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

[Alexei Starovoitov]

On Fri, Mar 22, 2024 at 9:31 AM Kumar Kartikeya Dwivedi
<memxor@xxxxxxxxx> wrote:
>
> 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.

Observation is correct. The patch is buggy,
but the suggestion to follow process_dynptr_func() will lead
to unnecessary complexity.
dynptr-s are on stack with plenty of extra checks.
In this case bpf_timer is in map_value.
Much simpler is to follow KF_ARG_PTR_TO_MAP approach.
Set ref_id, ref_t, ref_tname to bpf_timer and
process_kf_arg_ptr_to_btf_id() should work as-is.

You still need
+BTF_ID(struct, bpf_timer_kern)
to recognize that argument in a kfunc,
but the selftests will be using 'struct bpf_timer t',
so KF_ARG_PTR_TO_TIMER would need to match against 'struct bpf_timer'
and not against 'struct bpf_timer_kern'.