linux-kernel - [PATCH v3 1/2] libbpf: fix reuse of DEVMAP

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Message-ID: <20250814180113.1245565-3-yuka@yuka.dev>
Date: Thu, 14 Aug 2025 20:01:12 +0200
From: Yureka Lilian <yuka@...a.dev>
To: Alexei Starovoitov <ast@...nel.org>,
	Daniel Borkmann <daniel@...earbox.net>,
	Andrii Nakryiko <andrii@...nel.org>,
	Martin KaFai Lau <martin.lau@...ux.dev>,
	Eduard Zingerman <eddyz87@...il.com>,
	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 v3 1/2] 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@...a.dev>
---
 tools/lib/bpf/libbpf.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
index d41ee26b9..affcbb231 100644
--- a/tools/lib/bpf/libbpf.c
+++ b/tools/lib/bpf/libbpf.c
@@ -5088,6 +5088,17 @@ 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, ignore the BPF_F_RDONLY_PROG flag in the flags
+	 * returned from get_map_info 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 &&
-- 
2.50.1