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

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Message-ID: <20220523221954.dsqvy55ron4cfdqq@jup>
Date:   Mon, 23 May 2022 18:19:54 -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 03:02:31PM -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.

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
> >