netdev - Re: [PATCH v2] bpf: fix build warning

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Date:   Thu, 26 Mar 2020 16:54:26 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Jean-Philippe Menil <jpmenil@...il.com>
Cc:     yhs@...com, kernel-janitors@...r.kernel.org,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>,
        Andrii Nakryiko <andriin@...com>, netdev@...r.kernel.org,
        bpf@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] bpf: fix build warning - missing prototype

On Tue, Mar 24, 2020 at 08:22:31AM +0100, Jean-Philippe Menil wrote:
> Fix build warnings when building net/bpf/test_run.o with W=1 due
> to missing prototype for bpf_fentry_test{1..6}.
> 
> Declare prototypes in order to silence warnings.
> 
> Signed-off-by: Jean-Philippe Menil <jpmenil@...il.com>
> ---
>  net/bpf/test_run.c | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/net/bpf/test_run.c b/net/bpf/test_run.c
> index d555c0d8657d..cdf87fb0b6eb 100644
> --- a/net/bpf/test_run.c
> +++ b/net/bpf/test_run.c
> @@ -113,31 +113,37 @@ static int bpf_test_finish(const union bpf_attr *kattr,
>   * architecture dependent calling conventions. 7+ can be supported in the
>   * future.
>   */
> +int noinline bpf_fentry_test1(int a);
>  int noinline bpf_fentry_test1(int a)
>  {
>  	return a + 1;
>  }
>  
> +int noinline bpf_fentry_test2(int a, u64 b);
>  int noinline bpf_fentry_test2(int a, u64 b)
>  {
>  	return a + b;
>  }
>  
> +int noinline bpf_fentry_test3(char a, int b, u64 c);
>  int noinline bpf_fentry_test3(char a, int b, u64 c)
>  {
>  	return a + b + c;
>  }
>  
> +int noinline bpf_fentry_test4(void *a, char b, int c, u64 d);
>  int noinline bpf_fentry_test4(void *a, char b, int c, u64 d)
>  {
>  	return (long)a + b + c + d;
>  }
>  
> +int noinline bpf_fentry_test5(u64 a, void *b, short c, int d, u64 e);
>  int noinline bpf_fentry_test5(u64 a, void *b, short c, int d, u64 e)
>  {
>  	return a + (long)b + c + d + e;
>  }
>  
> +int noinline bpf_fentry_test6(u64 a, void *b, short c, int d, void *e, u64 f);
>  int noinline bpf_fentry_test6(u64 a, void *b, short c, int d, void *e, u64 f)

That's a bit too much of "watery water".
Have you considered
__diag_push();
__diag_ignore(GCC, "-Wwhatever specific flag will shut up this warn")
__diag_pop();
approach ?
It will be self documenting as well.