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bpf_fib_lookup() returns the FIB-resolved egress ifindex straight from the fib result. When the egress is a VLAN device, the returned ifindex is the VLAN netdev's, which has no XDP xmit handler; XDP programs that want to forward the frame (e.g. xdp-forward) must instead target the underlying physical device and push the VLAN tag themselves. Today the program has no way to learn either the underlying ifindex or the VLAN tag without maintaining its own VLAN-to-ifindex map in userspace and refreshing it on netlink events. Add BPF_FIB_LOOKUP_VLAN. When the caller sets this flag and the fib result is a VLAN device whose immediate parent is a real (non-VLAN) device in the same network namespace, populate the existing output fields params->h_vlan_proto and params->h_vlan_TCI from the VLAN device and replace params->ifindex with the parent's ifindex. params->h_vlan_TCI carries the VID only, with PCP and DEI bits zero; a consumer wanting to set egress priority writes PCP itself. params->smac is the VLAN device's own address, which can differ from the parent's. Only the immediate parent is resolved, via vlan_dev_priv(dev)->real_dev and not vlan_dev_real_dev(), which walks to the bottom of a stack. When the immediate parent is not a real device in the same namespace, the lookup returns BPF_FIB_LKUP_RET_VLAN_FAILURE and leaves params->ifindex at the input. This covers a stacked VLAN (QinQ), where the immediate parent is itself a VLAN device and one h_vlan_proto/h_vlan_TCI pair cannot describe two tags, and a parent in another network namespace (a VLAN device can be moved while its parent stays), whose ifindex would be meaningless in the caller's namespace. A program that wants the VLAN device's own ifindex re-issues the lookup, with a re-initialized params, without BPF_FIB_LOOKUP_VLAN, so the unreducible case stays distinct from a physical egress. That distinction matters for XDP: a program cannot xmit on a VLAN device, so a success carrying the VLAN ifindex would make it redirect to a device with no ndo_xdp_xmit and drop the frame at xdp_do_flush(). The swap and the vlan fields are written only on the reduce path; other output fields keep their existing behaviour, so a frag-needed result still reports the route mtu in params->mtu_result. BPF_FIB_LOOKUP_VLAN is only useful to XDP, which cannot redirect to a VLAN device. A tc program can redirect to the VLAN device directly, so bpf_skb_fib_lookup() rejects the flag with -EINVAL; bpf_xdp_fib_lookup() accepts it. When the flag is not set, behaviour is unchanged: h_vlan_proto and h_vlan_TCI are zeroed and ifindex is left at the FIB result. The new block is compiled only under CONFIG_VLAN_8021Q since vlan_dev_priv() is not defined otherwise; without that config is_vlan_dev() is constant false and the flag is accepted but never acts. That is safe because no VLAN device can exist there, so every egress is already physical. This lets an XDP redirect target the physical device and learn the tag to push in a single lookup, which xdp-forward's optional VLAN mode (xdp-project/xdp-tools#504) wants from the kernel side. The helper's input semantics are unchanged; the reverse direction (supplying a tag as lookup input) is added in the following patch. Suggested-by: Toke Høiland-Jørgensen <toke@redhat.com> Signed-off-by: Avinash Duduskar <avinash.duduskar@gmail.com> Reviewed-by: Toke Høiland-Jørgensen <toke@redhat.com> Reviewed-by: Emil Tsalapatis <emil@etsalapatis.com> Acked-by: David Ahern <dsahern@kernel.org> Link: https://lore.kernel.org/bpf/20260713162305.1237211-2-avinash.duduskar@gmail.com Signed-off-by: Kumar Kartikeya Dwivedi <memxor@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. 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