. Converting a double that is exactly representable as a subnormal float sets FPCR_UNF even though the result is exact, and an underflow trap additionally sets FPCR_INE even when the emulated operation turns out to be exact. alpha_fp_emul() only wrote the FPCR when soft-fp raised an exception, so whenever it determined that the instruction was exact the fabricated bits stayed in the FPCR and were reported to user space by fetestexcept(). Pass the exception summary register down from do_entArith() so the handler can tell which exceptions the hardware attributed to the trapping instruction, and always write the FPCR. Clear the exceptions that the trap reported but that soft-fp did not raise. EXC_SUM reports only the underflow or overflow when the hardware also set INE, so treat INE as a candidate in that case, and treat a trap with no reported exception as a denormal operand trap, for which the hardware can fabricate INE and UNF as well. Bits that software has already confirmed in ieee_state belong to this or an earlier instruction and are never cleared. The imprecise path passes no summary. There the trap was taken somewhere in the trap shadow, so EXC_SUM is not attribution for the instruction being re-executed -- and only EV6, which traps precisely and so never takes that path, has fabricated bits to clear. For the same reason the clearing is guarded by implver(), matching swcr_update_status(). On an UP1500 (EV68) this takes the glibc math testsuite from 831 failures to 28, the remainder being unrelated to exception status. This belongs with the preceding fix to ieee_swcr_to_fpcr(), and should not be backported without it -- nor it without this. That fix stops FPCR_DNOD being set unconditionally, so denormal operand traps start firing again. Those traps very often find an exact result, which is precisely the case where the old code left the FPCR unwritten and the fabricated bits visible. Applied alone it would make spurious exception flags more common, not less. One case cannot be resolved here: an inexact instruction without the software completion suffix never traps, so its INE reaches the FPCR without being recorded anywhere else. Such a bit is indistinguishable from an INE the hardware fabricated for a trapping instruction, and is lost if an underflow or overflow trap with an exact result follows it. The FPCR is the only record of those instructions and it carries no attribution. The bug predates the git history, so there is no commit to reference in a Fixes tag. Cc: stable@vger.kernel.org # 5.15+ Signed-off-by: Matt Turner <mattst88@gmail.com> Reviewed-by: Magnus Lindholm <linmag7@gmail.com> Tested-by: Magnus Lindholm <linmag7@gmail.com> Link: https://lore.kernel.org/r/20260803-alpha-fp-exceptions-v1-2-c99d75608e60@gmail.com Signed-off-by: Magnus Lindholm <linmag7@gmail.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