selftests/bpf: Test bpf_skb_adjust_room on CHECKSUM_PARTIAL

When the bpf_skb_adjust_room() shrinks the skb such that its csum_start
is invalid, the skb->ip_summed should be reset from CHECKSUM_PARTIAL to
CHECKSUM_NONE.

The commit 54c3f1a8 ("bpf: pull before calling skb_postpull_rcsum()")
fixed it.

This patch adds a test to ensure the skb->ip_summed changed from
CHECKSUM_PARTIAL to CHECKSUM_NONE after bpf_skb_adjust_room().
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Stanislav Fomichev <sdf@google.com>
Link: https://lore.kernel.org/bpf/20221221185653.1589961-1-martin.lau@linux.dev

tools/testing/selftests/bpf/DENYLIST.s390x

@@ -14,6 +14,7 @@ cgrp_kfunc                               # JIT does not support calling kernel f
 cgrp_local_storage                       # prog_attach unexpected error: -524                                          (trampoline)
 core_read_macros                         # unknown func bpf_probe_read#4                                               (overlapping)
 d_path                                   # failed to auto-attach program 'prog_stat': -524                             (trampoline)
+decap_sanity                             # JIT does not support calling kernel function                                (kfunc)
 deny_namespace                           # failed to attach: ERROR: strerror_r(-524)=22                                (trampoline)
 dummy_st_ops                             # test_run unexpected error: -524 (errno 524)                                 (trampoline)
 fentry_fexit                             # fentry attach failed: -524                                                  (trampoline)

tools/testing/selftests/bpf/prog_tests/decap_sanity.c

+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (c) 2022 Meta Platforms, Inc. and affiliates. */
+
+#include <sys/types.h>
+#include <sys/socket.h>
+#include <net/if.h>
+#include <linux/in6.h>
+
+#include "test_progs.h"
+#include "network_helpers.h"
+#include "decap_sanity.skel.h"
+
+#define SYS(fmt, ...)						\
+	({							\
+		char cmd[1024];					\
+		snprintf(cmd, sizeof(cmd), fmt, ##__VA_ARGS__);	\
+		if (!ASSERT_OK(system(cmd), cmd))		\
+			goto fail;				\
+	})
+
+#define NS_TEST "decap_sanity_ns"
+#define IPV6_IFACE_ADDR "face::1"
+#define UDP_TEST_PORT 7777
+
+void test_decap_sanity(void)
+{
+	LIBBPF_OPTS(bpf_tc_hook, qdisc_hook, .attach_point = BPF_TC_EGRESS);
+	LIBBPF_OPTS(bpf_tc_opts, tc_attach);
+	struct nstoken *nstoken = NULL;
+	struct decap_sanity *skel;
+	struct sockaddr_in6 addr;
+	socklen_t addrlen;
+	char buf[128] = {};
+	int sockfd, err;
+
+	skel = decap_sanity__open_and_load();
+	if (!ASSERT_OK_PTR(skel, "skel open_and_load"))
+		return;
+
+	SYS("ip netns add %s", NS_TEST);
+	SYS("ip -net %s -6 addr add %s/128 dev lo nodad", NS_TEST, IPV6_IFACE_ADDR);
+	SYS("ip -net %s link set dev lo up", NS_TEST);
+
+	nstoken = open_netns(NS_TEST);
+	if (!ASSERT_OK_PTR(nstoken, "open_netns"))
+		goto fail;
+
+	qdisc_hook.ifindex = if_nametoindex("lo");
+	if (!ASSERT_GT(qdisc_hook.ifindex, 0, "if_nametoindex lo"))
+		goto fail;
+
+	err = bpf_tc_hook_create(&qdisc_hook);
+	if (!ASSERT_OK(err, "create qdisc hook"))
+		goto fail;
+
+	tc_attach.prog_fd = bpf_program__fd(skel->progs.decap_sanity);
+	err = bpf_tc_attach(&qdisc_hook, &tc_attach);
+	if (!ASSERT_OK(err, "attach filter"))
+		goto fail;
+
+	addrlen = sizeof(addr);
+	err = make_sockaddr(AF_INET6, IPV6_IFACE_ADDR, UDP_TEST_PORT,
+			    (void *)&addr, &addrlen);
+	if (!ASSERT_OK(err, "make_sockaddr"))
+		goto fail;
+	sockfd = socket(AF_INET6, SOCK_DGRAM, 0);
+	if (!ASSERT_NEQ(sockfd, -1, "socket"))
+		goto fail;
+	err = sendto(sockfd, buf, sizeof(buf), 0, (void *)&addr, addrlen);
+	close(sockfd);
+	if (!ASSERT_EQ(err, sizeof(buf), "send"))
+		goto fail;
+
+	ASSERT_TRUE(skel->bss->init_csum_partial, "init_csum_partial");
+	ASSERT_TRUE(skel->bss->final_csum_none, "final_csum_none");
+	ASSERT_FALSE(skel->bss->broken_csum_start, "broken_csum_start");
+
+fail:
+	if (nstoken) {
+		bpf_tc_hook_destroy(&qdisc_hook);
+		close_netns(nstoken);
+	}
+	system("ip netns del " NS_TEST " >& /dev/null");
+	decap_sanity__destroy(skel);
+}

tools/testing/selftests/bpf/progs/bpf_tracing_net.h

@@ -50,6 +50,12 @@
 #define ICSK_TIME_LOSS_PROBE	5
 #define ICSK_TIME_REO_TIMEOUT	6
 
+#define ETH_HLEN		14
+#define ETH_P_IPV6		0x86DD
+
+#define CHECKSUM_NONE		0
+#define CHECKSUM_PARTIAL	3
+
 #define IFNAMSIZ		16
 
 #define RTF_GATEWAY		0x0002

tools/testing/selftests/bpf/progs/decap_sanity.c

+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (c) 2022 Meta Platforms, Inc. and affiliates. */
+
+#include "vmlinux.h"
+#include "bpf_tracing_net.h"
+#include <bpf/bpf_helpers.h>
+#include <bpf/bpf_endian.h>
+
+#define UDP_TEST_PORT 7777
+
+void *bpf_cast_to_kern_ctx(void *) __ksym;
+bool init_csum_partial = false;
+bool final_csum_none = false;
+bool broken_csum_start = false;
+
+static unsigned int skb_headlen(const struct sk_buff *skb)
+{
+	return skb->len - skb->data_len;
+}
+
+static unsigned int skb_headroom(const struct sk_buff *skb)
+{
+	return skb->data - skb->head;
+}
+
+static int skb_checksum_start_offset(const struct sk_buff *skb)
+{
+	return skb->csum_start - skb_headroom(skb);
+}
+
+SEC("tc")
+int decap_sanity(struct __sk_buff *skb)
+{
+	struct sk_buff *kskb;
+	struct ipv6hdr ip6h;
+	struct udphdr udph;
+	int err;
+
+	if (skb->protocol != __bpf_constant_htons(ETH_P_IPV6))
+		return TC_ACT_SHOT;
+
+	if (bpf_skb_load_bytes(skb, ETH_HLEN, &ip6h, sizeof(ip6h)))
+		return TC_ACT_SHOT;
+
+	if (ip6h.nexthdr != IPPROTO_UDP)
+		return TC_ACT_SHOT;
+
+	if (bpf_skb_load_bytes(skb, ETH_HLEN + sizeof(ip6h), &udph, sizeof(udph)))
+		return TC_ACT_SHOT;
+
+	if (udph.dest != __bpf_constant_htons(UDP_TEST_PORT))
+		return TC_ACT_SHOT;
+
+	kskb = bpf_cast_to_kern_ctx(skb);
+	init_csum_partial = (kskb->ip_summed == CHECKSUM_PARTIAL);
+	err = bpf_skb_adjust_room(skb, -(s32)(ETH_HLEN + sizeof(ip6h) + sizeof(udph)),
+				  1, BPF_F_ADJ_ROOM_FIXED_GSO);
+	if (err)
+		return TC_ACT_SHOT;
+	final_csum_none = (kskb->ip_summed == CHECKSUM_NONE);
+	if (kskb->ip_summed == CHECKSUM_PARTIAL &&
+	    (unsigned int)skb_checksum_start_offset(kskb) >= skb_headlen(kskb))
+		broken_csum_start = true;
+
+	return TC_ACT_SHOT;
+}
+
+char __license[] SEC("license") = "GPL";