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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>
Why we want a copy of kernel headers in tools?
==============================================
There used to be no copies, with tools/ code using kernel headers
directly. From time to time tools/perf/ broke due to legitimate kernel
hacking. At some point Linus complained about such direct usage. Then we
adopted the current model.
The way these headers are used in perf are not restricted to just
including them to compile something.
There are sometimes used in scripts that convert defines into string
tables, etc, so some change may break one of these scripts, or new MSRs
may use some different #define pattern, etc.
E.g.:
$ ls -1 tools/perf/trace/beauty/*.sh | head -5
tools/perf/trace/beauty/arch_errno_names.sh
tools/perf/trace/beauty/drm_ioctl.sh
tools/perf/trace/beauty/fadvise.sh
tools/perf/trace/beauty/fsconfig.sh
tools/perf/trace/beauty/fsmount.sh
$
$ tools/perf/trace/beauty/fadvise.sh
static const char *fadvise_advices[] = {
[0] = "NORMAL",
[1] = "RANDOM",
[2] = "SEQUENTIAL",
[3] = "WILLNEED",
[4] = "DONTNEED",
[5] = "NOREUSE",
};
$
The tools/perf/check-headers.sh script, part of the tools/ build
process, points out changes in the original files.
So its important not to touch the copies in tools/ when doing changes in
the original kernel headers, that will be done later, when
check-headers.sh inform about the change to the perf tools hackers.
Another explanation from Ingo Molnar:
It's better than all the alternatives we tried so far:
- Symbolic links and direct #includes: this was the original approach but
was pushed back on from the kernel side, when tooling modified the
headers and broke them accidentally for kernel builds.
- Duplicate self-defined ABI headers like glibc: double the maintenance
burden, double the chance for mistakes, plus there's no tech-driven
notification mechanism to look at new kernel side changes.
What we are doing now is a third option:
- A software-enforced copy-on-write mechanism of kernel headers to
tooling, driven by non-fatal warnings on the tooling side build when
kernel headers get modified:
Warning: Kernel ABI header differences:
diff -u tools/include/uapi/drm/i915_drm.h include/uapi/drm/i915_drm.h
diff -u tools/include/uapi/linux/fs.h include/uapi/linux/fs.h
diff -u tools/include/uapi/linux/kvm.h include/uapi/linux/kvm.h
...
The tooling policy is to always pick up the kernel side headers as-is,
and integate them into the tooling build. The warnings above serve as a
notification to tooling maintainers that there's changes on the kernel
side.
We've been using this for many years now, and it might seem hacky, but
works surprisingly well.