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

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Date:   Wed, 8 Jun 2022 17:22:34 +0200
From:   Daniel Borkmann <daniel@...earbox.net>
To:     James Hilliard <james.hilliard1@...il.com>
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 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
> 
>  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