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:
- https://github.com/cilium/ebpf/issues/925
- https://github.com/cilium/ebpf/pull/930

Fixes: 0cdbb4b09a06 ("devmap: Allow map lookups from eBPF")
Signed-off-by: Yureka Lilian <yuka@yuka.dev>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20250814180113.1245565-3-yuka@yuka.dev

diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
index 8f5a81b672e1b8546852d5d8dd34518b33443421..fe4fc5438678c51d3df036b1d54120ce7acd127b 100644 (file)
--- a/tools/lib/bpf/libbpf.c
+++ b/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 &&