Re: [PATCH bpf-next v5 2/6] bpf/verifier: add bpf_timer as a kfunc capable type

[Kumar Kartikeya Dwivedi]

On Fri, 22 Mar 2024 at 15:57, Benjamin Tissoires <bentiss@xxxxxxxxxx> wrote:
>
> We need to extend the bpf_timer API, but the way forward relies on kfuncs.
> So make bpf_timer known for kfuncs from the verifier PoV
>
> Signed-off-by: Benjamin Tissoires <bentiss@xxxxxxxxxx>
>
> ---
>
> changes in v5:
> - also check for the reg offset
>
> changes in v4:
> - enforce KF_ARG_PTR_TO_TIMER to be of type PTR_TO_MAP_VALUE
>
> new in v3 (split from v2 02/10)
> ---
>  kernel/bpf/verifier.c | 23 +++++++++++++++++++++++
>  1 file changed, 23 insertions(+)
>
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index 63749ad5ac6b..24a604e26ec7 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -10826,6 +10826,7 @@ enum {
>         KF_ARG_LIST_NODE_ID,
>         KF_ARG_RB_ROOT_ID,
>         KF_ARG_RB_NODE_ID,
> +       KF_ARG_TIMER_ID,
>  };
>
>  BTF_ID_LIST(kf_arg_btf_ids)
> @@ -10834,6 +10835,7 @@ BTF_ID(struct, bpf_list_head)
>  BTF_ID(struct, bpf_list_node)
>  BTF_ID(struct, bpf_rb_root)
>  BTF_ID(struct, bpf_rb_node)
> +BTF_ID(struct, bpf_timer_kern)
>
>  static bool __is_kfunc_ptr_arg_type(const struct btf *btf,
>                                     const struct btf_param *arg, int type)
> @@ -10877,6 +10879,12 @@ static bool is_kfunc_arg_rbtree_node(const struct btf *btf, const struct btf_par
>         return __is_kfunc_ptr_arg_type(btf, arg, KF_ARG_RB_NODE_ID);
>  }
>
> +static bool is_kfunc_arg_timer(const struct btf *btf, const struct btf_param *arg)
> +{
> +       bool ret = __is_kfunc_ptr_arg_type(btf, arg, KF_ARG_TIMER_ID);
> +       return ret;
> +}
> +
>  static bool is_kfunc_arg_callback(struct bpf_verifier_env *env, const struct btf *btf,
>                                   const struct btf_param *arg)
>  {
> @@ -10946,6 +10954,7 @@ enum kfunc_ptr_arg_type {
>         KF_ARG_PTR_TO_NULL,
>         KF_ARG_PTR_TO_CONST_STR,
>         KF_ARG_PTR_TO_MAP,
> +       KF_ARG_PTR_TO_TIMER,
>  };
>
>  enum special_kfunc_type {
> @@ -11102,6 +11111,9 @@ get_kfunc_ptr_arg_type(struct bpf_verifier_env *env,
>         if (is_kfunc_arg_map(meta->btf, &args[argno]))
>                 return KF_ARG_PTR_TO_MAP;
>
> +       if (is_kfunc_arg_timer(meta->btf, &args[argno]))
> +               return KF_ARG_PTR_TO_TIMER;
> +
>         if ((base_type(reg->type) == PTR_TO_BTF_ID || reg2btf_ids[base_type(reg->type)])) {
>                 if (!btf_type_is_struct(ref_t)) {
>                         verbose(env, "kernel function %s args#%d pointer type %s %s is not supported\n",
> @@ -11735,6 +11747,7 @@ static int check_kfunc_args(struct bpf_verifier_env *env, struct bpf_kfunc_call_
>                 case KF_ARG_PTR_TO_CALLBACK:
>                 case KF_ARG_PTR_TO_REFCOUNTED_KPTR:
>                 case KF_ARG_PTR_TO_CONST_STR:
> +               case KF_ARG_PTR_TO_TIMER:
>                         /* Trusted by default */
>                         break;
>                 default:
> @@ -12021,6 +12034,16 @@ static int check_kfunc_args(struct bpf_verifier_env *env, struct bpf_kfunc_call_
>                         if (ret)
>                                 return ret;
>                         break;
> +               case KF_ARG_PTR_TO_TIMER:
> +                       if (reg->type != PTR_TO_MAP_VALUE) {
> +                               verbose(env, "arg#%d doesn't point to a map value\n", i);
> +                               return -EINVAL;
> +                       }
> +                       if (reg->off) {
> +                               verbose(env, "arg#%d offset can not be greater than 0\n", i);
> +                               return -EINVAL;
> +                       }

This won't be correct. You don't really check whether the timer exists
at reg->off (and if you did, this would still restrict it to 0 offset,
and not check the variable offset which would be non-zero). What I
would suggest is calling process_timer_func (see how dynptr calls the
same underlying process_dynptr_func to enforce type invariants). This
would allow sharing the same checks and avoid bugs from creeping in.
It does all checks wrt constant/variable offset and looking up the
timer field offset and matching it against the one in the pointer.

> +                       break;
>                 }
>         }
>
>
> --
> 2.44.0
>
>