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    Subject[PATCH 5.4 010/102] samples/bpf: Fix broken xdp_rxq_info due to map order assumptions
    Date
    From: Jesper Dangaard Brouer <brouer@redhat.com>

    commit edbca120a8cdfa5a5793707e33497aa5185875ca upstream.

    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@redhat.com>
    Signed-off-by: Alexei Starovoitov <ast@kernel.org>
    Acked-by: Toke Høiland-Jørgensen <toke@redhat.com>
    Acked-by: Andrii Nakryiko <andriin@fb.com>
    Link: https://lore.kernel.org/bpf/157529025128.29832.5953245340679936909.stgit@firesoul
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

    ---
    samples/bpf/xdp_rxq_info_user.c | 6 +++---
    1 file changed, 3 insertions(+), 3 deletions(-)

    --- 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");
    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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