linux-kernel - Re: [PATCH] libbpf: avoid inline hint definition from 'linux/stddef.h'

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Date:   Tue, 16 Mar 2021 22:57:14 +0100
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Andrii Nakryiko <andrii.nakryiko@...il.com>
Cc:     Pedro Tammela <pctammela@...il.com>,
        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>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
        open list <linux-kernel@...r.kernel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>
Subject: Re: [PATCH] libbpf: avoid inline hint definition from
 'linux/stddef.h'

On 3/16/21 10:34 PM, Andrii Nakryiko wrote:
> On Tue, Mar 16, 2021 at 2:01 PM Daniel Borkmann <daniel@...earbox.net> wrote:
>> On 3/14/21 6:38 PM, Pedro Tammela wrote:
>>> Linux headers might pull 'linux/stddef.h' which defines
>>> '__always_inline' as the following:
>>>
>>>      #ifndef __always_inline
>>>      #define __always_inline __inline__
>>>      #endif
>>>
>>> This becomes an issue if the program picks up the 'linux/stddef.h'
>>> definition as the macro now just hints inline to clang.
>>
>> How did the program end up including linux/stddef.h ? Would be good to
>> also have some more details on how we got here for the commit desc.
> 
> It's an UAPI header, so why not? Is there anything special about
> stddef.h that makes it unsuitable to be included?

Hm, fair enough, looks like linux/types.h already pulls it in, so no. We
defined our own stddef.h longer time ago, so looks like we never ran into
this issue.

>>> This change now enforces the proper definition for BPF programs
>>> regardless of the include order.
>>>
>>> Signed-off-by: Pedro Tammela <pctammela@...il.com>
>>> ---
>>>    tools/lib/bpf/bpf_helpers.h | 7 +++++--
>>>    1 file changed, 5 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/tools/lib/bpf/bpf_helpers.h b/tools/lib/bpf/bpf_helpers.h
>>> index ae6c975e0b87..5fa483c0b508 100644
>>> --- a/tools/lib/bpf/bpf_helpers.h
>>> +++ b/tools/lib/bpf/bpf_helpers.h
>>> @@ -29,9 +29,12 @@
>>>     */
>>>    #define SEC(NAME) __attribute__((section(NAME), used))
>>>
>>> -#ifndef __always_inline
>>> +/*
>>> + * Avoid 'linux/stddef.h' definition of '__always_inline'.
>>> + */
>>
>> I think the comment should have more details on 'why' we undef it as in
>> few months looking at it again, the next question to dig into would be
>> what was wrong with linux/stddef.h. Providing a better rationale would
>> be nice for readers here.
> 
> So for whatever reason commit bot didn't send notification, but I've
> already landed this yesterday. To me, with #undef + #define it's
> pretty clear that we "force-define" __always_inline exactly how we
> want it, but we can certainly add clarifying comment in the follow up,
> if you think it's needed.

Up to you, but given you applied it it's probably not worth the trouble;
missed it earlier given I didn't see the patchbot message in the thread
initially. :/

>>> +#undef __always_inline
>>>    #define __always_inline inline __attribute__((always_inline))
>>> -#endif
>>> +
>>>    #ifndef __noinline
>>>    #define __noinline __attribute__((noinline))
>>>    #endif
>>>
>>