samples/bpf: Fix broken xdp_rxq_info due to map order assumptions

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

diff --git a/samples/bpf/xdp_rxq_info_user.c b/samples/bpf/xdp_rxq_info_user.c
index 51e0d810e0709b2d8800b7629b78a474f1b3a438..8fc3ad01de72c7bb8cf1c4cec0a4cbf17634758f 100644 (file)
--- 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;