bpf: generalize dynptr_get_spi to be usable for iters

Generalize the logic of fetching special stack slot object state using
spi (stack slot index). This will be used by STACK_ITER logic next.
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/r/20230302235015.2044271-12-andrii@kernel.orgSigned-off-by: Alexei Starovoitov <ast@kernel.org>

kernel/bpf/verifier.c

@@ -710,32 +710,38 @@ static bool is_spi_bounds_valid(struct bpf_func_state *state, int spi, int nr_sl
        return spi - nr_slots + 1 >= 0 && spi < allocated_slots;
 }
 
-static int dynptr_get_spi(struct bpf_verifier_env *env, struct bpf_reg_state *reg)
+static int stack_slot_obj_get_spi(struct bpf_verifier_env *env, struct bpf_reg_state *reg,
+			          const char *obj_kind, int nr_slots)
 {
 	int off, spi;
 
 	if (!tnum_is_const(reg->var_off)) {
-		verbose(env, "dynptr has to be at a constant offset\n");
+		verbose(env, "%s has to be at a constant offset\n", obj_kind);
 		return -EINVAL;
 	}
 
 	off = reg->off + reg->var_off.value;
 	if (off % BPF_REG_SIZE) {
-		verbose(env, "cannot pass in dynptr at an offset=%d\n", off);
+		verbose(env, "cannot pass in %s at an offset=%d\n", obj_kind, off);
 		return -EINVAL;
 	}
 
 	spi = __get_spi(off);
-	if (spi < 1) {
-		verbose(env, "cannot pass in dynptr at an offset=%d\n", off);
+	if (spi + 1 < nr_slots) {
+		verbose(env, "cannot pass in %s at an offset=%d\n", obj_kind, off);
 		return -EINVAL;
 	}
 
-	if (!is_spi_bounds_valid(func(env, reg), spi, BPF_DYNPTR_NR_SLOTS))
+	if (!is_spi_bounds_valid(func(env, reg), spi, nr_slots))
 		return -ERANGE;
 	return spi;
 }
 
+static int dynptr_get_spi(struct bpf_verifier_env *env, struct bpf_reg_state *reg)
+{
+	return stack_slot_obj_get_spi(env, reg, "dynptr", BPF_DYNPTR_NR_SLOTS);
+}
+
 static const char *kernel_type_name(const struct btf* btf, u32 id)
 {
 	return btf_name_by_offset(btf, btf_type_by_id(btf, id)->name_off);