netdev - Re: [PATCH bpf-next 06/13] bpf: Introduce BPF_MAP_TYPE_STRUCT

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Date:   Fri, 20 Dec 2019 10:41:20 -0800
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Martin Lau <kafai@...com>
Cc:     Yonghong Song <yhs@...com>,
        "bpf@...r.kernel.org" <bpf@...r.kernel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        David Miller <davem@...emloft.net>,
        Kernel Team <Kernel-team@...com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: [PATCH bpf-next 06/13] bpf: Introduce BPF_MAP_TYPE_STRUCT_OPS

On Fri, Dec 20, 2019 at 8:52 AM Martin Lau <kafai@...com> wrote:
>
> On Thu, Dec 19, 2019 at 11:22:17PM -0800, Martin Lau wrote:
>
> > [ ... ]
> >
> > > > +/* __bpf_##_name (e.g. __bpf_tcp_congestion_ops) is the map's value
> > > > + * exposed to the userspace and its btf-type-id is stored
> > > > + * at the map->btf_vmlinux_value_type_id.
> > > > + *
> > > > + * The *_name##_dummy is to ensure the BTF type is emitted.
> > > > + */
> > > > +
> > > >   #define BPF_STRUCT_OPS_TYPE(_name)                              \
> > > > -extern struct bpf_struct_ops bpf_##_name;
> > > > +extern struct bpf_struct_ops bpf_##_name;                        \
> > > > +                                                         \
> > > > +static struct __bpf_##_name {                                    \
> > > > + BPF_STRUCT_OPS_COMMON_VALUE;                            \
> > > > + struct _name data ____cacheline_aligned_in_smp;         \
> > > > +} *_name##_dummy;
> > >
> > > There are other ways to retain types in debug info without
> > > creating new variables. For example, you can use it in a cast
> > > like
> > >      (void *)(struct __bpf_##_name *)v
> > hmm... What is v?
> Got it.  "v" could be any dummy pointer in a function.
> I will use (void) instead of (void *) to avoid compiler warning.
>

This discussion inspired me to try this:

#define PRESERVE_TYPE_INFO(type) ((void)(type *)0)

... somewhere in any function ...

PRESERVE_TYPE_INFO(struct whatever_struct);

And it works! We should probably put this helper macro somewhere in
include/linux/bpf.h and use it consistently for cases like this.

> >
> > > Not sure whether we could easily find a place for such casting or not.
> This can be done in bpf_struct_ops_init().
>
> Thanks for the tips!