selftests/bpf: Add cgroup_skb/egress test for load_bytes_relative

When cgroup_skb/egress triggers the MAC header is not set. Added a
test that asserts reading MAC header is a -EFAULT but NET header
succeeds. The test result from within the eBPF program is stored in
an 1-element array map that the userspace then reads and asserts on.

Another assertion is added that reading from a large offset, past
the end of packet, returns -EFAULT.

Signed-off-by: YiFei Zhu <zhuyifei@google.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Reviewed-by: Stanislav Fomichev <sdf@google.com>
Link: https://lore.kernel.org/bpf/9028ccbea4385a620e69c0a104f469ffd655c01e.1591812755.git.zhuyifei@google.com

1 files changed, 48 insertions, 0 deletions

diff --git a/tools/testing/selftests/bpf/progs/load_bytes_relative.c b/tools/testing/selftests/bpf/progs/load_bytes_relative.c
new file mode 100644
index 000000000000..dc1d04a7a3d6
--- /dev/null
+++ b/tools/testing/selftests/bpf/progs/load_bytes_relative.c
@@ -0,0 +1,48 @@
+// SPDX-License-Identifier: GPL-2.0-only
+
+/*
+ * Copyright 2020 Google LLC.
+ */
+
+#include <errno.h>
+#include <linux/bpf.h>
+#include <linux/if_ether.h>
+#include <linux/ip.h>
+#include <bpf/bpf_helpers.h>
+
+struct {
+	__uint(type, BPF_MAP_TYPE_ARRAY);
+	__uint(max_entries, 1);
+	__type(key, __u32);
+	__type(value, __u32);
+} test_result SEC(".maps");
+
+SEC("cgroup_skb/egress")
+int load_bytes_relative(struct __sk_buff *skb)
+{
+	struct ethhdr eth;
+	struct iphdr iph;
+
+	__u32 map_key = 0;
+	__u32 test_passed = 0;
+
+	/* MAC header is not set by the time cgroup_skb/egress triggers */
+	if (bpf_skb_load_bytes_relative(skb, 0, &eth, sizeof(eth),
+					BPF_HDR_START_MAC) != -EFAULT)
+		goto fail;
+
+	if (bpf_skb_load_bytes_relative(skb, 0, &iph, sizeof(iph),
+					BPF_HDR_START_NET))
+		goto fail;
+
+	if (bpf_skb_load_bytes_relative(skb, 0xffff, &iph, sizeof(iph),
+					BPF_HDR_START_NET) != -EFAULT)
+		goto fail;
+
+	test_passed = 1;
+
+fail:
+	bpf_map_update_elem(&test_result, &map_key, &test_passed, BPF_ANY);
+
+	return 1;
+}