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Date:   Mon, 2 Dec 2019 09:35:44 -0800
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Jesper Dangaard Brouer <brouer@...hat.com>
Cc:     bpf <bpf@...r.kernel.org>,
        Jakub Kicinski <jakub.kicinski@...ronome.com>,
        Networking <netdev@...r.kernel.org>,
        Daniel Borkmann <borkmann@...earbox.net>,
        "Daniel T. Lee" <danieltimlee@...il.com>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>
Subject: Re: [bpf PATCH] samples/bpf: fix broken xdp_rxq_info due to map order assumptions

On Mon, Dec 2, 2019 at 4:37 AM Jesper Dangaard Brouer <brouer@...hat.com> wrote:
>
> In the days of using bpf_load.c the order in which the 'maps' sections
> were defines in BPF side (*_kern.c) file, were used by userspace side
> to identify the map via using the map order as an index. In effect the
> order-index is created based on the order the maps sections are stored
> in the ELF-object file, by the LLVM compiler.
>
> This have also carried over in libbpf via API bpf_map__next(NULL, obj)
> to extract maps in the order libbpf parsed the ELF-object file.
>
> When BTF based maps were introduced a new section type ".maps" were
> created. I found that the LLVM compiler doesn't create the ".maps"
> sections in the order they are defined in the C-file. The order in the
> ELF file is based on the order the map pointer is referenced in the code.
>
> This combination of changes lead to xdp_rxq_info mixing up the map
> file-descriptors in userspace, resulting in very broken behaviour, but
> without warning the user.
>
> This patch fix issue by instead using bpf_object__find_map_by_name()
> to find maps via their names. (Note, this is the ELF name, which can
> be longer than the name the kernel retains).
>
> Fixes: be5bca44aa6b ("samples: bpf: convert some XDP samples from bpf_load to libbpf")
> Fixes: 451d1dc886b5 ("samples: bpf: update map definition to new syntax BTF-defined map")
> Signed-off-by: Jesper Dangaard Brouer <brouer@...hat.com>
> ---
>  samples/bpf/xdp_rxq_info_user.c |    6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
>
> diff --git a/samples/bpf/xdp_rxq_info_user.c b/samples/bpf/xdp_rxq_info_user.c
> index 51e0d810e070..8fc3ad01de72 100644
> --- a/samples/bpf/xdp_rxq_info_user.c
> +++ b/samples/bpf/xdp_rxq_info_user.c
> @@ -489,9 +489,9 @@ int main(int argc, char **argv)
>         if (bpf_prog_load_xattr(&prog_load_attr, &obj, &prog_fd))
>                 return EXIT_FAIL;
>
> -       map = bpf_map__next(NULL, obj);
> -       stats_global_map = bpf_map__next(map, obj);
> -       rx_queue_index_map = bpf_map__next(stats_global_map, obj);
> +       map =  bpf_object__find_map_by_name(obj, "config_map");
> +       stats_global_map = bpf_object__find_map_by_name(obj, "stats_global_map");
> +       rx_queue_index_map = bpf_object__find_map_by_name(obj, "rx_queue_index_map");

Yeah, relying on relative order of maps as instantiated by libbpf
internally is extremely fragile. Thanks for fixing this.

Acked-by: Andrii Nakryiko <andriin@...com>

>         if (!map || !stats_global_map || !rx_queue_index_map) {
>                 printf("finding a map in obj file failed\n");
>                 return EXIT_FAIL;
>

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