netdev - Re: [PATCH v2 1/1] libbpf: replace typeof with __typeof_

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Date:   Thu, 9 Jun 2022 00:14:57 -0600
From:   James Hilliard <james.hilliard1@...il.com>
To:     Daniel Borkmann <daniel@...earbox.net>
Cc:     bpf <bpf@...r.kernel.org>, Alexei Starovoitov <ast@...nel.org>,
        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>,
        Björn Töpel <bjorn@...nel.org>,
        Magnus Karlsson <magnus.karlsson@...el.com>,
        Maciej Fijalkowski <maciej.fijalkowski@...el.com>,
        Jonathan Lemon <jonathan.lemon@...il.com>,
        "open list:BPF (Safe dynamic programs and tools)" 
        <netdev@...r.kernel.org>, open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 1/1] libbpf: replace typeof with __typeof__

On Wed, Jun 8, 2022 at 9:22 AM Daniel Borkmann <daniel@...earbox.net> wrote:
>
> On 6/8/22 3:04 PM, James Hilliard wrote:
> > On Wed, Jun 8, 2022 at 6:50 AM Daniel Borkmann <daniel@...earbox.net> 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@...il.com>
> >>> ---
> >>> 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

Ok, looks like this does appear to work, although just switching to __typeof__
may be preferable as it should work everywhere.

> >
> >  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 think my build env was using stale headers, I think I figured out
what's going on,
the typeof issue is triggered by building with -std=c17, the macro
issue is unrelated
and limited to the SEC macro it would appear.

I suppose running builds with -std=c17 would prevent typeof() usage from
slipping through.

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