On Fri, Dec 20, 2019 at 8:52 AM Martin Lau <kafai@xxxxxx> 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!
