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

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Message-ID: <f8215e80-ff1b-4911-a5b1-417630f9cdde@linux.dev>
Date: Mon, 11 Aug 2025 12:38:04 -0700
From: Martin KaFai Lau <martin.lau@...ux.dev>
To: Yureka Lilian <yuka@...a.dev>
Cc: Andrii Nakryiko <andrii@...nel.org>, Eduard Zingerman
 <eddyz87@...il.com>, Alexei Starovoitov <ast@...nel.org>,
 Daniel Borkmann <daniel@...earbox.net>, 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>, Toke Høiland-Jørgensen
 <toke@...hat.com>, bpf@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] bpf: fix reuse of DEVMAP

On 8/11/25 2:39 AM, Yureka Lilian wrote:
> 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>

This should be a fix for this commit?

Fixes: 0cdbb4b09a06 ("devmap: Allow map lookups from eBPF")

> ---
>   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;

Does it break older kernels that do not set the RDONLY_PROG bit implicitly by 
itself?

> +
>   	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);
>   }
>