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

[Andrii Nakryiko]

On Fri, Aug 21, 2020 at 5:43 PM Hao Luo <haoluo@xxxxxxxxxx> wrote:
>
> 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

Huh? Are we talking about the same bpf_core_types_are_compat() (there
is no bpf_core_types_compat, I think it's a typo)?
bpf_core_types_are_compat() doesn't even compare any name, so I'm not
sure what you are talking about. Some of btf_dedup functions do string
comparisons using name_off directly, but that's a special and very
careful case, it's not relevant here.


> 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

bpf_core_types_are_compat() didn't land until this week, so you must
be confusing something. Please take another look.

> 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.

public APIs are those that users of libbpf are supposed to use,
internal one is just for libbpf internal use. The former can't change,
the latter can be refactor as much as we need to.

>
> Thanks,
> Hao