netdev - Re: [PATCH v4 bpf-next 07/14] bpf: Allow nested BTF object to be refferenced by BTF object + offset

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Date:   Tue, 30 Jun 2020 13:07:17 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Jiri Olsa <jolsa@...nel.org>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        Martin KaFai Lau <kafai@...com>,
        David Miller <davem@...hat.com>,
        John Fastabend <john.fastabend@...il.com>,
        Wenbo Zhang <ethercflow@...il.com>,
        KP Singh <kpsingh@...omium.org>,
        Andrii Nakryiko <andriin@...com>,
        Brendan Gregg <bgregg@...flix.com>,
        Florent Revest <revest@...omium.org>,
        Al Viro <viro@...iv.linux.org.uk>
Subject: Re: [PATCH v4 bpf-next 07/14] bpf: Allow nested BTF object to be
 refferenced by BTF object + offset

On Tue, Jun 30, 2020 at 1:05 PM Andrii Nakryiko
<andrii.nakryiko@...il.com> wrote:
>
> On Thu, Jun 25, 2020 at 4:49 PM Jiri Olsa <jolsa@...nel.org> wrote:
> >
> > Adding btf_struct_address function that takes 2 BTF objects
> > and offset as arguments and checks whether object A is nested
> > in object B on given offset.
> >
> > This function will be used when checking the helper function
> > PTR_TO_BTF_ID arguments. If the argument has an offset value,
> > the btf_struct_address will check if the final address is
> > the expected BTF ID.
> >
> > This way we can access nested BTF objects under PTR_TO_BTF_ID
> > pointer type and pass them to helpers, while they still point
> > to valid kernel BTF objects.
> >
> > Using btf_struct_access to implement new btf_struct_address
> > function, because it already walks down the given BTF object.
> >
> > Signed-off-by: Jiri Olsa <jolsa@...nel.org>
> > ---
> >  include/linux/bpf.h   |  3 ++
> >  kernel/bpf/btf.c      | 67 ++++++++++++++++++++++++++++++++++++++-----
> >  kernel/bpf/verifier.c | 37 +++++++++++++++---------
> >  3 files changed, 87 insertions(+), 20 deletions(-)
> >

[...]

> >
> >  error:
> >                 bpf_log(log, "access beyond struct %s at off %u size %u\n",
> > @@ -4043,9 +4054,21 @@ int btf_struct_access(struct bpf_verifier_log *log,
> >
> >                         /* adjust offset we're looking for */
> >                         off -= moff;
> > +
> > +                       /* We are nexting into another struct,
> > +                        * check if we are crossing expected ID.
> > +                        */
> > +                       if (data->op == ACCESS_EXPECT && !off && t == data->exp_type)
>
> before you can do this type check, you need to btf_type_skip_modifiers() first.
>

Ignore this part, btf_resolve_size() (somewhat unexpectedly) already does that.

> > +                               return 0;
> >                         goto again;
> >                 }
> >
> > +               /* We are interested only in structs for expected ID,
> > +                * bail out.
> > +                */
> > +               if (data->op == ACCESS_EXPECT)
> > +                       return -EINVAL;
> > +
> >                 if (btf_type_is_ptr(mtype)) {
> >                         const struct btf_type *stype;
> >                         u32 id;

[...]