selftests/bpf: Test new __sk_buff field gso_size

Analogous to the gso_segs selftests introduced in commit d9ff286a
("bpf: allow BPF programs access skb_shared_info->gso_segs field").
Signed-off-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20200303200503.226217-4-willemdebruijn.kernel@gmail.com

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

@@ -14,6 +14,7 @@ void test_skb_ctx(void)
 		.wire_len = 100,
 		.gso_segs = 8,
 		.mark = 9,
+		.gso_size = 10,
 	};
 	struct bpf_prog_test_run_attr tattr = {
 		.data_in = &pkt_v4,

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

@@ -23,6 +23,8 @@ int process(struct __sk_buff *skb)
 		return 1;
 	if (skb->gso_segs != 8)
 		return 1;
+	if (skb->gso_size != 10)
+		return 1;
 
 	return 0;
 }

tools/testing/selftests/bpf/verifier/ctx_skb.c

@@ -1010,6 +1010,53 @@
 	.result = ACCEPT,
 	.prog_type = BPF_PROG_TYPE_SCHED_CLS,
 },
+{
+	"read gso_size from CGROUP_SKB",
+	.insns = {
+	BPF_LDX_MEM(BPF_W, BPF_REG_0, BPF_REG_1,
+		    offsetof(struct __sk_buff, gso_size)),
+	BPF_MOV64_IMM(BPF_REG_0, 0),
+	BPF_EXIT_INSN(),
+	},
+	.result = ACCEPT,
+	.prog_type = BPF_PROG_TYPE_CGROUP_SKB,
+},
+{
+	"read gso_size from CGROUP_SKB",
+	.insns = {
+	BPF_LDX_MEM(BPF_W, BPF_REG_1, BPF_REG_1,
+		    offsetof(struct __sk_buff, gso_size)),
+	BPF_MOV64_IMM(BPF_REG_0, 0),
+	BPF_EXIT_INSN(),
+	},
+	.result = ACCEPT,
+	.prog_type = BPF_PROG_TYPE_CGROUP_SKB,
+},
+{
+	"write gso_size from CGROUP_SKB",
+	.insns = {
+	BPF_MOV64_IMM(BPF_REG_0, 0),
+	BPF_STX_MEM(BPF_W, BPF_REG_1, BPF_REG_0,
+		    offsetof(struct __sk_buff, gso_size)),
+	BPF_MOV64_IMM(BPF_REG_0, 0),
+	BPF_EXIT_INSN(),
+	},
+	.result = REJECT,
+	.result_unpriv = REJECT,
+	.errstr = "invalid bpf_context access off=176 size=4",
+	.prog_type = BPF_PROG_TYPE_CGROUP_SKB,
+},
+{
+	"read gso_size from CLS",
+	.insns = {
+	BPF_LDX_MEM(BPF_W, BPF_REG_0, BPF_REG_1,
+		    offsetof(struct __sk_buff, gso_size)),
+	BPF_MOV64_IMM(BPF_REG_0, 0),
+	BPF_EXIT_INSN(),
+	},
+	.result = ACCEPT,
+	.prog_type = BPF_PROG_TYPE_SCHED_CLS,
+},
 {
 	"check wire_len is not readable by sockets",
 	.insns = {