netdev - Re: [PATCH] bpftool: mmaped fields missing map structure in generated skeletons

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Date:   Mon, 23 May 2022 19:08:54 -0400
From:   Michael Mullin <masmullin@...il.com>
To:     Andrii Nakryiko <andrii.nakryiko@...il.com>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>,
        Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] bpftool: mmaped fields missing map structure in
 generated skeletons

On Mon, May 23, 2022 at 03:38:03PM -0700, Andrii Nakryiko wrote:
> On Mon, May 23, 2022 at 3:19 PM Michael Mullin <masmullin@...il.com> wrote:
> >
> > On Mon, May 23, 2022 at 03:02:31PM -0700, Andrii Nakryiko wrote:
> > > On Mon, May 23, 2022 at 1:26 PM Michael Mullin <masmullin@...il.com> wrote:
> > > >
> > > > When generating a skeleton which has an mmaped map field, bpftool's
> > > > output is missing the map structure.  This causes a compile break when
> > > > the generated skeleton is compiled as the field belongs to the internal
> > > > struct maps, not directly to the obj.
> > > >
> > > > Signed-off-by: Michael Mullin <masmullin@...il.com>
> > > > ---
> > > >  tools/bpf/bpftool/gen.c | 2 +-
> > > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > > >
> > > > diff --git a/tools/bpf/bpftool/gen.c b/tools/bpf/bpftool/gen.c
> > > > index f158dc1c2149..b49293795ba0 100644
> > > > --- a/tools/bpf/bpftool/gen.c
> > > > +++ b/tools/bpf/bpftool/gen.c
> > > > @@ -853,7 +853,7 @@ codegen_maps_skeleton(struct bpf_object *obj, size_t map_cnt, bool mmaped)
> > > >                         i, bpf_map__name(map), i, ident);
> > > >                 /* memory-mapped internal maps */
> > > >                 if (mmaped && is_internal_mmapable_map(map, ident, sizeof(ident))) {
> > > > -                       printf("\ts->maps[%zu].mmaped = (void **)&obj->%s;\n",
> > > > +                       printf("\ts->maps[%zu].mmaped = (void **)&obj->maps.%s;\n",
> > >
> > > That's not right. maps.my_map is struct bpf_map *, but mmaped is
> > > supposed to be a blob of memory that is memory-mapped into map.
> > >
> > > Can you elaborate on how you trigger that compilation error with a
> > > small example?
> >
> > I have an a very small example on github. I have added some sed fixes to
> > my Makefile make my sample program compile.
> > https://github.com/masmullin2000/libbpf-sample/tree/main/c/simple
> >
> > https://github.com/masmullin2000/libbpf-sample/tree/02c7f945bf9027daedec04f485a320ba2df28204/c/simple
> > contains broken code.
> 
> You are missing -g when calling clang to generate debug info (DWARF
> and subsequently BTF). When BPF object file doesn't have BTF, we don't
> generate those bss/data/rodata/etc sections of the skeleton. You
> should be good just by adding -g.
> 

Thank you. By using -g, I am also unable to trigger the segfault
I was trying to fix with the patch
https://lore.kernel.org/bpf/20220523194917.igkgorco42537arb@jup/T/#u

I am not sure if there would be other ways to trigger the segfault.

> >
> > I apologize if this is an incorrect way to share external code.  I
> > haven't found the proper way to share example code from
> > kernelnewbies.org
> >
> > >
> > > >                                 i, ident);
> > > >                 }
> > > >                 i++;
> > > > --
> > > > 2.36.1
> > > >