libbpf: Fix reuse of DEVMAP

Previously, re-using pinned DEVMAP maps would always fail, because
get_map_info on a DEVMAP always returns flags with BPF_F_RDONLY_PROG set,
but BPF_F_RDONLY_PROG being set on a map during creation is invalid.

Thus, ignore the BPF_F_RDONLY_PROG flag in the flags returned from
get_map_info when checking for compatibility with an existing DEVMAP.

The same problem is handled in a third-party ebpf library:
- cilium/ebpf#925
- cilium/ebpf#930

Fixes: 0cdbb4b ("devmap: Allow map lookups from eBPF")
Signed-off-by: Yureka Lilian <[email protected]>
Signed-off-by: Andrii Nakryiko <[email protected]>
Link: https://lore.kernel.org/bpf/[email protected]

Author: yuyuyureka

tools/lib/bpf/libbpf.c

@@ -5093,6 +5093,16 @@ static bool map_is_reuse_compat(const struct bpf_map *map, int map_fd)
 		return false;
 	}
 
+	/*
+	 * bpf_get_map_info_by_fd() for DEVMAP will always return flags with
+	 * BPF_F_RDONLY_PROG set, but it generally is not set at map creation time.
+	 * Thus, ignore the BPF_F_RDONLY_PROG flag in the flags returned from
+	 * bpf_get_map_info_by_fd() when checking for compatibility with an
+	 * existing DEVMAP.
+	 */
+	if (map->def.type == BPF_MAP_TYPE_DEVMAP || map->def.type == BPF_MAP_TYPE_DEVMAP_HASH)
+		map_info.map_flags &= ~BPF_F_RDONLY_PROG;
+
 	return (map_info.type == map->def.type &&
 		map_info.key_size == map->def.key_size &&
 		map_info.value_size == map->def.value_size &&