selftests/net: test sk_filter support for SKF_NET_OFF on frags

Verify that a classic BPF linux socket filter correctly matches
packet contents. Including when accessing contents in an
skb_frag.

1. Open a SOCK_RAW socket with a classic BPF filter on UDP dport 8000.
2. Open a tap device with IFF_NAPI_FRAGS to inject skbs with frags.
3. Send a packet for which the UDP header is in frag[0].
4. Receive this packet to demonstrate that the socket accepted it.

Acked-by: Stanislav Fomichev <sdf@fomichev.me>
Signed-off-by: Willem de Bruijn <willemb@google.com>
Link: https://lore.kernel.org/r/20250408132833.195491-3-willemdebruijn.kernel@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

tools/testing/selftests/net/.gitignore

@@ -39,6 +39,7 @@ scm_rights
 sk_bind_sendto_listen
 sk_connect_zero_addr
 sk_so_peek_off
+skf_net_off
 socket
 so_incoming_cpu
 so_netns_cookie

tools/testing/selftests/net/Makefile

@@ -106,6 +106,8 @@ TEST_PROGS += ipv6_route_update_soft_lockup.sh
 TEST_PROGS += busy_poll_test.sh
 TEST_GEN_PROGS += proc_net_pktgen
 TEST_PROGS += lwt_dst_cache_ref_loop.sh
+TEST_PROGS += skf_net_off.sh
+TEST_GEN_FILES += skf_net_off
 
 # YNL files, must be before "include ..lib.mk"
 YNL_GEN_FILES := busy_poller netlink-dumps

tools/testing/selftests/net/skf_net_off.c

@@ -0,0 +1,244 @@
+// SPDX-License-Identifier: GPL-2.0
+
+/* Open a tun device.
+ *
+ * [modifications: use IFF_NAPI_FRAGS, add sk filter]
+ *
+ * Expects the device to have been configured previously, e.g.:
+ *   sudo ip tuntap add name tap1 mode tap
+ *   sudo ip link set tap1 up
+ *   sudo ip link set dev tap1 addr 02:00:00:00:00:01
+ *   sudo ip -6 addr add fdab::1 peer fdab::2 dev tap1 nodad
+ *
+ * And to avoid premature pskb_may_pull:
+ *
+ *   sudo ethtool -K tap1 gro off
+ *   sudo bash -c 'echo 0 > /proc/sys/net/ipv4/ip_early_demux'
+ */
+
+#define _GNU_SOURCE
+
+#include <arpa/inet.h>
+#include <errno.h>
+#include <error.h>
+#include <fcntl.h>
+#include <getopt.h>
+#include <linux/filter.h>
+#include <linux/if.h>
+#include <linux/if_packet.h>
+#include <linux/if_tun.h>
+#include <linux/ipv6.h>
+#include <netinet/if_ether.h>
+#include <netinet/in.h>
+#include <netinet/ip.h>
+#include <netinet/ip6.h>
+#include <netinet/udp.h>
+#include <poll.h>
+#include <signal.h>
+#include <stdbool.h>
+#include <stddef.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <sys/ioctl.h>
+#include <sys/socket.h>
+#include <sys/poll.h>
+#include <sys/types.h>
+#include <sys/uio.h>
+#include <unistd.h>
+
+static bool cfg_do_filter;
+static bool cfg_do_frags;
+static int cfg_dst_port = 8000;
+static char *cfg_ifname;
+
+static int tun_open(const char *tun_name)
+{
+	struct ifreq ifr = {0};
+	int fd, ret;
+
+	fd = open("/dev/net/tun", O_RDWR);
+	if (fd == -1)
+		error(1, errno, "open /dev/net/tun");
+
+	ifr.ifr_flags = IFF_TAP;
+	if (cfg_do_frags)
+		ifr.ifr_flags |= IFF_NAPI | IFF_NAPI_FRAGS;
+
+	strncpy(ifr.ifr_name, tun_name, IFNAMSIZ - 1);
+
+	ret = ioctl(fd, TUNSETIFF, &ifr);
+	if (ret)
+		error(1, ret, "ioctl TUNSETIFF");
+
+	return fd;
+}
+
+static void sk_set_filter(int fd)
+{
+	const int offset_proto = offsetof(struct ip6_hdr, ip6_nxt);
+	const int offset_dport = sizeof(struct ip6_hdr) + offsetof(struct udphdr, dest);
+
+	/* Filter UDP packets with destination port cfg_dst_port */
+	struct sock_filter filter_code[] = {
+		BPF_STMT(BPF_LD  + BPF_B   + BPF_ABS, SKF_AD_OFF + SKF_AD_PKTTYPE),
+		BPF_JUMP(BPF_JMP + BPF_JEQ + BPF_K, PACKET_HOST, 0, 4),
+		BPF_STMT(BPF_LD  + BPF_B   + BPF_ABS, SKF_NET_OFF + offset_proto),
+		BPF_JUMP(BPF_JMP + BPF_JEQ + BPF_K, IPPROTO_UDP, 0, 2),
+		BPF_STMT(BPF_LD  + BPF_H   + BPF_ABS, SKF_NET_OFF + offset_dport),
+		BPF_JUMP(BPF_JMP + BPF_JEQ + BPF_K, cfg_dst_port, 1, 0),
+		BPF_STMT(BPF_RET + BPF_K, 0),
+		BPF_STMT(BPF_RET + BPF_K, 0xFFFF),
+	};
+
+	struct sock_fprog filter = {
+		sizeof(filter_code) / sizeof(filter_code[0]),
+		filter_code,
+	};
+
+	if (setsockopt(fd, SOL_SOCKET, SO_ATTACH_FILTER, &filter, sizeof(filter)))
+		error(1, errno, "setsockopt attach filter");
+}
+
+static int raw_open(void)
+{
+	int fd;
+
+	fd = socket(PF_INET6, SOCK_RAW, IPPROTO_UDP);
+	if (fd == -1)
+		error(1, errno, "socket raw (udp)");
+
+	if (cfg_do_filter)
+		sk_set_filter(fd);
+
+	return fd;
+}
+
+static void tun_write(int fd)
+{
+	const char eth_src[] = { 0x02, 0x00, 0x00, 0x00, 0x00, 0x02 };
+	const char eth_dst[] = { 0x02, 0x00, 0x00, 0x00, 0x00, 0x01 };
+	struct tun_pi pi = {0};
+	struct ipv6hdr ip6h = {0};
+	struct udphdr uh = {0};
+	struct ethhdr eth = {0};
+	uint32_t payload;
+	struct iovec iov[5];
+	int ret;
+
+	pi.proto = htons(ETH_P_IPV6);
+
+	memcpy(eth.h_source, eth_src, sizeof(eth_src));
+	memcpy(eth.h_dest, eth_dst, sizeof(eth_dst));
+	eth.h_proto = htons(ETH_P_IPV6);
+
+	ip6h.version = 6;
+	ip6h.payload_len = htons(sizeof(uh) + sizeof(uint32_t));
+	ip6h.nexthdr = IPPROTO_UDP;
+	ip6h.hop_limit = 8;
+	if (inet_pton(AF_INET6, "fdab::2", &ip6h.saddr) != 1)
+		error(1, errno, "inet_pton src");
+	if (inet_pton(AF_INET6, "fdab::1", &ip6h.daddr) != 1)
+		error(1, errno, "inet_pton src");
+
+	uh.source = htons(8000);
+	uh.dest = htons(cfg_dst_port);
+	uh.len = ip6h.payload_len;
+	uh.check = 0;
+
+	payload = htonl(0xABABABAB);		/* Covered in IPv6 length */
+
+	iov[0].iov_base = &pi;
+	iov[0].iov_len  = sizeof(pi);
+	iov[1].iov_base = &eth;
+	iov[1].iov_len  = sizeof(eth);
+	iov[2].iov_base = &ip6h;
+	iov[2].iov_len  = sizeof(ip6h);
+	iov[3].iov_base = &uh;
+	iov[3].iov_len  = sizeof(uh);
+	iov[4].iov_base = &payload;
+	iov[4].iov_len  = sizeof(payload);
+
+	ret = writev(fd, iov, sizeof(iov) / sizeof(iov[0]));
+	if (ret <= 0)
+		error(1, errno, "writev");
+}
+
+static void raw_read(int fd)
+{
+	struct timeval tv = { .tv_usec = 100 * 1000 };
+	struct msghdr msg = {0};
+	struct iovec iov[2];
+	struct udphdr uh;
+	uint32_t payload[2];
+	int ret;
+
+	if (setsockopt(fd, SOL_SOCKET, SO_RCVTIMEO, &tv, sizeof(tv)))
+		error(1, errno, "setsockopt rcvtimeo udp");
+
+	iov[0].iov_base = &uh;
+	iov[0].iov_len = sizeof(uh);
+
+	iov[1].iov_base = payload;
+	iov[1].iov_len = sizeof(payload);
+
+	msg.msg_iov = iov;
+	msg.msg_iovlen = sizeof(iov) / sizeof(iov[0]);
+
+	ret = recvmsg(fd, &msg, 0);
+	if (ret <= 0)
+		error(1, errno, "read raw");
+	if (ret != sizeof(uh) + sizeof(payload[0]))
+		error(1, errno, "read raw: len=%d\n", ret);
+
+	fprintf(stderr, "raw recv: 0x%x\n", payload[0]);
+}
+
+static void parse_opts(int argc, char **argv)
+{
+	int c;
+
+	while ((c = getopt(argc, argv, "fFi:")) != -1) {
+		switch (c) {
+		case 'f':
+			cfg_do_filter = true;
+			printf("bpf filter enabled\n");
+			break;
+		case 'F':
+			cfg_do_frags = true;
+			printf("napi frags mode enabled\n");
+			break;
+		case 'i':
+			cfg_ifname = optarg;
+			break;
+		default:
+			error(1, 0, "unknown option %c", optopt);
+			break;
+		}
+	}
+
+	if (!cfg_ifname)
+		error(1, 0, "must specify tap interface name (-i)");
+}
+
+int main(int argc, char **argv)
+{
+	int fdt, fdr;
+
+	parse_opts(argc, argv);
+
+	fdr = raw_open();
+	fdt = tun_open(cfg_ifname);
+
+	tun_write(fdt);
+	raw_read(fdr);
+
+	if (close(fdt))
+		error(1, errno, "close tun");
+	if (close(fdr))
+		error(1, errno, "close udp");
+
+	fprintf(stderr, "OK\n");
+	return 0;
+}
+

tools/testing/selftests/net/skf_net_off.sh

@@ -0,0 +1,30 @@
+#!/bin/bash
+# SPDX-License-Identifier: GPL-2.0
+
+readonly NS="ns-$(mktemp -u XXXXXX)"
+
+cleanup() {
+	ip netns del $NS
+}
+
+ip netns add $NS
+trap cleanup EXIT
+
+ip -netns $NS link set lo up
+ip -netns $NS tuntap add name tap1 mode tap
+ip -netns $NS link set tap1 up
+ip -netns $NS link set dev tap1 addr 02:00:00:00:00:01
+ip -netns $NS -6 addr add fdab::1 peer fdab::2 dev tap1 nodad
+ip netns exec $NS ethtool -K tap1 gro off
+
+# disable early demux, else udp_v6_early_demux pulls udp header into linear
+ip netns exec $NS sysctl -w net.ipv4.ip_early_demux=0
+
+echo "no filter"
+ip netns exec $NS ./skf_net_off -i tap1
+
+echo "filter, linear skb (-f)"
+ip netns exec $NS ./skf_net_off -i tap1 -f
+
+echo "filter, fragmented skb (-f) (-F)"
+ip netns exec $NS ./skf_net_off -i tap1 -f -F