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e2e3224122
Improve the 'unknown reason' comment, with an actual explaination of why
the ctx pkt-data pointers need to be loaded after the helper function
bpf_xdp_adjust_meta(). Based on the explaination Daniel gave.
Fixes: 36e04a2d78 ("samples/bpf: xdp2skb_meta shows transferring info from XDP to SKB")
Reported-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
106 lines
3.0 KiB
C
106 lines
3.0 KiB
C
/* SPDX-License-Identifier: GPL-2.0
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* Copyright (c) 2018 Jesper Dangaard Brouer, Red Hat Inc.
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*
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* Example howto transfer info from XDP to SKB, e.g. skb->mark
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* -----------------------------------------------------------
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* This uses the XDP data_meta infrastructure, and is a cooperation
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* between two bpf-programs (1) XDP and (2) clsact at TC-ingress hook.
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*
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* Notice: This example does not use the BPF C-loader (bpf_load.c),
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* but instead rely on the iproute2 TC tool for loading BPF-objects.
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*/
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#include <uapi/linux/bpf.h>
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#include <uapi/linux/pkt_cls.h>
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#include "bpf_helpers.h"
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/*
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* This struct is stored in the XDP 'data_meta' area, which is located
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* just in-front-of the raw packet payload data. The meaning is
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* specific to these two BPF programs that use it as a communication
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* channel. XDP adjust/increase the area via a bpf-helper, and TC use
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* boundary checks to see if data have been provided.
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*
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* The struct must be 4 byte aligned, which here is enforced by the
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* struct __attribute__((aligned(4))).
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*/
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struct meta_info {
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__u32 mark;
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} __attribute__((aligned(4)));
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SEC("xdp_mark")
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int _xdp_mark(struct xdp_md *ctx)
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{
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struct meta_info *meta;
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void *data, *data_end;
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int ret;
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/* Reserve space in-front of data pointer for our meta info.
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* (Notice drivers not supporting data_meta will fail here!)
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*/
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ret = bpf_xdp_adjust_meta(ctx, -(int)sizeof(*meta));
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if (ret < 0)
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return XDP_ABORTED;
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/* Notice: Kernel-side verifier requires that loading of
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* ctx->data MUST happen _after_ helper bpf_xdp_adjust_meta(),
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* as pkt-data pointers are invalidated. Helpers that require
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* this are determined/marked by bpf_helper_changes_pkt_data()
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*/
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data = (void *)(unsigned long)ctx->data;
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/* Check data_meta have room for meta_info struct */
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meta = (void *)(unsigned long)ctx->data_meta;
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if (meta + 1 > data)
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return XDP_ABORTED;
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meta->mark = 42;
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return XDP_PASS;
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}
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SEC("tc_mark")
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int _tc_mark(struct __sk_buff *ctx)
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{
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void *data = (void *)(unsigned long)ctx->data;
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void *data_end = (void *)(unsigned long)ctx->data_end;
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void *data_meta = (void *)(unsigned long)ctx->data_meta;
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struct meta_info *meta = data_meta;
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/* Check XDP gave us some data_meta */
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if (meta + 1 > data) {
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ctx->mark = 41;
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/* Skip "accept" if no data_meta is avail */
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return TC_ACT_OK;
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}
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/* Hint: See func tc_cls_act_is_valid_access() for BPF_WRITE access */
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ctx->mark = meta->mark; /* Transfer XDP-mark to SKB-mark */
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return TC_ACT_OK;
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}
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/* Manually attaching these programs:
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export DEV=ixgbe2
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export FILE=xdp2skb_meta_kern.o
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# via TC command
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tc qdisc del dev $DEV clsact 2> /dev/null
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tc qdisc add dev $DEV clsact
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tc filter add dev $DEV ingress prio 1 handle 1 bpf da obj $FILE sec tc_mark
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tc filter show dev $DEV ingress
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# XDP via IP command:
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ip link set dev $DEV xdp off
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ip link set dev $DEV xdp obj $FILE sec xdp_mark
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# Use iptable to "see" if SKBs are marked
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iptables -I INPUT -p icmp -m mark --mark 41 # == 0x29
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iptables -I INPUT -p icmp -m mark --mark 42 # == 0x2a
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# Hint: catch XDP_ABORTED errors via
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perf record -e xdp:*
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perf script
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*/
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