bpf: emit frameno for PTR_TO_STACK regs if it differs from current one

It's possible to pass a pointer to parent's stack to child subprogs. In
such case verifier state output is ambiguous not showing whether
register container a pointer to "current" stack, belonging to current
subprog (frame), or it's actually a pointer to one of parent frames.

So emit this information if frame number differs between the state which
register is part of. E.g., if current state is in frame 2 and it has
a register pointing to stack in grand parent state (frame #0), we'll see
something like 'R1=fp[0]-16', while "local stack pointer" will be just
'R2=fp-16'.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>

kernel/bpf/log.c

@@ -615,18 +615,18 @@ static bool type_is_map_ptr(enum bpf_reg_type t) {
 	}
 }
 
-static void print_reg_state(struct bpf_verifier_env *env, const struct bpf_reg_state *reg)
+static void print_reg_state(struct bpf_verifier_env *env,
+			    const struct bpf_func_state *state,
+			    const struct bpf_reg_state *reg)
 {
 	enum bpf_reg_type t;
 	const char *sep = "";
 
 	t = reg->type;
 	if (t == SCALAR_VALUE && reg->precise)
 		verbose(env, "P");
-	if ((t == SCALAR_VALUE || t == PTR_TO_STACK) &&
-	    tnum_is_const(reg->var_off)) {
+	if (t == SCALAR_VALUE && tnum_is_const(reg->var_off)) {
 		/* reg->off should be 0 for SCALAR_VALUE */
-		verbose(env, "%s", t == SCALAR_VALUE ? "" : reg_type_str(env, t));
 		verbose_snum(env, reg->var_off.value + reg->off);
 		return;
 	}
@@ -637,6 +637,14 @@ static void print_reg_state(struct bpf_verifier_env *env, const struct bpf_reg_s
 #define verbose_a(fmt, ...) ({ verbose(env, "%s" fmt, sep, ##__VA_ARGS__); sep = ","; })
 
 	verbose(env, "%s", reg_type_str(env, t));
+	if (t == PTR_TO_STACK) {
+		if (state->frameno != reg->frameno)
+			verbose(env, "[%d]", reg->frameno);
+		if (tnum_is_const(reg->var_off)) {
+			verbose_snum(env, reg->var_off.value + reg->off);
+			return;
+		}
+	}
 	if (base_type(t) == PTR_TO_BTF_ID)
 		verbose(env, "%s", btf_type_name(reg->btf, reg->btf_id));
 	verbose(env, "(");
@@ -694,7 +702,7 @@ void print_verifier_state(struct bpf_verifier_env *env, const struct bpf_func_st
 		verbose(env, " R%d", i);
 		print_liveness(env, reg->live);
 		verbose(env, "=");
-		print_reg_state(env, reg);
+		print_reg_state(env, state, reg);
 	}
 	for (i = 0; i < state->allocated_stack / BPF_REG_SIZE; i++) {
 		char types_buf[BPF_REG_SIZE + 1];
@@ -727,7 +735,7 @@ void print_verifier_state(struct bpf_verifier_env *env, const struct bpf_func_st
 			verbose(env, " fp%d", (-i - 1) * BPF_REG_SIZE);
 			print_liveness(env, reg->live);
 			verbose(env, "=%s", types_buf);
-			print_reg_state(env, reg);
+			print_reg_state(env, state, reg);
 			break;
 		case STACK_DYNPTR:
 			/* skip to main dynptr slot */