bpf: fix max call depth check

fix off by one error in max call depth check
and add a test

Fixes: f4d7e40a5b ("bpf: introduce function calls (verification)")
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

kernel/bpf/verifier.c

@@ -2126,9 +2126,9 @@ static int check_func_call(struct bpf_verifier_env *env, struct bpf_insn *insn,
 	struct bpf_func_state *caller, *callee;
 	int i, subprog, target_insn;
 
-	if (state->curframe >= MAX_CALL_FRAMES) {
+	if (state->curframe + 1 >= MAX_CALL_FRAMES) {
 		verbose(env, "the call stack of %d frames is too deep\n",
-			state->curframe);
+			state->curframe + 2);
 		return -E2BIG;
 	}
 

tools/testing/selftests/bpf/test_verifier.c

@@ -8884,6 +8884,41 @@ static struct bpf_test tests[] = {
 		.result = REJECT,
 		.errstr = "combined stack",
 	},
+	{
+		"calls: stack depth check using three frames. test5",
+		.insns = {
+			/* main */
+			BPF_RAW_INSN(BPF_JMP|BPF_CALL, 0, 1, 0, 1), /* call A */
+			BPF_EXIT_INSN(),
+			/* A */
+			BPF_RAW_INSN(BPF_JMP|BPF_CALL, 0, 1, 0, 1), /* call B */
+			BPF_EXIT_INSN(),
+			/* B */
+			BPF_RAW_INSN(BPF_JMP|BPF_CALL, 0, 1, 0, 1), /* call C */
+			BPF_EXIT_INSN(),
+			/* C */
+			BPF_RAW_INSN(BPF_JMP|BPF_CALL, 0, 1, 0, 1), /* call D */
+			BPF_EXIT_INSN(),
+			/* D */
+			BPF_RAW_INSN(BPF_JMP|BPF_CALL, 0, 1, 0, 1), /* call E */
+			BPF_EXIT_INSN(),
+			/* E */
+			BPF_RAW_INSN(BPF_JMP|BPF_CALL, 0, 1, 0, 1), /* call F */
+			BPF_EXIT_INSN(),
+			/* F */
+			BPF_RAW_INSN(BPF_JMP|BPF_CALL, 0, 1, 0, 1), /* call G */
+			BPF_EXIT_INSN(),
+			/* G */
+			BPF_RAW_INSN(BPF_JMP|BPF_CALL, 0, 1, 0, 1), /* call H */
+			BPF_EXIT_INSN(),
+			/* H */
+			BPF_MOV64_IMM(BPF_REG_0, 0),
+			BPF_EXIT_INSN(),
+		},
+		.prog_type = BPF_PROG_TYPE_XDP,
+		.errstr = "call stack",
+		.result = REJECT,
+	},
 	{
 		"calls: spill into caller stack frame",
 		.insns = {