This patch adds verifier side tests for the new bpf_mptcp_sock() helper.

Here are the new tests :
- NULL bpf_sock is correctly handled
- We cannot access a field from bpf_mptcp_sock if the latter is NULL
- We can access a field from bpf_mptcp_sock if the latter is not NULL
- We cannot modify a field from bpf_mptcp_sock.

Note that "token" is currently the only field in bpf_mptcp_sock.

Currently, there is no easy way to test the token field since we cannot
get back the mptcp_sock in userspace, this could be a future amelioration.

Acked-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Acked-by: Song Liu <songliubraving@fb.com>
Signed-off-by: Nicolas Rybowski <nicolas.rybowski@tessares.net>
---

Notes:
    v1 -> v2:
    - new patch: mandatory selftests (Alexei)

 tools/testing/selftests/bpf/verifier/sock.c | 63 +++++++++++++++++++++
 1 file changed, 63 insertions(+)

Author: nrybowski

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

@@ -631,3 +631,66 @@
 	.prog_type = BPF_PROG_TYPE_SK_REUSEPORT,
 	.result = ACCEPT,
 },
+{
+	"bpf_mptcp_sock(skops->sk): no !skops->sk check",
+	.insns = {
+	BPF_LDX_MEM(BPF_DW, BPF_REG_1, BPF_REG_1, offsetof(struct bpf_sock_ops, sk)),
+	BPF_EMIT_CALL(BPF_FUNC_mptcp_sock),
+	BPF_MOV64_IMM(BPF_REG_0, 0),
+	BPF_EXIT_INSN(),
+	},
+	.prog_type = BPF_PROG_TYPE_SOCK_OPS,
+	.result = REJECT,
+	.errstr = "type=sock_or_null expected=sock_common",
+},
+{
+	"bpf_mptcp_sock(skops->sk): no NULL check on ret",
+	.insns = {
+	BPF_LDX_MEM(BPF_DW, BPF_REG_1, BPF_REG_1, offsetof(struct bpf_sock_ops, sk)),
+	BPF_JMP_IMM(BPF_JNE, BPF_REG_1, 0, 2),
+	BPF_MOV64_IMM(BPF_REG_0, 0),
+	BPF_EXIT_INSN(),
+	BPF_EMIT_CALL(BPF_FUNC_mptcp_sock),
+	BPF_LDX_MEM(BPF_W, BPF_REG_0, BPF_REG_0, offsetof(struct bpf_mptcp_sock, token)),
+	BPF_MOV64_IMM(BPF_REG_0, 0),
+	BPF_EXIT_INSN(),
+	},
+	.prog_type = BPF_PROG_TYPE_SOCK_OPS,
+	.result = REJECT,
+	.errstr = "invalid mem access 'mptcp_sock_or_null'",
+},
+{
+	"bpf_mptcp_sock(skops->sk): msk->token",
+	.insns = {
+	BPF_LDX_MEM(BPF_DW, BPF_REG_1, BPF_REG_1, offsetof(struct bpf_sock_ops, sk)),
+	BPF_JMP_IMM(BPF_JNE, BPF_REG_1, 0, 2),
+	BPF_MOV64_IMM(BPF_REG_0, 0),
+	BPF_EXIT_INSN(),
+	BPF_EMIT_CALL(BPF_FUNC_mptcp_sock),
+	BPF_JMP_IMM(BPF_JNE, BPF_REG_0, 0, 1),
+	BPF_EXIT_INSN(),
+	BPF_LDX_MEM(BPF_W, BPF_REG_0, BPF_REG_0, offsetof(struct bpf_mptcp_sock, token)),
+	BPF_MOV64_IMM(BPF_REG_0, 0),
+	BPF_EXIT_INSN(),
+	},
+	.prog_type = BPF_PROG_TYPE_SOCK_OPS,
+	.result = ACCEPT,
+},
+{
+	"bpf_mptcp_sock(skops->sk): msk->token cannot be modified",
+	.insns = {
+	BPF_LDX_MEM(BPF_DW, BPF_REG_1, BPF_REG_1, offsetof(struct bpf_sock_ops, sk)),
+	BPF_JMP_IMM(BPF_JNE, BPF_REG_1, 0, 2),
+	BPF_MOV64_IMM(BPF_REG_0, 0),
+	BPF_EXIT_INSN(),
+	BPF_EMIT_CALL(BPF_FUNC_mptcp_sock),
+	BPF_JMP_IMM(BPF_JNE, BPF_REG_0, 0, 1),
+	BPF_EXIT_INSN(),
+	BPF_ST_MEM(BPF_W, BPF_REG_0, offsetof(struct bpf_mptcp_sock, token), 0x2a),
+	BPF_MOV64_IMM(BPF_REG_0, 0),
+	BPF_EXIT_INSN(),
+	},
+	.prog_type = BPF_PROG_TYPE_SOCK_OPS,
+	.result = REJECT,
+	.errstr = "cannot write into mptcp_sock",
+},