netdev - Re: [PATCH net-next] libbpf: use map

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Date:   Wed, 27 Sep 2017 09:29:12 -0700
From:   Alexei Starovoitov <ast@...com>
To:     Craig Gallek <kraigatgoog@...il.com>,
        Daniel Borkmann <daniel@...earbox.net>,
        "David S . Miller" <davem@...emloft.net>
CC:     Chonggang Li <chonggangli@...gle.com>, <netdev@...r.kernel.org>
Subject: Re: [PATCH net-next] libbpf: use map_flags when creating maps

On 9/27/17 7:04 AM, Craig Gallek wrote:
> From: Craig Gallek <kraig@...gle.com>
>
> This extends struct bpf_map_def to include a flags field.  Note that
> this has the potential to break the validation logic in
> bpf_object__validate_maps and bpf_object__init_maps as they use
> sizeof(struct bpf_map_def) as a minimal allowable size of a map section.
> Any bpf program compiled with a smaller struct bpf_map_def will fail this
> check.
>
> I don't believe this will be an issue in practice as both compile-time
> definitions of struct bpf_map_def (in samples/bpf/bpf_load.h and
> tools/testing/selftests/bpf/bpf_helpers.h) have always been larger
> than this newly updated version in libbpf.h.
>
> Signed-off-by: Craig Gallek <kraig@...gle.com>
> ---
>  tools/lib/bpf/libbpf.c | 2 +-
>  tools/lib/bpf/libbpf.h | 1 +
>  2 files changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> index 35f6dfcdc565..6bea85f260a3 100644
> --- a/tools/lib/bpf/libbpf.c
> +++ b/tools/lib/bpf/libbpf.c
> @@ -874,7 +874,7 @@ bpf_object__create_maps(struct bpf_object *obj)
>  				      def->key_size,
>  				      def->value_size,
>  				      def->max_entries,
> -				      0);
> +				      def->map_flags);
>  		if (*pfd < 0) {
>  			size_t j;
>  			int err = *pfd;
> diff --git a/tools/lib/bpf/libbpf.h b/tools/lib/bpf/libbpf.h
> index 7959086eb9c9..6e20003109e0 100644
> --- a/tools/lib/bpf/libbpf.h
> +++ b/tools/lib/bpf/libbpf.h
> @@ -207,6 +207,7 @@ struct bpf_map_def {
>  	unsigned int key_size;
>  	unsigned int value_size;
>  	unsigned int max_entries;
> +	unsigned int map_flags;
>  };

yes it will break loading of pre-compiled .o
Instead of breaking, let's fix the loader to do it the way
samples/bpf/bpf_load.c does.
See commit 156450d9d964 ("samples/bpf: make bpf_load.c code compatible 
with ELF maps section changes")