Add xdp_vlan01_kern.c to investigate compiler optimizations

Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>

Author: netoptimizer

common/parsing_helpers.h

@@ -56,7 +56,7 @@ struct icmphdr_common {
 
 /* Allow users of header file to redefine VLAN max depth */
 #ifndef VLAN_MAX_DEPTH
-#define VLAN_MAX_DEPTH 2
+#define VLAN_MAX_DEPTH 4
 #endif
 
 static __always_inline int proto_is_vlan(__u16 h_proto)

packet-solutions/Makefile

@@ -1,6 +1,7 @@
 # SPDX-License-Identifier: (GPL-2.0 OR BSD-2-Clause)
 
 XDP_TARGETS  := xdp_prog_kern_02 xdp_prog_kern_03
+XDP_TARGETS  += xdp_vlan01_kern
 USER_TARGETS := xdp_prog_user
 
 LIBBPF_DIR = ../libbpf/src/

packet-solutions/xdp_prog_kern_02.c

@@ -1,6 +1,8 @@
 /* SPDX-License-Identifier: GPL-2.0 */
 #include <linux/bpf.h>
 #include <linux/in.h>
+
+#include <linux/if_ether.h>
 #include "bpf_helpers.h"
 #include "bpf_endian.h"
 

packet-solutions/xdp_vlan01_kern.c

@@ -0,0 +1,115 @@
+/* SPDX-License-Identifier: (LGPL-2.1 OR BSD-2-Clause) */
+#include <linux/bpf.h>
+#include <linux/if_ether.h>
+
+#include "bpf_helpers.h"
+#include "bpf_endian.h"
+
+/* NOTICE: Re-defining VLAN header levels to parse */
+#define VLAN_MAX_DEPTH 10
+//#include "../common/parsing_helpers.h"
+/*
+ * NOTICE: Copied over parts of ../common/parsing_helpers.h
+ *         to make it easier to point out compiler optimizations
+ */
+
+/* Header cursor to keep track of current parsing position */
+struct hdr_cursor {
+	void *pos;
+};
+
+static __always_inline int proto_is_vlan(__u16 h_proto)
+{
+        return !!(h_proto == bpf_htons(ETH_P_8021Q) ||
+                  h_proto == bpf_htons(ETH_P_8021AD));
+}
+
+/*
+ * 	struct vlan_hdr - vlan header
+ * 	@h_vlan_TCI: priority and VLAN ID
+ *	@h_vlan_encapsulated_proto: packet type ID or len
+ */
+struct vlan_hdr {
+	__be16	h_vlan_TCI;
+	__be16	h_vlan_encapsulated_proto; /* NOTICE: unsigned type */
+};
+
+/* Notice, parse_ethhdr() will skip VLAN tags, by advancing nh->pos and returns
+ * next header EtherType, BUT the ethhdr pointer supplied still points to the
+ * Ethernet header. Thus, caller can look at eth->h_proto to see if this was a
+ * VLAN tagged packet.
+ */
+static __always_inline int parse_ethhdr(struct hdr_cursor *nh, void *data_end,
+					struct ethhdr **ethhdr)
+{
+	struct ethhdr *eth = nh->pos;
+	int hdrsize = sizeof(*eth);
+        struct vlan_hdr *vlh;
+        __u16 h_proto;
+        int i;
+
+	/* Byte-count bounds check; check if current pointer + size of header
+	 * is after data_end.
+	 */
+	if (nh->pos + hdrsize > data_end)
+		return -1;
+
+	nh->pos += hdrsize;
+	*ethhdr = eth;
+        vlh = nh->pos;
+        h_proto = eth->h_proto;
+
+        /* Use loop unrolling to avoid the verifier restriction on loops;
+         * support up to VLAN_MAX_DEPTH layers of VLAN encapsulation.
+         */
+        #pragma unroll
+        for (i = 0; i < VLAN_MAX_DEPTH; i++) {
+                if (!proto_is_vlan(h_proto))
+                        break;
+
+                if (vlh + 1 > data_end)
+                        break;
+
+                h_proto = vlh->h_vlan_encapsulated_proto;
+                vlh++;
+        }
+
+        nh->pos = vlh;
+	return bpf_ntohs(h_proto);
+}
+
+SEC("xdp_vlan01")
+int xdp_vlan_01(struct xdp_md *ctx)
+{
+	void *data_end = (void *)(long)ctx->data_end;
+	void *data = (void *)(long)ctx->data;
+
+	/* These keep track of the next header type and iterator pointer */
+	struct hdr_cursor nh;
+	int nh_type;
+	nh.pos = data;
+
+	struct ethhdr *eth;
+	nh_type = parse_ethhdr(&nh, data_end, &eth);
+	if (nh_type < 0)
+		return XDP_ABORTED;
+
+	/* The LLVM compiler is very clever, and will remove above walking of
+	 * VLAN headers (the loop unroll).
+	 *
+	 * The returned value nh_type, variable (__u16) h_proto in
+	 * parse_ethhdr(), is only compared against a negative value (signed).
+	 * The compile see that it can remove the VLAN loop, because:
+	 *  1. h_proto = vlh->h_vlan_encapsulated_proto can only be >= 0
+	 *  2. we never read nh->pos (so it removes nh->pos = vlh;).
+	 */
+
+	/* Accessing eth pointer is still valid after compiler optimization */
+	if (proto_is_vlan(eth->h_proto))
+		return XDP_DROP;
+
+	/* Hint: to inspect BPF byte-code run:
+	 *  llvm-objdump -S xdp_vlan01_kern.o
+	 */
+	return XDP_PASS;
+}