Re: [PATCH bpf-next v1 3/8] bpf: Introduce help function to validate ksym's type.

[Hao Luo]

On Fri, Aug 21, 2020 at 2:50 PM Andrii Nakryiko
<andrii.nakryiko@xxxxxxxxx> wrote:
>
> On Thu, Aug 20, 2020 at 10:22 AM Yonghong Song <yhs@xxxxxx> wrote:
> >
> >
> >
> > On 8/19/20 3:40 PM, Hao Luo wrote:
> > > For a ksym to be safely dereferenced and accessed, its type defined in
> > > bpf program should basically match its type defined in kernel. Implement
> > > a help function for a quick matching, which is used by libbpf when
> > > resolving the kernel btf_id of a ksym.
> > >
> > > Signed-off-by: Hao Luo <haoluo@xxxxxxxxxx>
> > > ---
[...]
> > > +/*
> > > + * Match a ksym's type defined in bpf programs against its type encoded in
> > > + * kernel btf.
> > > + */
> > > +bool btf_ksym_type_match(const struct btf *ba, __u32 id_a,
> > > +                      const struct btf *bb, __u32 id_b)
> > > +{
>
> [...]
>
> > > +                     }
> > > +             }
> >
> > I am wondering whether this is too strict and how this can co-work with
> > CO-RE. Forcing users to write almost identical structure definition to
> > the underlying kernel will not be user friendly and may not work cross
> > kernel versions even if the field user cares have not changed.
> >
> > Maybe we can relax the constraint here. You can look at existing
> > libbpf CO-RE code.
>
> Right. Hao, can you just re-use bpf_core_types_are_compat() instead?
> See if semantics makes sense, but I think it should. BPF CO-RE has
> been permissive in terms of struct size and few other type aspects,
> because it handles relocations so well. This approach allows to not
> have to exactly match all possible variations of some struct
> definition, which is a big problem with ever-changing kernel data
> structures.
>

I have to say I hate myself writing another type comparison instead of
reusing the existing one. The issue is that when bpf_core_types_compat
compares names, it uses t1->name_off == t2->name_off. It is also used
in bpf_equal_common(). In my case, because these types are from two
different BTFs, their name_off are not expected to be the same, right?
I didn't find a good solution to refactor before posting this patch. I
think I can adapt bpf_core_type_compat() and pay more attention to
CO-RE.

> >
> > > +             break;
> > > +     }
>
> [...]
>
> > > +
> > >   struct btf_ext_sec_setup_param {
> > >       __u32 off;
> > >       __u32 len;
> > > diff --git a/tools/lib/bpf/btf.h b/tools/lib/bpf/btf.h
> > > index 91f0ad0e0325..5ef220e52485 100644
> > > --- a/tools/lib/bpf/btf.h
> > > +++ b/tools/lib/bpf/btf.h
> > > @@ -52,6 +52,8 @@ LIBBPF_API int btf__get_map_kv_tids(const struct btf *btf, const char *map_name,
> > >                                   __u32 expected_key_size,
> > >                                   __u32 expected_value_size,
> > >                                   __u32 *key_type_id, __u32 *value_type_id);
> > > +LIBBPF_API bool btf_ksym_type_match(const struct btf *ba, __u32 id_a,
> > > +                                 const struct btf *bb, __u32 id_b);
> > >
> > >   LIBBPF_API struct btf_ext *btf_ext__new(__u8 *data, __u32 size);
> > >   LIBBPF_API void btf_ext__free(struct btf_ext *btf_ext);
> >
> > The new API function should be added to libbpf.map.
>
> My question is why does this even have to be a public API?

I can fix. Please pardon my ignorance, what is the difference between
public and internal APIs? I wasn't sure, so used it improperly.

Thanks,
Hao