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Date:   Mon, 02 Dec 2019 13:37:31 +0100
From:   Jesper Dangaard Brouer <brouer@...hat.com>
To:     bpf@...r.kernel.org
Cc:     jakub.kicinski@...ronome.com, netdev@...r.kernel.org,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        Daniel Borkmann <borkmann@...earbox.net>,
        danieltimlee@...il.com,
        Alexei Starovoitov <alexei.starovoitov@...il.com>
Subject: [bpf PATCH] 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@...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");
 	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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