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

[Hao Luo]

Ah, I see bpf_core_types_are_compat() after sync'ing my local repo. It
seems the perfect fit for my use case. I only found the
btf_equal_xxx() defined in btf.c when posting these patches. I can
test and use bpf_core_types_are_compat() in v2. Thanks for pointing it
out and explaining the public APIs.

Hao

On Fri, Aug 21, 2020 at 7:43 PM Andrii Nakryiko
<andrii.nakryiko@xxxxxxxxx> wrote:
>
> 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