samples/bpf/xdp2skb_meta_kern.c

Source file repositories/reference/linux-study-clean/samples/bpf/xdp2skb_meta_kern.c

File Facts

System
Linux kernel
Corpus path
samples/bpf/xdp2skb_meta_kern.c
Extension
.c
Size
3027 bytes
Lines
105
Domain
Support Tooling And Documentation
Bucket
samples
Inferred role
Support Tooling And Documentation: implementation source
Status
source implementation candidate

Why This File Exists

Repository support layer: documentation, build tooling, samples, user-space helper tools, generated initramfs support, licenses, and validation utilities.

Dependency Surface

Detected Declarations

Annotated Snippet

struct meta_info {
	__u32 mark;
} __attribute__((aligned(4)));

SEC("xdp_mark")
int _xdp_mark(struct xdp_md *ctx)
{
	struct meta_info *meta;
	void *data;
	int ret;

	/* Reserve space in-front of data pointer for our meta info.
	 * (Notice drivers not supporting data_meta will fail here!)
	 */
	ret = bpf_xdp_adjust_meta(ctx, -(int)sizeof(*meta));
	if (ret < 0)
		return XDP_ABORTED;

	/* Notice: Kernel-side verifier requires that loading of
	 * ctx->data MUST happen _after_ helper bpf_xdp_adjust_meta(),
	 * as pkt-data pointers are invalidated.  Helpers that require
	 * this are determined/marked by bpf_helper_changes_pkt_data()
	 */
	data = (void *)(unsigned long)ctx->data;

	/* Check data_meta have room for meta_info struct */
	meta = (void *)(unsigned long)ctx->data_meta;
	if (meta + 1 > data)
		return XDP_ABORTED;

	meta->mark = 42;

	return XDP_PASS;
}

SEC("tc_mark")
int _tc_mark(struct __sk_buff *ctx)
{
	void *data      = (void *)(unsigned long)ctx->data;
	void *data_meta = (void *)(unsigned long)ctx->data_meta;
	struct meta_info *meta = data_meta;

	/* Check XDP gave us some data_meta */
	if (meta + 1 > data) {
		ctx->mark = 41;
		 /* Skip "accept" if no data_meta is avail */
		return TC_ACT_OK;
	}

	/* Hint: See func tc_cls_act_is_valid_access() for BPF_WRITE access */
	ctx->mark = meta->mark; /* Transfer XDP-mark to SKB-mark */

	return TC_ACT_OK;
}

/* Manually attaching these programs:
export DEV=ixgbe2
export FILE=xdp2skb_meta_kern.o

# via TC command
tc qdisc del dev $DEV clsact 2> /dev/null
tc qdisc add dev $DEV clsact
tc filter  add dev $DEV ingress prio 1 handle 1 bpf da obj $FILE sec tc_mark
tc filter show dev $DEV ingress

# XDP via IP command:
ip link set dev $DEV xdp off
ip link set dev $DEV xdp obj $FILE sec xdp_mark

# Use iptable to "see" if SKBs are marked
iptables -I INPUT -p icmp -m mark --mark 41  # == 0x29
iptables -I INPUT -p icmp -m mark --mark 42  # == 0x2a

# Hint: catch XDP_ABORTED errors via
perf record -e xdp:*
perf script

*/

Annotation

Implementation Notes