Re: [PATCH v2 1/1] libbpf: replace typeof with __typeof__

[Daniel Borkmann]

On 6/8/22 3:04 PM, James Hilliard wrote:
> On Wed, Jun 8, 2022 at 6:50 AM Daniel Borkmann <daniel@xxxxxxxxxxxxx> wrote:
> > On 6/8/22 8:40 AM, James Hilliard wrote:
> > > It seems the gcc preprocessor breaks when typeof is used with
> > > macros.
> > >
> > > Fixes errors like:
> > > error: expected identifier or '(' before '#pragma'
> > >     106 | SEC("cgroup/bind6")
> > >         | ^~~
> > >
> > > error: expected '=', ',', ';', 'asm' or '__attribute__' before '#pragma'
> > >     114 | char _license[] SEC("license") = "GPL";
> > >         | ^~~
> > >
> > > Signed-off-by: James Hilliard <james.hilliard1@xxxxxxxxx>
> > > ---
> > > Changes v1 -> v2:
> > >     - replace typeof with __typeof__ instead of changing pragma macros
> > > ---
> > >    tools/lib/bpf/bpf_core_read.h   | 16 ++++++++--------
> > >    tools/lib/bpf/bpf_helpers.h     |  4 ++--
> > >    tools/lib/bpf/bpf_tracing.h     | 24 ++++++++++++------------
> > >    tools/lib/bpf/btf.h             |  4 ++--
> > >    tools/lib/bpf/libbpf_internal.h |  6 +++---
> > >    tools/lib/bpf/usdt.bpf.h        |  6 +++---
> > >    tools/lib/bpf/xsk.h             | 12 ++++++------
> > >    7 files changed, 36 insertions(+), 36 deletions(-)
> > >
> > > diff --git a/tools/lib/bpf/bpf_core_read.h b/tools/lib/bpf/bpf_core_read.h
> > > index fd48b1ff59ca..d3a88721c9e7 100644
> > > --- a/tools/lib/bpf/bpf_core_read.h
> > > +++ b/tools/lib/bpf/bpf_core_read.h
> > > @@ -111,7 +111,7 @@ enum bpf_enum_value_kind {
> > >    })
> > >
> > >    #define ___bpf_field_ref1(field)    (field)
> > > -#define ___bpf_field_ref2(type, field)       (((typeof(type) *)0)->field)
> > > +#define ___bpf_field_ref2(type, field)       (((__typeof__(type) *)0)->field)
> > >    #define ___bpf_field_ref(args...)                                       \
> > >        ___bpf_apply(___bpf_field_ref, ___bpf_narg(args))(args)
> >
> > Can't we just add the below?
> >
> > #ifndef typeof
> > # define typeof __typeof__
> > #endif
>
>  From what I can tell it's not actually missing, but rather is
> preprocessed differently
> as the errors seem to be macro related.

Are you saying that the above suggestion wouldn't work? Do you have some more
details? I'm mainly wondering if there's a way where we could prevent letting
typeof() usage slip through in future given from kernel side people are used
to it.

> I did also find this change which seems related:
> https://github.com/torvalds/linux/commit/8faf7fc597d59b142af41ddd4a2d59485f75f88a
>
> > Thanks,
> > Daniel