netdev - Re: [PATCH] bpf: use flexible array members, not zero-length

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Message-ID: <02a551bc-7551-7c0e-0215-5ac8856b0512@embeddedor.com>
Date:   Sun, 29 Sep 2019 00:49:59 -0500
From:   "Gustavo A. R. Silva" <gustavo@...eddedor.com>
To:     Stephen Kitt <steve@....org>, Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>
Cc:     linux-doc@...r.kernel.org, netdev@...r.kernel.org,
        bpf@...r.kernel.org
Subject: Re: [PATCH] bpf: use flexible array members, not zero-length



On 9/28/19 09:48, Stephen Kitt wrote:
> This switches zero-length arrays in variable-length structs to C99
> flexible array members. GCC will then ensure that the arrays are
> always the last element in the struct.
> 
> Coccinelle:
> @@
> identifier S, fld;
> type T;
> @@
> 
> struct S {
>   ...
> - T fld[0];
> + T fld[];
>   ...
> };
> 
> Signed-off-by: Stephen Kitt <steve@....org>
> ---
>  Documentation/bpf/btf.rst       | 2 +-
>  tools/lib/bpf/libbpf.c          | 2 +-
>  tools/lib/bpf/libbpf_internal.h | 2 +-
>  3 files changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/Documentation/bpf/btf.rst b/Documentation/bpf/btf.rst
> index 4d565d202ce3..24ce50fc1fc1 100644
> --- a/Documentation/bpf/btf.rst
> +++ b/Documentation/bpf/btf.rst
> @@ -670,7 +670,7 @@ func_info for each specific ELF section.::
>          __u32   sec_name_off; /* offset to section name */
>          __u32   num_info;
>          /* Followed by num_info * record_size number of bytes */
> -        __u8    data[0];
> +        __u8    data[];
>       };
>  
>  Here, num_info must be greater than 0.
> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> index e0276520171b..c02ea0e1a588 100644
> --- a/tools/lib/bpf/libbpf.c
> +++ b/tools/lib/bpf/libbpf.c
> @@ -5577,7 +5577,7 @@ static struct perf_buffer *__perf_buffer__new(int map_fd, size_t page_cnt,
>  struct perf_sample_raw {
>  	struct perf_event_header header;
>  	uint32_t size;
> -	char data[0];
> +	char data[];
>  };
>  
>  struct perf_sample_lost {
> diff --git a/tools/lib/bpf/libbpf_internal.h b/tools/lib/bpf/libbpf_internal.h
> index 2e83a34f8c79..26eaa3f594aa 100644
> --- a/tools/lib/bpf/libbpf_internal.h
> +++ b/tools/lib/bpf/libbpf_internal.h
> @@ -86,7 +86,7 @@ struct btf_ext_info_sec {
>  	__u32	sec_name_off;
>  	__u32	num_info;
>  	/* Followed by num_info * record_size number of bytes */
> -	__u8	data[0];
> +	__u8 data[];

I think you should preserve the tab here.

--
Gustavo

>  };
>  
>  /* The minimum bpf_func_info checked by the loader */
>