bpf: selftest: Ensure the return value of bpf_skc_to helpers must be checked

This patch tests:

int bpf_cls(struct __sk_buff *skb)
{
	/* REG_6: sk
	 * REG_7: tp
	 * REG_8: req_sk
	 */

	sk = skb->sk;
	if (!sk)
		return 0;

	tp = bpf_skc_to_tcp_sock(sk);
	req_sk = bpf_skc_to_tcp_request_sock(sk);
	if (!req_sk)
		return 0;

	/* !tp has not been tested, so verifier should reject. */
	return *(__u8 *)tp;
}

Signed-off-by: Martin KaFai Lau <kafai@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20201019194219.1051314-1-kafai@fb.com

Author: iamkafai

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

@@ -631,3 +631,28 @@
 	.prog_type = BPF_PROG_TYPE_SK_REUSEPORT,
 	.result = ACCEPT,
 },
+{
+	"mark null check on return value of bpf_skc_to helpers",
+	.insns = {
+	BPF_LDX_MEM(BPF_DW, BPF_REG_1, BPF_REG_1, offsetof(struct __sk_buff, sk)),
+	BPF_JMP_IMM(BPF_JNE, BPF_REG_1, 0, 2),
+	BPF_MOV64_IMM(BPF_REG_0, 0),
+	BPF_EXIT_INSN(),
+	BPF_MOV64_REG(BPF_REG_6, BPF_REG_1),
+	BPF_EMIT_CALL(BPF_FUNC_skc_to_tcp_sock),
+	BPF_MOV64_REG(BPF_REG_7, BPF_REG_0),
+	BPF_MOV64_REG(BPF_REG_1, BPF_REG_6),
+	BPF_EMIT_CALL(BPF_FUNC_skc_to_tcp_request_sock),
+	BPF_MOV64_REG(BPF_REG_8, BPF_REG_0),
+	BPF_JMP_IMM(BPF_JNE, BPF_REG_8, 0, 2),
+	BPF_MOV64_IMM(BPF_REG_0, 0),
+	BPF_EXIT_INSN(),
+	BPF_LDX_MEM(BPF_B, BPF_REG_0, BPF_REG_7, 0),
+	BPF_EXIT_INSN(),
+	},
+	.prog_type = BPF_PROG_TYPE_SCHED_CLS,
+	.result = REJECT,
+	.errstr = "invalid mem access",
+	.result_unpriv = REJECT,
+	.errstr_unpriv = "unknown func",
+},