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Tejun Heo says: ==================== This makes BPF arena memory directly dereferenceable from kernel code (struct_ops callbacks, kfuncs). Each arena gets a per-arena scratch page that an arch fault hook installs into empty PTEs on kernel-side faults, after KFENCE. The faulting instruction retries and the violation is reported through the program's BPF stream. v4: - Patch 1: note that the strict-zero cmpxchg is narrower than pte_none() in inline comments on both x86 and arm64. (Andrea) - Patch 2: stub bpf_arena_handle_page_fault() for !CONFIG_BPF_SYSCALL via a new include/linux/bpf_defs.h. (lkp) - Patch 7: scx_arena_alloc() retries via a loop instead of a single retry on pool growth. (Andrea) - Picked up Reviewed-by tags from Emil and Andrea. v3: https://lore.kernel.org/r/20260520235052.4180316-1-tj@kernel.org v2: https://lore.kernel.org/r/20260517211232.1670594-1-tj@kernel.org v1 (RFC): https://lore.kernel.org/r/20260427105109.2554518-1-tj@kernel.org Motivation ---------- sched_ext's ops_cid.set_cmask() hands the BPF scheduler a struct scx_cmask *. The kernel translates a kernel cpumask to a cmask, but it had no way to write into the arena, so the cmask lived in kernel memory and was passed as a trusted pointer. BPF cmask helpers all operate on arena cmasks though, so the BPF side had to word-by-word probe-read the kernel cmask into an arena cmask via cmask_copy_from_kernel() before any helper could touch it. It works, but is clumsy. The shape isn't unique to set_cmask. Sub-scheduler support is on the way and more sched_ext callbacks will want to pass structured data to BPF. Anywhere a kfunc or struct_ops callback wants to hand a struct to a BPF program, arena residence is the natural answer. Approach -------- Each arena gets a per-arena scratch page. Arenas stay sparsely mapped as today - PTEs are populated only for allocated pages. A new arch fault hook (bpf_arena_handle_page_fault) is wired into x86 page_fault_oops() and arm64 __do_kernel_fault(), after KFENCE. When a kernel-side access faults inside an arena's kern_vm range, the helper walks the stack to find the BPF program responsible, range-checks the fault address against prog->aux->arena, and atomically installs the scratch page into the empty PTE via the new ptep_try_set() wrapper. The kernel instruction retries and reads/writes the scratch page. Free paths and map destruction treat scratch as non-owned. Real allocation refuses to overwrite scratch (apply_range_set_cb returns -EBUSY). A scratched address stays dead until map destroy, since its presence means the BPF program has already malfunctioned. The mechanism is default behavior - no UAPI flag. What this preserves ------------------- All the debugging properties of today's sparse-PTE design are preserved: * BPF programs still fault on unmapped arena accesses. The fault semantics (instruction retry with rdst = 0) and the violation report through bpf_streams are unchanged for prog-side accesses. * The first kernel-side touch of an unmapped address is reported via bpf_streams the same way as a prog-side fault, with the stack walk attributing it to the originating prog. * User-side fault on a never-scratched address still lazy-allocates a real page (or returns SIGSEGV under BPF_F_SEGV_ON_FAULT). User-side fault on a scratched address SIGSEGVs. What changes for the kernel-side caller is just that an unmapped deref no longer oopses - it retries through the scratch page and emits a violation report. The same shape today's BPF instruction faults have. Patches 1-2 (atomic PTE install + arena scratch-page recovery) -------------------------------------------------------------- mm: Add ptep_try_set() for lockless empty-slot installs bpf: Recover arena kernel faults with scratch page Patches 3-5 (helpers used by struct_ops registration) ----------------------------------------------------- bpf: Add sleepable variant of bpf_arena_alloc_pages for kernel callers bpf: Add bpf_struct_ops_for_each_prog() bpf/arena: Add bpf_arena_map_kern_vm_start() and bpf_prog_arena() ==================== Link: https://lore.kernel.org/bpf/20260522172219.1423324-1-tj@kernel.org/ Signed-off-by: Alexei Starovoitov <ast@kernel.org> |
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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