netdev - Re: [PATCH bpf] libbpf: add null pointer check in bpf_object__init_user_btf

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Message-ID: <CAEf4BzaQdv8s4cGp=ouitxczzWV1E1WeuxktDTp5JFkXXkRU=w@mail.gmail.com>
Date:   Thu, 12 Mar 2020 10:54:48 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Daniel Borkmann <daniel@...earbox.net>
Cc:     Quentin Monnet <quentin@...valent.com>,
        Alexei Starovoitov <ast@...nel.org>, bpf <bpf@...r.kernel.org>,
        Networking <netdev@...r.kernel.org>,
        Andrii Nakryiko <andriin@...com>,
        Michal Rostecki <mrostecki@...nsuse.org>
Subject: Re: [PATCH bpf] libbpf: add null pointer check in bpf_object__init_user_btf_maps()

On Thu, Mar 12, 2020 at 8:38 AM Daniel Borkmann <daniel@...earbox.net> wrote:
>
> On 3/12/20 3:03 PM, Quentin Monnet wrote:
> > When compiling bpftool with clang 7, after the addition of its recent
> > "bpftool prog profile" feature, Michal reported a segfault. This
> > occurred while the build process was attempting to generate the
> > skeleton needed for the profiling program, with the following command:
> >
> >      ./_bpftool gen skeleton skeleton/profiler.bpf.o > profiler.skel.h
> >
> > Tracing the error showed that bpf_object__init_user_btf_maps() does no
> > verification on obj->btf before passing it to btf__get_nr_types(), where
> > btf is dereferenced. Libbpf considers BTF information should be here
> > because of the presence of a ".maps" section in the object file (hence
> > the check on "obj->efile.btf_maps_shndx < 0" fails and we do not exit
> > from the function early), but it was unable to load BTF info as there is
> > no .BTF section.
> >
> > Add a null pointer check and error out if the pointer is null. The final
> > bpftool executable still fails to build, but at least we have a proper
> > error and no more segfault.
> >
> > Fixes: abd29c931459 ("libbpf: allow specifying map definitions using BTF")
> > Cc: Andrii Nakryiko <andriin@...com>
> > Reported-by: Michal Rostecki <mrostecki@...nsuse.org>
> > Signed-off-by: Quentin Monnet <quentin@...valent.com>
>
> Applied to bpf-next, thanks! Note ...

I don't think this is the right fix. The problem was in my
5327644614a1 ("libbpf: Relax check whether BTF is mandatory") commit.
I've removed "mandatory" status of BTF if .maps is present. But that's
not right. We have the need for BTF at two levels: for libbpf itself
and for kernel, those are overlapping, but not exactly the same. BTF
is needed for libbpf when .maps, .struct_ops and externs are present.
But kernel needs it only for when .struct_ops are present. Right now
those checks are conflated together. Proper fix would be to separate
them. Can we please undo this patch? I'll post a proper fix shortly.

>
> > ---
> >   tools/lib/bpf/libbpf.c | 4 ++++
> >   1 file changed, 4 insertions(+)
> >
> > diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> > index 223be01dc466..19c0c40e8a80 100644
> > --- a/tools/lib/bpf/libbpf.c
> > +++ b/tools/lib/bpf/libbpf.c
> > @@ -2140,6 +2140,10 @@ static int bpf_object__init_user_btf_maps(struct bpf_object *obj, bool strict,
> >               return -EINVAL;
> >       }
> >
> > +     if (!obj->btf) {
> > +             pr_warn("failed to retrieve BTF for map");
>
> I've added a '\n' here, otherwise it looks like:
>
> [...]
>    LINK     _bpftool
>    CLANG    skeleton/profiler.bpf.o
>    GEN      profiler.skel.h
> libbpf: failed to retrieve BTF for mapError: failed to open BPF object file: 0
> Makefile:129: recipe for target 'profiler.skel.h' failed
>
> Fixed version:
>
>    LINK     _bpftool
>    GEN      profiler.skel.h
> libbpf: failed to retrieve BTF for map
> Error: failed to open BPF object file: 0
> Makefile:129: recipe for target 'profiler.skel.h' failed
>
> Thanks,
> Daniel