libbpf: Make bpf_map order and indices stable

Currently, libbpf re-sorts bpf_map structs after all the maps are added and
initialized, which might change their relative order and invalidate any
bpf_map pointer or index taken before that. This is inconvenient and
error-prone. For instance, it can cause .kconfig map index to point to a wrong
map.

Furthermore, libbpf itself doesn't rely on any specific ordering of bpf_maps,
so it's just an unnecessary complication right now. This patch drops sorting
of maps and makes their relative positions fixed. If efficient index is ever
needed, it's better to have a separate array of pointers as a search index,
instead of reordering bpf_map struct in-place. This will be less error-prone
and will allow multiple independent orderings, if necessary (e.g., either by
section index or by name).

Fixes: 166750bc ("libbpf: Support libbpf-provided extern variables")
Reported-by: Martin KaFai Lau <kafai@fb.com>
Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20200110034247.1220142-1-andriin@fb.com

tools/lib/bpf/libbpf.c

@@ -1123,16 +1123,6 @@ bpf_object__init_kversion(struct bpf_object *obj, void *data, size_t size)
 	return 0;
 }
 
-static int compare_bpf_map(const void *_a, const void *_b)
-{
-	const struct bpf_map *a = _a;
-	const struct bpf_map *b = _b;
-
-	if (a->sec_idx != b->sec_idx)
-		return a->sec_idx - b->sec_idx;
-	return a->sec_offset - b->sec_offset;
-}
-
 static bool bpf_map_type__is_map_in_map(enum bpf_map_type type)
 {
 	if (type == BPF_MAP_TYPE_ARRAY_OF_MAPS ||
@@ -2196,10 +2186,6 @@ static int bpf_object__init_maps(struct bpf_object *obj,
 	if (err)
 		return err;
 
-	if (obj->nr_maps) {
-		qsort(obj->maps, obj->nr_maps, sizeof(obj->maps[0]),
-		      compare_bpf_map);
-	}
 	return 0;
 }