linux-kernel - [PATCH] bpf: fix reuse of DEVMAP

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Message-ID: <20250811093945.41028-1-yuka@yuka.dev>
Date: Mon, 11 Aug 2025 11:39:44 +0200
From: Yureka Lilian <yuka@...a.dev>
To: Andrii Nakryiko <andrii@...nel.org>,
	Eduard Zingerman <eddyz87@...il.com>,
	Alexei Starovoitov <ast@...nel.org>,
	Daniel Borkmann <daniel@...earbox.net>,
	Martin KaFai Lau <martin.lau@...ux.dev>,
	Song Liu <song@...nel.org>,
	Yonghong Song <yonghong.song@...ux.dev>,
	John Fastabend <john.fastabend@...il.com>,
	KP Singh <kpsingh@...nel.org>,
	Stanislav Fomichev <sdf@...ichev.me>,
	Hao Luo <haoluo@...gle.com>,
	Jiri Olsa <jolsa@...nel.org>
Cc: Yureka Lilian <yuka@...a.dev>,
	bpf@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: [PATCH] bpf: 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,
it BPF_F_RDONLY_PROG being set on a map being created is invalid.

Thus, match the BPF_F_RDONLY_PROG flag being set on the new map when
checking for compatibility with an existing DEVMAP

The same problem is handled in third-party ebpf library:
- https://github.com/cilium/ebpf/issues/925
- https://github.com/cilium/ebpf/pull/930

Signed-off-by: Yureka Lilian <yuka@...a.dev>
---
 tools/lib/bpf/libbpf.c | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
index fb4d92c5c..a554d7fff 100644
--- a/tools/lib/bpf/libbpf.c
+++ b/tools/lib/bpf/libbpf.c
@@ -5081,6 +5081,7 @@ static bool map_is_reuse_compat(const struct bpf_map *map, int map_fd)
 {
 	struct bpf_map_info map_info;
 	__u32 map_info_len = sizeof(map_info);
+	__u32 map_flags_for_check = map->def.map_flags;
 	int err;
 
 	memset(&map_info, 0, map_info_len);
@@ -5093,11 +5094,20 @@ static bool map_is_reuse_compat(const struct bpf_map *map, int map_fd)
 		return false;
 	}
 
+	/* get_map_info on a DEVMAP will always return flags with
+	 * BPF_F_RDONLY_PROG set, but it will never be set on a map
+	 * being created.
+	 * Thus, match the BPF_F_RDONLY_PROG flag being set on the new
+	 * map 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_flags_for_check |= 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 &&
 		map_info.max_entries == map->def.max_entries &&
-		map_info.map_flags == map->def.map_flags &&
+		map_info.map_flags == map_flags_for_check &&
 		map_info.map_extra == map->map_extra);
 }
 
-- 
2.50.1